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Document values and JSON pointers of json_view

Browse Source 
- API pages for get, get_to, get_string, number_token, and value of
  basic_json_view; JSON pointer overloads of operator[], at, and
  contains; links both ways with the basic_json pages
- the feature page describes which conversions copy nothing
- the examples show when the view helps: strings without copies, numbers
  exactly as written, and paths into a large text

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-28 23:10:06 +02:00
parent 8b58c8cd7c
commit d3d2ad3e4a
34 changed files with 1007 additions and 16 deletions
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5
docs/docset/docSet.sql
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@@ -154,6 +154,9 @@ INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::empty', 'Met
INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::end', 'Method', 'api/basic_json_view/end/index.html');
INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::find', 'Method', 'api/basic_json_view/find/index.html');
INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::front', 'Method', 'api/basic_json_view/front/index.html');
INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::get', 'Method', 'api/basic_json_view/get/index.html');
INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::get_string', 'Method', 'api/basic_json_view/get_string/index.html');
INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::get_to', 'Method', 'api/basic_json_view/get_to/index.html');
INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::is_array', 'Method', 'api/basic_json_view/is_array/index.html');
INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::is_binary', 'Method', 'api/basic_json_view/is_binary/index.html');
INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::is_boolean', 'Method', 'api/basic_json_view/is_boolean/index.html');
@@ -169,12 +172,14 @@ INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::is_string',
INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::is_structured', 'Method', 'api/basic_json_view/is_structured/index.html');
INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::items', 'Method', 'api/basic_json_view/items/index.html');
INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::materialize', 'Method', 'api/basic_json_view/materialize/index.html');
INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::number_token', 'Method', 'api/basic_json_view/number_token/index.html');
INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::operator bool', 'Method', 'api/basic_json_view/operator_bool/index.html');
INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::operator[]', 'Operator', 'api/basic_json_view/operator[]/index.html');
INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::size', 'Method', 'api/basic_json_view/size/index.html');
INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::source_offset', 'Method', 'api/basic_json_view/source_offset/index.html');
INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::type', 'Method', 'api/basic_json_view/type/index.html');
INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::type_name', 'Method', 'api/basic_json_view/type_name/index.html');
INSERT INTO searchIndex(name, type, path) VALUES ('basic_json_view::value', 'Method', 'api/basic_json_view/value/index.html');
INSERT INTO searchIndex(name, type, path) VALUES ('json', 'Class', 'api/json/index.html');
INSERT INTO searchIndex(name, type, path) VALUES ('json_document', 'Class', 'api/json_document/index.html');
INSERT INTO searchIndex(name, type, path) VALUES ('json_view', 'Class', 'api/json_view/index.html');

2
docs/mkdocs/docs/api/basic_json/get.md
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@@ -163,6 +163,8 @@ overload (3).
- [get_ref](get_ref.md) get a reference to the stored value
- [operator ValueType](operator_ValueType.md) get a value via implicit conversion
- [Converting values](../../features/conversions.md) - the type conversions article
- [basic_json_view::get](../basic_json_view/get.md) - the same conversion on a zero-copy view (many types are
converted without ever building a `basic_json` value)
## Version history

2
docs/mkdocs/docs/api/basic_json/get_ref.md
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@@ -61,6 +61,8 @@ Constant.
## See also
- [get_ptr()](get_ptr.md) get a pointer value
- [basic_json_view::get_string](../basic_json_view/get_string.md) - the closest counterpart on a zero-copy view: a
string without a copy, but as a view rather than a reference to a value that must already exist
## Version history

1
docs/mkdocs/docs/api/basic_json/get_to.md
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@@ -67,6 +67,7 @@ Depends on the `json_serializer<ValueType>::from_json()` implementation.
- [get_ref](get_ref.md) get a reference to the stored value
- [get_ptr](get_ptr.md) get a pointer to the stored value
- [Converting values](../../features/conversions.md) - the type conversions article
- [basic_json_view::get_to](../basic_json_view/get_to.md) - the same conversion on a zero-copy view
## Version history

1
docs/mkdocs/docs/api/basic_json/value.md
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@@ -181,6 +181,7 @@ changes to any JSON value.
- see [`at`](at.md) for access by reference with range checking
- see [`operator[]`](operator%5B%5D.md) for unchecked access by reference
- [basic_json_view::value](../basic_json_view/value.md) - the same access on a zero-copy view
## Version history

46
docs/mkdocs/docs/api/basic_json_view/at.md
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@@ -9,11 +9,15 @@ basic_json_view at(const string_t& key) const;
// (2)
basic_json_view at(size_type idx) const;
basic_json_view at(int idx) const;
// (3)
basic_json_view at(const json_pointer& ptr) const;
```
1. Returns the value of the object member with key `key` -- the first one, should the key occur more than once (see
[Notes on duplicate keys](operator[].md#notes)).
2. Returns the array element at index `idx`.
3. Returns the value a JSON pointer `ptr` refers to, starting at this value.
## Parameters
@@ -23,10 +27,14 @@ basic_json_view at(int idx) const;
`idx` (in)
: index of the element to access
`ptr` (in)
: JSON pointer to the element to access
## Return value
1. the value of the first member with key `key`
2. the element at index `idx`
3. the value `ptr` resolves to, starting at this value
## Exception safety
@@ -42,6 +50,20 @@ Strong exception safety: if an exception is thrown, there are no changes to the
[`BasicJsonType::at`](../basic_json/at.md):
- Throws [`type_error.304`](../../home/exceptions.md#jsonexceptiontype_error304) if the value is not an array.
- Throws [`out_of_range.401`](../../home/exceptions.md#jsonexceptionout_of_range401) if `#!cpp idx >= size()`.
3. The function can throw the following exceptions, all with the same message as the corresponding call to
[`BasicJsonType::at`](../basic_json/at.md):
- Throws [`parse_error.106`](../../home/exceptions.md#jsonexceptionparse_error106) if an array index in `ptr`
begins with `#!cpp '0'`.
- Throws [`parse_error.109`](../../home/exceptions.md#jsonexceptionparse_error109) if an array index in `ptr` is
not a number.
- Throws [`out_of_range.401`](../../home/exceptions.md#jsonexceptionout_of_range401) if an array index in `ptr`
is out of range.
- Throws [`out_of_range.402`](../../home/exceptions.md#jsonexceptionout_of_range402) if a reference token is
`#!cpp "-"` at an array -- `at` never inserts an element, so `#!cpp "-"` is always invalid.
- Throws [`out_of_range.403`](../../home/exceptions.md#jsonexceptionout_of_range403) if a reference token names
an object member that does not exist.
- Throws [`out_of_range.404`](../../home/exceptions.md#jsonexceptionout_of_range404) if `ptr` cannot be resolved
because a reference token is used on a primitive value.
None of these exceptions carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics.md) path: the view has no
`BasicJsonType` value to point at, so the exception is created without one, even if `BasicJsonType` was built with
@@ -54,16 +76,20 @@ None of these exceptions carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics
already known from the index, without reading the key bytes -- before comparing its content.
2. Linear in `idx`: elements are skipped one at a time from the first one, since they are not a fixed size in the
index (unlike `BasicJsonType`'s array, which is random-access).
3. Linear in the number of reference tokens of `ptr` and, for each token, in the number of members of the object at
that level (as 1.) or the index into the array (as 2.).
## Notes
Unlike [`operator[]`](operator[].md), which returns a [discarded](is_discarded.md) view for a missing key or an
out-of-range index, `at` always throws -- exactly as `BasicJsonType::at` does, and with the same messages, so
existing error handling written against `BasicJsonType::at` keeps working unchanged when switched to a view.
existing error handling written against `BasicJsonType::at` keeps working unchanged when switched to a view. This
also holds for overload 3: unlike [`operator[]`](operator[].md) with a JSON pointer, which returns a discarded view
for a missing key or an out-of-range index, `at` throws for those too (`out_of_range.403`/`out_of_range.401`).
## Examples
??? example
??? example "Example: (1)/(2) access specified element with bounds checking"
The example below reads required fields out of a service configuration with `at`, and shows that the exceptions
it throws -- for a wrong type and for a missing key -- carry the same messages
@@ -79,11 +105,27 @@ existing error handling written against `BasicJsonType::at` keeps working unchan
--8<-- "examples/basic_json_view__at.output"
```
??? example "Example: (3) access specified element via JSON pointer with bounds checking"
The example below shows that `at` with a JSON pointer throws exactly the exceptions, with exactly the messages,
that [`BasicJsonType::at`](../basic_json/at.md) throws for the same pointer and the same document.
```cpp
--8<-- "examples/basic_json_view__at_json_pointer.cpp"
```
Output:
```json
--8<-- "examples/basic_json_view__at_json_pointer.output"
```
## See also
- [operator[]](operator[].md) - access specified element (returns a discarded view instead of throwing)
- [front](front.md), [back](back.md) - access the first or last element
- [`BasicJsonType::at`](../basic_json/at.md) - the corresponding function of `basic_json`
- [`json_pointer`](../json_pointer/index.md) - JSON pointer type used by overload 3
## Version history

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@@ -1,21 +1,30 @@
# <small>nlohmann::basic_json_view::</small>contains
```cpp
// (1)
bool contains(string_view_t key) const;
bool contains(const char* key) const;
bool contains(const string_t& key) const;
// (2)
bool contains(const json_pointer& ptr) const;
```
Checks whether the value is an object with a member with key `key`.
1. Checks whether the value is an object with a member with key `key`.
2. Checks whether a JSON pointer `ptr` can be resolved, starting at this value.
## Parameters
`key` (in)
: key value to check its existence
`ptr` (in)
: JSON pointer to check its existence
## Return value
`#!cpp true` if the value is an object and has a member with key `key`, `#!cpp false` otherwise.
1. `#!cpp true` if the value is an object and has a member with key `key`, `#!cpp false` otherwise
2. `#!cpp true` if `ptr` can be resolved to a value starting at this view, `#!cpp false` otherwise
## Exception safety
@@ -23,22 +32,34 @@ No-throw guarantee: this function never throws exceptions.
## Complexity
Linear in the number of members: as for [`ordered_json`](../ordered_json.md), members are compared one after
another, in document order, stopping at the first match. Each comparison first checks the key's length -- already
known from the index, without reading the key bytes -- before comparing its content.
1. Linear in the number of members: as for [`ordered_json`](../ordered_json.md), members are compared one after
another, in document order, stopping at the first match. Each comparison first checks the key's length -- already
known from the index, without reading the key bytes -- before comparing its content.
2. Linear in the number of reference tokens of `ptr` and, for each token, in the number of members of the object at
that level or the index into the array -- as for [`operator[]`](operator[].md#complexity) and
[`at`](at.md#complexity) with a JSON pointer.
## Notes
This method always returns `#!cpp false` when the value is not an object -- including a [discarded](is_discarded.md)
Overload 1 always returns `#!cpp false` when the value is not an object -- including a [discarded](is_discarded.md)
view.
!!! info "Postconditions"
If `#!cpp v.contains(key)` returns `#!cpp true`, then `#!cpp v[key]` is not [discarded](is_discarded.md).
If `#!cpp v.contains(key)` returns `#!cpp true`, then `#!cpp v[key]` is not [discarded](is_discarded.md). If
`#!cpp v.contains(ptr)` returns `#!cpp true`, then `#!cpp v[ptr]` is not discarded and `#!cpp v.at(ptr)` does not
throw.
!!! info "Overload 2 never throws"
Unlike [`BasicJsonType::contains(const json_pointer&)`](../basic_json/contains.md), which can throw for certain
malformed pointers (for instance an empty array-index reference token), overload 2 never throws: a missing key,
an out-of-range or malformed array index, a `#!cpp "-"` index, or a reference token used on a primitive all
simply make it return `#!cpp false`.
## Examples
??? example
??? example "Example: (1) check with key"
The example below counts how many of a batch of records carry an optional `retry_of` field, using `contains()`
to check without ever materializing a single record of the batch.
@@ -53,10 +74,26 @@ view.
--8<-- "examples/basic_json_view__contains.output"
```
??? example "Example: (2) check with JSON pointer"
The example below checks an optional, nested field with a JSON pointer, and shows two pointers that
`#!cpp contains()` resolves to `#!cpp false` without throwing.
```cpp
--8<-- "examples/basic_json_view__contains_json_pointer.cpp"
```
Output:
```json
--8<-- "examples/basic_json_view__contains_json_pointer.output"
```
## See also
- [find](find.md) - find a value in an object
- [count](count.md) - returns the number of occurrences of a key
- [at](at.md), [operator[]](operator[].md) - resolve a JSON pointer and throw, or return a discarded view
- [`BasicJsonType::contains`](../basic_json/contains.md) - the corresponding function of `basic_json`
## Version history

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@@ -0,0 +1,130 @@
# <small>nlohmann::basic_json_view::</small>get
```cpp
template<typename T>
T get() const;
```
Converts the value to `T`.
For the types below, the conversion works directly on the flat index -- no `BasicJsonType` value is built for it:
- `#!cpp bool`
- arithmetic types other than `#!cpp bool` (from a number; from a boolean, as `#!cpp 0`/`#!cpp 1`, exactly as
[`BasicJsonType::get<T>()`](../basic_json/get.md) converts a boolean)
- `#!cpp std::nullptr_t`
- `#!cpp std::basic_string<char, Traits, Alloc>` (including `string_t`) -- a copy of the string
- [`string_view_t`](index.md#member-types) -- **no copy**: the returned view points into the document's
[`source()`](../basic_json_document/source.md) text, or, for a string that contains escape sequences, into the
document's own buffer of decoded strings (see [`get_string()`](get_string.md))
- `BasicJsonType` -- equivalent to [`materialize()`](materialize.md)
- `basic_json_view` -- returns `#!cpp *this`
- `#!cpp std::vector<U, A>` -- element by element, each converted with `#!cpp get<U>()`; `#!cpp
std::vector<basic_json_view>` keeps a view of every element instead of a value
- `#!cpp std::map<K, V, C, A>` and `#!cpp std::unordered_map<K, V, H, E, A>`, if `K` is constructible from a `#!cpp
(const char*, std::size_t)` pair -- member by member, each value converted with `#!cpp get<V>()`; with a repeated
key, the *last* value is kept, as [`BasicJsonType::parse()`](../basic_json/parse.md) (and
[`materialize()`](materialize.md)) does; `#!cpp std::map<std::string, basic_json_view>` keeps views of the members
instead of values
Every other `T` -- `#!cpp std::list`, `#!cpp std::pair`, `#!cpp std::array`, enumerations, user types with a
`from_json()`, ... -- is converted by `#!cpp materialize().get<T>()`: the subtree is built into a real `BasicJsonType`
value first (as [`BasicJsonType::parse()`](../basic_json/parse.md) would), and converted from there exactly as
[`BasicJsonType::get<T>()`](../basic_json/get.md) would convert it.
## Template parameters
`T`
: the type to convert the value to
## Return value
the value, converted to `T`
## Exception safety
Strong exception safety: if an exception is thrown, there are no changes to the view or the document it refers to.
## Exceptions
- For the directly-converted types listed above (other than `BasicJsonType` and `basic_json_view`, which never
throw): throws [`type_error.302`](../../home/exceptions.md#jsonexceptiontype_error302) if the value's type does not
match `T` -- the same exception, with the same message, that [`BasicJsonType::get<T>()`](../basic_json/get.md)
throws for the same JSON type and `T`.
- For `#!cpp std::vector<U, A>`: throws `type_error.302` if the value is not an array; otherwise, whatever converting
an element to `U` throws.
- For `#!cpp std::map`/`#!cpp std::unordered_map`: throws `type_error.302` if the value is not an object; otherwise,
whatever converting a member to the mapped type throws.
- For every other `T`: whatever [`materialize().get<T>()`](../basic_json/get.md) throws -- typically `type_error.302`,
or whatever a user-provided `from_json()` throws.
None of the exceptions thrown directly by this function (the first three bullets above) carry a
[`JSON_DIAGNOSTICS`](../macros/json_diagnostics.md) path: the view has no `BasicJsonType` value to point at. An
exception thrown while converting through `materialize()` (the last bullet) is different: it is thrown by a real
`BasicJsonType` value, so it **does** carry a `JSON_DIAGNOSTICS` path if `BasicJsonType` was built with it enabled.
## Complexity
- `#!cpp bool`, arithmetic types, `#!cpp std::nullptr_t`, [`string_view_t`](index.md#member-types), `basic_json_view`:
constant.
- `#!cpp std::basic_string<char, Traits, Alloc>`: constant, plus one allocation and a copy of the string's bytes.
- `BasicJsonType`: linear in the size of the subtree, see [`materialize()`](materialize.md).
- `#!cpp std::vector<U, A>`: linear in the number of elements, times the complexity of converting one element to `U`.
- `#!cpp std::map`/`#!cpp std::unordered_map`: linear in the number of members for walking them, plus the container's
own insertion cost per member (logarithmic for `#!cpp std::map`, amortized constant for `#!cpp
std::unordered_map`), times the complexity of converting one member to the mapped type.
- every other `T`: linear in the size of the subtree (building the `BasicJsonType` value), plus the complexity of
[`BasicJsonType::get<T>()`](../basic_json/get.md) on it.
## Notes
!!! info "Floating-point values"
A floating-point `T` is converted from the same digits the lexer would see during `#!cpp BasicJsonType::parse()`,
using the same conversion, so the result is bit-for-bit identical to `#!cpp BasicJsonType::parse(text).get<T>()`
for the same source text.
!!! info "Duplicate keys"
`#!cpp std::map`/`#!cpp std::unordered_map` conversions keep the *last* value of a repeated key, like
[`materialize()`](materialize.md) and [`BasicJsonType::parse()`](../basic_json/parse.md) do. This is the opposite
of [`operator[]`](operator[].md)/[`at`](at.md)/[`find`](find.md)/[`contains`](contains.md), which resolve to the
*first* occurrence (see the [Notes on duplicate keys](operator[].md#notes)).
!!! info "No pointers, references, or implicit conversion"
Unlike `BasicJsonType`, `basic_json_view` has no stored value anywhere to hand out a pointer or a reference to, so
it provides neither `#!cpp get_ptr()`, `#!cpp get_ref()`, nor `#!cpp operator ValueType()`.
[`get_string()`](get_string.md) (equivalently, `#!cpp get<string_view_t>()`) is the zero-copy alternative for
strings.
## Examples
??? example
The example below reads typed fields straight into C++ variables, collects a view of every array element with
`#!cpp get<std::vector<basic_json_view>>()` instead of a value, and converts a nested object into a user type
through its `from_json()` -- which runs on a `BasicJsonType` value `materialize()` builds for just that one
member.
```cpp
--8<-- "examples/basic_json_view__get.cpp"
```
Output:
```json
--8<-- "examples/basic_json_view__get.output"
```
## See also
- [get_to](get_to.md) - convert and write into a passed value
- [get_string](get_string.md) - the string, without a copy
- [number_token](number_token.md) - a number's token text, without a copy
- [materialize](materialize.md) - build the `BasicJsonType` value of this subtree
- [`BasicJsonType::get`](../basic_json/get.md) - the corresponding function of `basic_json`
## Version history
- Added in version 3.13.0.

71
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@@ -0,0 +1,71 @@
# <small>nlohmann::basic_json_view::</small>get_string
```cpp
string_view_t get_string() const;
```
Returns the string value as a [`string_view_t`](index.md#member-types), without copying it.
## Return value
The string, as a [`string_view_t`](index.md#member-types) that points either into the document's
[`source()`](../basic_json_document/source.md) text (a string with no escape sequences), or into the document's own
buffer of decoded strings (a string that contains escape sequences, such as `#!json "\n"` or `#!json "\u00e9"`, which
had to be decoded once when the document was parsed).
## Exception safety
Strong exception safety: if an exception is thrown, there are no changes to the view or the document it refers to.
## Exceptions
Throws [`type_error.302`](../../home/exceptions.md#jsonexceptiontype_error302) if the value is not a string; example:
`"type must be string, but is array"`.
This exception does not carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics.md) path: the view has no
`BasicJsonType` value to point at, so the exception is created without one, even if `BasicJsonType` was built with
`JSON_DIAGNOSTICS` enabled.
## Complexity
Constant.
## Notes
`basic_json_view` has no `BasicJsonType` value stored anywhere, so unlike `BasicJsonType`, it has no `get_ref()` to
hand out a reference to a stored `string_t`. `get_string()` (equivalently, [`get<string_view_t>()`](get.md)) is the
zero-copy alternative: [`BasicJsonType::get_ref<const string_t&>()`](../basic_json/get_ref.md) is its closest
counterpart, except that it returns a view instead of a reference to a value that must already exist.
The returned [`string_view_t`](index.md#member-types) is valid exactly as long as the view that produced it -- see the
[validity rules](index.md) of `basic_json_view` -- and, for a string with no escapes, for as long as the document's
source text.
## Examples
??? example
The example below pulls one field out of a JSON text that stands in for a large API response, and shows that no
`#!cpp std::string` was allocated for it: the returned view still points inside the original buffer. A field that
contains an escape sequence cannot point into the original text -- it was decoded once into the document's own
buffer instead -- but still avoids a per-field allocation.
```cpp
--8<-- "examples/basic_json_view__get_string.cpp"
```
Output:
```json
--8<-- "examples/basic_json_view__get_string.output"
```
## See also
- [get](get.md) - convert the value to a given type (`#!cpp get<string_view_t>()` is equivalent to this function)
- [number_token](number_token.md) - a number's token text, without a copy
- [`BasicJsonType::get_ref`](../basic_json/get_ref.md) - the closest counterpart of `basic_json`
## Version history
- Added in version 3.13.0.

65
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@@ -0,0 +1,65 @@
# <small>nlohmann::basic_json_view::</small>get_to
```cpp
template<typename T>
T& get_to(T& v) const;
```
Converts the value to `T` and assigns it to `v`. Equivalent to
```cpp
v = get<T>();
return v;
```
## Template parameters
`T`
: the type to convert the value to
## Parameters
`v` (out)
: the variable to store the converted value in
## Return value
`v`, allowing calls to chain
## Exception safety
Strong exception safety: if an exception is thrown, `v` is not modified.
## Exceptions
Whatever [`get<T>()`](get.md) throws for the same value and `T`.
## Complexity
Whatever [`get<T>()`](get.md) has for the same `T`.
## Examples
??? example
The example below reads several fields of a service configuration directly into existing variables, then uses
the returned reference to fold the `#!cpp host`/`#!cpp port` pair into a single string in the same expression.
```cpp
--8<-- "examples/basic_json_view__get_to.cpp"
```
Output:
```json
--8<-- "examples/basic_json_view__get_to.output"
```
## See also
- [get](get.md) - convert the value to a given type
- [`BasicJsonType::get_to`](../basic_json/get_to.md) - the corresponding function of `basic_json`
## Version history
- Added in version 3.13.0.

12
docs/mkdocs/docs/api/basic_json_view/index.md
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@@ -20,8 +20,11 @@ Moving the document itself does not invalidate its views: the index is heap-allo
`basic_json_document` object.
`basic_json_view` provides the read-only part of the `BasicJsonType` interface: the type-inspection functions, element
access, lookup, iteration, and [`materialize()`](materialize.md) to build the `BasicJsonType` value of a subtree on
demand. It does not (yet) provide `get<T>()`, JSON Pointer support, `dump()`, or comparison.
access, lookup, iteration, and conversion -- [`get<T>()`](get.md), [`get_string()`](get_string.md),
[`number_token()`](number_token.md), and [`materialize()`](materialize.md) to build the `BasicJsonType` value of a
subtree on demand. [`operator[]`](operator%5B%5D.md), [`at`](at.md), [`contains`](contains.md), and
[`value`](value.md) also accept a [`json_pointer`](../json_pointer/index.md). It does not (yet) provide `dump()` or
comparison.
## Template parameters
@@ -72,6 +75,7 @@ demand. It does not (yet) provide `get<T>()`, JSON Pointer support, `dump()`, or
- [**at**](at.md) - access specified element with bounds checking
- [**operator[]**](operator[].md) - access specified element
- [**value**](value.md) - access specified element with default value
- [**front**](front.md) - access the first element
- [**back**](back.md) - access the last element
@@ -96,6 +100,10 @@ demand. It does not (yet) provide `get<T>()`, JSON Pointer support, `dump()`, or
### Conversion
- [**get**](get.md) - get a value
- [**get_to**](get_to.md) - get a value and write it to a destination
- [**get_string**](get_string.md) - get a string value without a copy
- [**number_token**](number_token.md) - get a number's token text without a copy
- [**materialize**](materialize.md) - build the `BasicJsonType` value of this subtree
### Source access

74
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@@ -0,0 +1,74 @@
# <small>nlohmann::basic_json_view::</small>number_token
```cpp
string_view_t number_token() const;
```
Returns the number exactly as it appears in the source text, without parsing or rounding it.
## Return value
The number's token text, as a [`string_view_t`](index.md#member-types) into the document's
[`source()`](../basic_json_document/source.md) text -- for example `#!cpp "1.50"`, `#!cpp "1E2"`, `#!cpp "-0"`, or an
integer literal with more digits than any number type holds (such as a 30-digit integer, which [`get<T>()`](get.md)
and [`materialize()`](materialize.md) can only represent approximately, as a `number_float_t`).
## Exception safety
Strong exception safety: if an exception is thrown, there are no changes to the view or the document it refers to.
## Exceptions
Throws [`type_error.302`](../../home/exceptions.md#jsonexceptiontype_error302) if the value is not a number; example:
`"type must be number, but is string"`.
This exception does not carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics.md) path: the view has no
`BasicJsonType` value to point at, so the exception is created without one, even if `BasicJsonType` was built with
`JSON_DIAGNOSTICS` enabled.
## Complexity
Constant.
## Notes
`BasicJsonType` has no counterpart to this function: once a number is parsed into `number_integer_t`,
`number_unsigned_t`, or `number_float_t`, its original textual form (leading zeros aside, which are already rejected
by the grammar; trailing zeros in the fraction; the case and sign of the exponent; ...) is gone. `number_token()` is
useful precisely where that form must survive -- a price or an identifier that must be reproduced exactly, or a
number too large for any of `BasicJsonType`'s number types to hold without loss.
The returned [`string_view_t`](index.md#member-types) always points into the document's
[`source()`](../basic_json_document/source.md) text -- numbers are never decoded into the document's separate string
buffer -- and is valid exactly as long as that text is, see the [validity rules](index.md) of `basic_json_view`.
## Examples
??? example
The example below keeps a price and an order ID exactly as they were written in an incoming order, where
converting them with [`get<T>()`](get.md) would lose information: the price picks up floating-point rounding, and
the order ID -- more digits than a 64-bit integer holds -- can only be approximated as a `#!cpp double` once
`#!cpp materialize()`d.
```cpp
--8<-- "examples/basic_json_view__number_token.cpp"
```
Output:
```json
--8<-- "examples/basic_json_view__number_token.output"
```
## See also
- [get](get.md) - convert the value to a given type
- [get_string](get_string.md) - the string, without a copy
- [`BasicJsonType::number_integer_t`](../basic_json/number_integer_t.md),
[`number_unsigned_t`](../basic_json/number_unsigned_t.md), [`number_float_t`](../basic_json/number_float_t.md) - the
number types `#!cpp get<T>()` and `materialize()` convert into
## Version history
- Added in version 3.13.0.

57
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@@ -9,12 +9,18 @@ basic_json_view operator[](const string_t& key) const;
// (2)
basic_json_view operator[](size_type idx) const;
basic_json_view operator[](int idx) const;
// (3)
basic_json_view operator[](const json_pointer& ptr) const;
```
1. Returns the value of the object member with key `key` -- the first one, should the key occur more than once (see
the [Notes](#notes) below) -- or a [discarded](is_discarded.md) view if there is no such member.
2. Returns the array element at index `idx`, or a [discarded](is_discarded.md) view if `idx` is out of range. (The
`#!cpp int` overload only exists so that an integer literal is not ambiguous between this overload and 1.)
3. Returns the value a JSON pointer `ptr` refers to, starting at this value, or a [discarded](is_discarded.md) view
wherever resolving it further is not possible without inserting into or extending the document (see
[Return value](#return-value) and [Exceptions](#exceptions) below).
## Parameters
@@ -24,11 +30,18 @@ basic_json_view operator[](int idx) const;
`idx` (in)
: index of the element to access
`ptr` (in)
: JSON pointer to the element to access
## Return value
1. the value of the first member with key `key`, or a discarded view if `#!cpp is_object()` is `#!cpp false` or no
member has this key
2. the element at index `idx`, or a discarded view if `#!cpp is_array()` is `#!cpp false` or `#!cpp idx >= size()`
3. the value `ptr` resolves to, starting at this value, or a discarded view for exactly the reference tokens where the
**const** overload of [`BasicJsonType::operator[]`](../basic_json/operator%5B%5D.md) invokes undefined behavior for
the same pointer and the same document: an object member that does not exist, or an array index that is out of
range
## Exception safety
@@ -42,9 +55,19 @@ Strong exception safety: if an exception is thrown, there are no changes to the
2. Throws [`type_error.305`](../../home/exceptions.md#jsonexceptiontype_error305) if the value is not an array --
the same exception, with the same message, that the **const** overload of
[`BasicJsonType::operator[]`](../basic_json/operator%5B%5D.md) throws for a numeric argument on a non-array value.
3. Throws the same exceptions, with the same messages, that the **const** overload of
[`BasicJsonType::operator[]`](../basic_json/operator%5B%5D.md) throws for the same pointer and the same document:
- [`out_of_range.402`](../../home/exceptions.md#jsonexceptionout_of_range402) if a reference token is `#!cpp "-"`
at an array.
- [`out_of_range.404`](../../home/exceptions.md#jsonexceptionout_of_range404) if a reference token cannot be
resolved because it is used on a primitive value.
- [`parse_error.106`](../../home/exceptions.md#jsonexceptionparse_error106) if an array index in `ptr` begins
with `#!cpp '0'`.
- [`parse_error.109`](../../home/exceptions.md#jsonexceptionparse_error109) if an array index in `ptr` is not a
number.
Neither exception carries a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics.md) path: the view has no `BasicJsonType`
value to point at, so the exception is created without one, even if `BasicJsonType` was built with
None of these exceptions carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics.md) path: the view has no
`BasicJsonType` value to point at, so the exception is created without one, even if `BasicJsonType` was built with
`JSON_DIAGNOSTICS` enabled.
## Complexity
@@ -55,6 +78,8 @@ value to point at, so the exception is created without one, even if `BasicJsonTy
different length than `key` is rejected without touching the source text.
2. Linear in `idx`: elements are skipped one at a time from the first one, since they are not a fixed size in the
index (unlike `BasicJsonType`'s array, which is random-access).
3. Linear in the number of reference tokens of `ptr` and, for each token, in the number of members of the object at
that level (as 1.) or the index into the array (as 2.).
## Notes
@@ -74,9 +99,18 @@ document.
early. [`begin()`](begin.md)/[`end()`](end.md) and [`items()`](items.md) iterate over *all* members, including
duplicates, in document order. See the example below and [`size()`](size.md#notes).
!!! info "JSON pointer resolution"
Overload 3 walks `ptr` one reference token at a time, starting at this value, the same way [`at`](at.md) and
[`contains`](contains.md) do. It only ever returns a discarded view where the **const** overload of
`BasicJsonType::operator[]` would be undefined behavior for the same pointer -- a missing object member or an
out-of-range array index -- and still throws for every other way `ptr` can fail to resolve. See
[`at`](at.md#exceptions) for the checked version, which throws in every case instead, and
[`contains`](contains.md) for a version that never throws.
## Examples
??? example
??? example "Example: (1)/(2) access specified element"
The example below reads a couple of fields out of a batch of user records without ever materializing a full
`BasicJsonType` value for the batch. `operator[]` is used both to look up an optional object member and to index
@@ -93,12 +127,29 @@ document.
--8<-- "examples/basic_json_view__operator[].output"
```
??? example "Example: (3) access specified element via JSON pointer"
The example below reaches straight into one deeply nested field of a large document with a single JSON pointer,
without ever building a tree for the rest of it, and shows the discarded-view and throwing outcomes of a pointer
that cannot be fully resolved.
```cpp
--8<-- "examples/basic_json_view__operator[]_json_pointer.cpp"
```
Output:
```json
--8<-- "examples/basic_json_view__operator[]_json_pointer.output"
```
## See also
- [at](at.md) - access specified element with bounds checking (throws instead of returning a discarded view)
- [front](front.md), [back](back.md) - access the first or last element
- [find](find.md), [contains](contains.md) - look up a member without throwing
- [`BasicJsonType::operator[]`](../basic_json/operator%5B%5D.md) - the corresponding function of `basic_json`
- [`json_pointer`](../json_pointer/index.md) - JSON pointer type used by overload 3
## Version history

131
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@@ -0,0 +1,131 @@
# <small>nlohmann::basic_json_view::</small>value
```cpp
// (1)
template<typename T>
T value(string_view_t key, const T& default_value) const;
string_t value(string_view_t key, const char* default_value) const;
// (2)
template<typename T>
T value(const json_pointer& ptr, const T& default_value) const;
string_t value(const json_pointer& ptr, const char* default_value) const;
```
1. Returns the value of the object member with key `key` -- the first one, should the key occur more than once (see
[Notes on duplicate keys](operator[].md#notes)) -- converted to `T`, or `default_value` if there is no such member.
2. Returns the value a JSON pointer `ptr` refers to, starting at this value, converted to `T`, or `default_value` if
`ptr` cannot be resolved.
Both overloads have a dedicated `#!cpp const char*` overload, so `#!cpp v.value(key, "default")` (and the JSON pointer
equivalent) deduce `string_t`, not `const char*`, for their return type and for the comparison used to pick between
`key` and `default_value`.
## Template parameters
`T`
: the type to convert the found value to; also the type of `default_value`
## Parameters
`key` (in)
: object key of the element to access
`ptr` (in)
: JSON pointer to the element to access
`default_value` (in)
: the value to return if `key`/`ptr` resolves to no value
## Return value
1. the first member with key `key`, converted to `T`, or `default_value`
2. the value `ptr` resolves to, converted to `T`, or `default_value`
## Exception safety
Strong exception safety: if an exception is thrown, there are no changes to the view or the document it refers to.
## Exceptions
1. Throws [`type_error.306`](../../home/exceptions.md#jsonexceptiontype_error306) if the value is not an object --
the same exception, with the same message, that [`BasicJsonType::value`](../basic_json/value.md) throws for the
same call. If a member with key `key` is found, throws whatever converting it to `T` throws (typically
[`type_error.302`](../../home/exceptions.md#jsonexceptiontype_error302), with the same message
[`BasicJsonType::value`](../basic_json/value.md) throws for the same mismatch); a missing member never throws.
2. Throws [`type_error.306`](../../home/exceptions.md#jsonexceptiontype_error306) if this value -- not the value `ptr`
resolves to -- is neither an object nor an array. Throws [`parse_error.106`](../../home/exceptions.md#jsonexceptionparse_error106)
or [`parse_error.109`](../../home/exceptions.md#jsonexceptionparse_error109) if `ptr` contains a malformed array
index. If `ptr` resolves to a value, throws whatever converting it to `T` throws. Every other way `ptr` can fail to
resolve -- a missing key, an out-of-range or "`-`" array index, an unresolvable token on a primitive -- yields
`default_value` instead of throwing, exactly as [`BasicJsonType::value`](../basic_json/value.md) catches
`out_of_range` and returns `default_value`.
None of these exceptions carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics.md) path: the view has no
`BasicJsonType` value to point at, so the exception is created without one, even if `BasicJsonType` was built with
`JSON_DIAGNOSTICS` enabled.
## Complexity
1. Linear in the number of members: as for [`operator[]`](operator[].md#complexity), members are compared one after
another, in document order, stopping at the first match. Plus the complexity of converting the found member to
`T` (see [`get`](get.md)).
2. Linear in the number of reference tokens of `ptr` and, for each token, in the number of members of the object at
that level or the index into the array -- as for the [`operator[]`](operator[].md#complexity) and
[`at`](at.md#complexity) overloads that take a JSON pointer. Plus the complexity of converting the resolved value
to `T`.
## Notes
!!! info "Differences to `at` and `operator[]`"
Unlike [`at`](at.md), this function does not throw if `key`/`ptr` resolves to no value. Unlike
[`operator[]`](operator[].md), it never returns a [discarded](is_discarded.md) view -- it always returns a `T` --
and it is available on any view, since it never needs to insert a missing element the way the non-const
`BasicJsonType::operator[]` would.
!!! info "Which values can be asked"
As for [`BasicJsonType::value`](../basic_json/value.md), the key overload (1) requires an object, and the JSON
pointer overload (2) an object or an array.
## Examples
??? example "Example: (1) access specified object element with default value"
The example below reads a couple of optional configuration fields with a default, so that a missing key never
needs a `#!cpp try`/`#!cpp catch` of its own.
```cpp
--8<-- "examples/basic_json_view__value.cpp"
```
Output:
```json
--8<-- "examples/basic_json_view__value.output"
```
??? example "Example: (2) access specified element via JSON pointer with default value"
The example below reads an optional, nested configuration value with a default, given as a JSON pointer.
```cpp
--8<-- "examples/basic_json_view__value_json_pointer.cpp"
```
Output:
```json
--8<-- "examples/basic_json_view__value_json_pointer.output"
```
## See also
- [at](at.md) - access specified element with bounds checking (throws instead of returning a default value)
- [operator[]](operator[].md) - access specified element (returns a discarded view instead of a default value)
- [`BasicJsonType::value`](../basic_json/value.md) - the corresponding function of `basic_json`
## Version history
- Added in version 3.13.0.

34
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@@ -0,0 +1,34 @@
#include <iostream>
#include <nlohmann/json_view.hpp>
using json_document = nlohmann::json_document;
using json_pointer = nlohmann::json::json_pointer;
int main()
{
json_document doc = json_document::parse(R"({"region": "eu", "servers": ["eu-1", "eu-2"]})");
const auto root = doc.root();
std::cout << root.at(json_pointer("/servers/1")).materialize().dump() << '\n';
// at() throws for every resolution failure -- with the very same
// message json::at(ptr) would throw for the same pointer and the same
// document
try
{
static_cast<void>(root.at(json_pointer("/servers/5")));
}
catch (const nlohmann::json::out_of_range& e)
{
std::cout << e.what() << '\n';
}
try
{
static_cast<void>(root.at(json_pointer("/missing")));
}
catch (const nlohmann::json::out_of_range& e)
{
std::cout << e.what() << '\n';
}
}

3
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@@ -0,0 +1,3 @@
"eu-2"
[json.exception.out_of_range.401] array index 5 is out of range
[json.exception.out_of_range.403] key 'missing' not found

39
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@@ -0,0 +1,39 @@
#include <iostream>
#include <nlohmann/json_view.hpp>
using json_document = nlohmann::json_document;
using json_pointer = nlohmann::json::json_pointer;
int main()
{
// "retry_of" is only present on some records, nested a level down
// inside "meta"
json_document batch = json_document::parse(R"(
[
{"id": 1, "meta": {}},
{"id": 2, "meta": {"retry_of": 1}}
]
)");
const auto records = batch.root();
const json_pointer retry_of("/meta/retry_of");
for (std::size_t i = 0; i < records.size(); ++i)
{
const auto record = records[i];
if (record.contains(retry_of))
{
std::cout << "record " << i << " is a retry of " << record[retry_of].get<int>() << '\n';
}
else
{
std::cout << "record " << i << " is original\n";
}
}
// contains() with a JSON pointer never throws -- not even for a
// pointer that indexes into a primitive ("/0/id/x") or uses a
// malformed array index ("/01"), either of which would need a
// try/catch with json::contains(ptr)
std::cout << std::boolalpha << records.contains(json_pointer("/0/id/x")) << '\n';
std::cout << std::boolalpha << records.contains(json_pointer("/01")) << '\n';
}

4
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@@ -0,0 +1,4 @@
record 0 is original
record 1 is a retry of 1
false
false

55
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@@ -0,0 +1,55 @@
#include <iostream>
#include <nlohmann/json_view.hpp>
#include <string>
#include <vector>
using json_document = nlohmann::json_document;
using json_view = nlohmann::json_view;
// address has no direct conversion in get<T>(), so get<address>() falls back
// to materialize().get<address>() -- a real nlohmann::json value is built
// for just this one member, and its own from_json() runs on that
struct address
{
std::string city;
int zip = 0;
};
void from_json(const nlohmann::json& j, address& a)
{
j.at("city").get_to(a.city);
j.at("zip").get_to(a.zip);
}
int main()
{
json_document doc = json_document::parse(R"(
{
"name": "Alice",
"active": true,
"orders": [1, 2, 3],
"address": {"city": "Berlin", "zip": 10115}
}
)");
const json_view customer = doc.root();
// read typed fields straight into C++ variables -- none of these build
// a nlohmann::json value
const std::string name = customer["name"].get<std::string>();
const bool active = customer["active"].get<bool>();
std::cout << name << (active ? " (active)" : " (inactive)") << '\n';
// std::vector<json_view> keeps views of the array elements instead of
// copies of their values
bool first = true;
for (const json_view order : customer["orders"].get<std::vector<json_view>>())
{
std::cout << (first ? "" : " ") << order.get<int>();
first = false;
}
std::cout << '\n';
// everything else goes through materialize()
const address a = customer["address"].get<address>();
std::cout << a.city << ' ' << a.zip << '\n';
}

3
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@@ -0,0 +1,3 @@
Alice (active)
1 2 3
Berlin 10115

31
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@@ -0,0 +1,31 @@
#include <iostream>
#include <nlohmann/json_view.hpp>
#include <string>
using json_document = nlohmann::json_document;
using json_view = nlohmann::json_view;
int main()
{
// a "large response" stand-in: only the "id" field is ever read out of it
const std::string text =
R"({"id": "8f14e45f-ceea-467e-bb92-963f5e3c7a08", "note": "created via API\n", "payload": "..."})";
const json_document doc = json_document::parse(text);
const json_view response = doc.root();
const json_view::string_view_t id = response["id"].get_string();
std::cout << id << '\n';
// no std::string was allocated for "id": its bytes still live inside
// the original buffer, so id's data lies inside [text.data(),
// text.data() + text.size())
const bool id_in_source = id.data() >= text.data() && id.data() + id.size() <= text.data() + text.size();
std::cout << std::boolalpha << id_in_source << '\n';
// "note" contains an escape sequence ('\n'), so it was decoded once
// into the document's own buffer -- get_string() still avoids a copy
// into a new std::string, but the bytes no longer live inside "text"
const json_view::string_view_t note = response["note"].get_string();
const bool note_in_source = note.data() >= text.data() && note.data() + note.size() <= text.data() + text.size();
std::cout << std::boolalpha << note_in_source << '\n';
}

3
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@@ -0,0 +1,3 @@
8f14e45f-ceea-467e-bb92-963f5e3c7a08
true
false

28
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@@ -0,0 +1,28 @@
#include <iostream>
#include <nlohmann/json_view.hpp>
#include <string>
using json_document = nlohmann::json_document;
using json_view = nlohmann::json_view;
int main()
{
json_document doc = json_document::parse(R"({"host": "db.example.com", "port": 5432, "ssl": true})");
const json_view config = doc.root();
// get_to() writes directly into existing variables -- handy for filling
// in the members of a struct one field at a time, without an
// intermediate value from get<T>() for each one
std::string host;
int port = 0;
bool ssl = false;
config["host"].get_to(host);
config["port"].get_to(port);
config["ssl"].get_to(ssl);
std::cout << host << ':' << port << (ssl ? " (tls)" : "") << '\n';
// the return value is a reference to the argument, so a call can be
// used directly in a larger expression
std::string other_host;
std::cout << config["host"].get_to(other_host).size() << '\n';
}

2
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@@ -0,0 +1,2 @@
db.example.com:5432 (tls)
14

29
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@@ -0,0 +1,29 @@
#include <iostream>
#include <nlohmann/json_view.hpp>
using json_document = nlohmann::json_document;
using json_view = nlohmann::json_view;
int main()
{
// a price and an order id from an incoming order -- both need to be
// reproduced exactly, e.g. for an invoice or an audit log
json_document doc = json_document::parse(R"(
{"price": 19.90, "order_id": 1234567890123456789012345, "quantity": 3}
)");
const json_view order = doc.root();
// number_token() returns the number exactly as written in the source
std::cout << order["price"].number_token() << '\n';
std::cout << order["order_id"].number_token() << '\n';
// get<double>() converts it instead -- the exact source text is gone:
// "19.90" becomes the double closest to 19.9, printed without the
// trailing zero, and the 25-digit order id -- far beyond any 64-bit
// integer -- can only be approximated as a double
std::cout << order["price"].get<double>() << '\n';
std::cout << order.materialize()["order_id"].dump() << '\n';
// an ordinary quantity has nothing to lose either way
std::cout << order["quantity"].number_token() << " == " << order["quantity"].get<int>() << '\n';
}

5
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@@ -0,0 +1,5 @@
19.90
1234567890123456789012345
19.9
1.2345678901234568e+24
3 == 3

47
docs/mkdocs/docs/examples/basic_json_view__operator[]_json_pointer.cpp Normal file
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@@ -0,0 +1,47 @@
#include <iostream>
#include <nlohmann/json_view.hpp>
using json_document = nlohmann::json_document;
using json_pointer = nlohmann::json::json_pointer;
int main()
{
// a larger document; operator[] with a JSON pointer reaches straight to
// one deeply nested field, without ever building a tree for the rest
json_document doc = json_document::parse(R"(
{
"region": {
"servers": [
{"name": "eu-1", "metrics": {"cpu": 0.42}},
{"name": "eu-2", "metrics": {"cpu": 0.71}}
]
}
}
)");
const auto root = doc.root();
std::cout << root[json_pointer("/region/servers/1/metrics/cpu")].materialize().dump() << '\n';
// a missing key or an out-of-range index along the path gives a
// discarded view, exactly where const json::operator[] would be
// undefined behavior for the same pointer
if (const auto missing = root[json_pointer("/region/servers/5/metrics/cpu")])
{
std::cout << missing.materialize().dump() << '\n';
}
else
{
std::cout << "no such server\n";
}
// indexing into a primitive still throws, as basic_json::operator[]
// does for the same pointer
try
{
static_cast<void>(root[json_pointer("/region/servers/0/name/x")]);
}
catch (const nlohmann::json::out_of_range& e)
{
std::cout << e.what() << '\n';
}
}

3
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@@ -0,0 +1,3 @@
0.71
no such server
[json.exception.out_of_range.404] unresolved reference token 'x'

29
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@@ -0,0 +1,29 @@
#include <iostream>
#include <nlohmann/json_view.hpp>
#include <string>
using json_document = nlohmann::json_document;
using json_view = nlohmann::json_view;
int main()
{
json_document doc = json_document::parse(R"({"server": {"host": "localhost"}})");
const json_view server = doc.root()["server"];
// "port" is missing -- value() returns the default instead of
// throwing, so optional configuration fields never need their own
// try/catch
std::cout << server.value("host", std::string("0.0.0.0")) << '\n';
std::cout << server.value("port", 8080) << '\n';
// a present but wrong-typed default still throws -- value() only
// replaces "not found", not "wrong type", exactly as basic_json::value
try
{
static_cast<void>(server.value("host", 0));
}
catch (const nlohmann::json::type_error& e)
{
std::cout << e.what() << '\n';
}
}

3
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@@ -0,0 +1,3 @@
localhost
8080
[json.exception.type_error.302] type must be number, but is string

21
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@@ -0,0 +1,21 @@
#include <iostream>
#include <nlohmann/json_view.hpp>
using json_document = nlohmann::json_document;
using json_view = nlohmann::json_view;
using json_pointer = nlohmann::json::json_pointer;
int main()
{
json_document doc = json_document::parse(R"({"server": {"host": "localhost", "limits": {"connections": 100}}})");
const json_view config = doc.root();
// a nested, optional setting read with a default -- no exception, even
// though "timeout" is missing several levels down
std::cout << config.value(json_pointer("/server/limits/connections"), 10) << '\n';
std::cout << config.value(json_pointer("/server/limits/timeout"), 30) << '\n';
// an out-of-range array index also falls back to the default
json_document list_doc = json_document::parse(R"({"servers": ["a", "b"]})");
std::cout << list_doc.root().value(json_pointer("/servers/5"), std::string("none")) << '\n';
}

3
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@@ -0,0 +1,3 @@
100
30
none

26
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@@ -139,9 +139,33 @@ whenever any of the other conditions above was not met.
element access and lookup functions never carry the JSON Pointer path `JSON_DIAGNOSTICS` would otherwise add: the
view has no `basic_json` value to point at, so the exception is created without one, regardless of how
`BasicJsonType` was built.
- **`get<T>()`, JSON Pointer, `dump()`, and comparison are not (yet) provided** by `basic_json_view`. For now,
- **`dump()` and comparison are not (yet) provided** by `basic_json_view`. For now,
[`materialize()`](../api/basic_json_view/materialize.md) is the way to get a value you can do those things with.
## Getting values out without copying
[`get<T>()`](../api/basic_json_view/get.md) converts many `T` directly from the flat index, without ever building a
`basic_json` value for the conversion: `#!cpp bool`, arithmetic types, `#!cpp std::nullptr_t`,
`#!cpp std::string`/other `#!cpp std::basic_string`s (copied once), `basic_json`/`ordered_json` (via
[`materialize()`](../api/basic_json_view/materialize.md)), `basic_json_view` itself, `#!cpp std::vector<U>`, and
`#!cpp std::map`/`#!cpp std::unordered_map` with string-like keys. Every other type -- `#!cpp std::list`,
`#!cpp std::pair`, `#!cpp std::array`, enumerations, user types with a `from_json()` -- goes through
[`materialize()`](../api/basic_json_view/materialize.md)`.get<T>()` instead: the subtree is built into a real
`basic_json` value first, exactly as [`parse()`](../api/basic_json/parse.md) would, and converted from there.
Two conversions never copy at all:
- [`get_string()`](../api/basic_json_view/get_string.md) (equivalently, `#!cpp get<string_view_t>()`) returns a
string as a `string_view_t` pointing into the document's [`source()`](../api/basic_json_document/source.md) text --
or, for a string that contains escape sequences, into the document's own buffer of decoded strings -- instead of
allocating a new `#!cpp std::string`.
- [`number_token()`](../api/basic_json_view/number_token.md) returns a number exactly as it was written in the
source, e.g. `#!cpp "1.50"`, `#!cpp "1E2"`, or an integer with more digits than any number type holds, instead of
rounding it into a `#!cpp double`/`#!cpp int64_t` the way `#!cpp get<T>()` (and
[`basic_json::parse()`](../api/basic_json/parse.md)) would.
Both results are only valid as long as the view -- and, for a string with no escapes, the borrowed source text -- is.
## Choosing between `json`, `ordered_json`, the SAX interface, and `json_view`
| | [`json`](../api/json.md) / [`ordered_json`](../api/ordered_json.md) | [SAX interface](parsing/sax_interface.md) | [`json_document`](../api/json_document.md) / [`json_view`](../api/json_view.md) |

5
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@@ -257,6 +257,9 @@ nav:
- 'end': api/basic_json_view/end.md
- 'find': api/basic_json_view/find.md
- 'front': api/basic_json_view/front.md
- 'get': api/basic_json_view/get.md
- 'get_string': api/basic_json_view/get_string.md
- 'get_to': api/basic_json_view/get_to.md
- 'is_array': api/basic_json_view/is_array.md
- 'is_binary': api/basic_json_view/is_binary.md
- 'is_boolean': api/basic_json_view/is_boolean.md
@@ -272,12 +275,14 @@ nav:
- 'is_structured': api/basic_json_view/is_structured.md
- 'items': api/basic_json_view/items.md
- 'materialize': api/basic_json_view/materialize.md
- 'number_token': api/basic_json_view/number_token.md
- 'operator bool': api/basic_json_view/operator_bool.md
- 'operator[]': api/basic_json_view/operator[].md
- 'size': api/basic_json_view/size.md
- 'source_offset': api/basic_json_view/source_offset.md
- 'type': api/basic_json_view/type.md
- 'type_name': api/basic_json_view/type_name.md
- 'value': api/basic_json_view/value.md
- byte_container_with_subtype:
- 'Overview': api/byte_container_with_subtype/index.md
- '(constructor)': api/byte_container_with_subtype/byte_container_with_subtype.md