77acd4563c
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>
2.8 KiB
nlohmann::basic_json_view::get_string
string_view_t get_string() const;
Returns the string value as a string_view_t, without copying it.
Return value
The string, as a string_view_t that points either into the document's
source() 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 if the value is not a string; example:
"type must be string, but is array".
This exception does not carry a JSON_DIAGNOSTICS 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>()) is the
zero-copy alternative: BasicJsonType::get_ref<const string_t&>() 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 is valid exactly as long as the view that produced it -- see the
validity rules 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 - convert the value to a given type (
#!cpp get<string_view_t>()is equivalent to this function) - number_token - a number's token text, without a copy
BasicJsonType::get_ref- the closest counterpart ofbasic_json
Version history
- Added in version 3.13.0.