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Niels Lohmann 77acd4563c Add element access, iteration, values, and JSON pointers to json_view

Give basic_json_view the read-only access functions of basic_json:
operator[] and at() with keys and indices, front()/back(), find(),
contains(), count(), begin()/end() and cbegin()/cend(), items()
with structured bindings from C++17 on, and type_name().

Exceptions have the ids and messages of the const functions of
basic_json. Where basic_json has undefined behavior the view
answers safely: operator[] with a missing key or an out-of-range
index returns a discarded view, and front()/back() of an empty
container throw invalid_iterator.214. Objects are iterated in
document order, and all members are visited; duplicate-key lookups
find the first member (as yyjson and simdjson do), while parse(),
materialize(), and the map conversions keep the last value, as
parse() does. Keys of up to 16 bytes are compared with two
overlapping loads.

Add value conversions: get<T>()/get_to() for arithmetic types,
bool, nullptr_t, strings (std::basic_string copied,
string_view_t without a copy), BasicJsonType, views, std::vector,
and maps with string keys; get_string() for the string without a
copy; number_token() for the number exactly as written in the
source; value() with keys and JSON pointers; and operator[]/at()/
contains() with JSON pointers. Everything else, including types
with from_json(), goes through materialize() of that subtree.
get<T>() of arithmetic types is inlined down to the conversion, so
reading an integer needs no call.

detail::json_pointer_access exposes a pointer's reference tokens
to code outside basic_json.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

2026-10-07 16:42:26 +02:00

1.2 KiB

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nlohmann::basic_json_view::get_to

template<typename T>
T& get_to(T& v) const;

Converts the value to T and assigns it to v. Equivalent to

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>() throws for the same value and T.

Complexity

Whatever get<T>() 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"
```

Version history

Added in version 3.13.0.

1.2 KiB Raw Blame History