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From gregmarr's review of #5622: - count() counts every member with the key, so count(key) > 1 detects duplicate keys; it was contains() as 0/1 before. - get_to() passes types that get<T>() does not convert directly to from_json with the existing value, as basic_json::get_to() does, so that a from_json which sets only the fields present keeps the others. - array_index() checks the empty token and "-" first; the results are unchanged. - Docs: no "if an exception is thrown" where a view never changes anything; the borrowed source must also stay unmodified; items() on a temporary view is safe; back()/front() notes shortened; the basic_json see-also entries say how the view's results can differ. Signed-off-by: Niels Lohmann <mail@nlohmann.me>
2.6 KiB
2.6 KiB
nlohmann::basic_json_view::get_to
template<typename T>
T& get_to(T& v) const;
Converts the value to T and stores it in v, in one of two ways:
- For the types that
get<T>()converts directly -- arithmetic types,#!cpp bool,#!cpp std::nullptr_t,string_view_t,#!cpp std::basic_string,BasicJsonType,basic_json_view,#!cpp std::vector, and#!cpp std::map/#!cpp std::unordered_mapwith string keys --vis assigned:#!cpp v = get<T>(). - For every other type, the subtree is built into a
BasicJsonTypevalue and#!cpp materialize().get_to(v)is called, i.e.from_json()is invoked with the existingv, exactly asBasicJsonType::get_to()does.
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
- For the directly converted types: strong exception safety. If an exception is thrown,
vis not modified. - For every other type: as for
BasicJsonType::get_to().materialize()throws beforevis touched, but afrom_json()that throws may leavevpartly modified.
Exceptions
- For the directly converted types: whatever
get<T>()throws for the same value andT. - For every other type: whatever
materialize()andBasicJsonType::get_to()throw.
Complexity
- For the directly converted types: whatever
get<T>()has for the sameT. - For every other type: that of
materialize(), plus that ofBasicJsonType::get_to().
Notes
The difference between the two ways matters for a from_json() that sets only the fields present in the JSON: since
it runs on the existing v, the other fields of v keep their values, whereas assigning get<T>() would replace
them with those of a default-constructed 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 - convert the value to a given type
BasicJsonType::get_to- the corresponding function ofbasic_json
Version history
- Added in version 3.13.0.