forked from Research/WhisperCom
Dominik Meyer
cc002ebfbb
git-subtree-dir: libs/json git-subtree-split: f42a74b8f53cc308647123d49d33d1c8122e3f42
137 lines
4.2 KiB
Markdown
137 lines
4.2 KiB
Markdown
# basic_json::get
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```cpp
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// (1)
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template<typename ValueType>
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ValueType get() const noexcept(
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noexcept(JSONSerializer<ValueType>::from_json(
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std::declval<const basic_json_t&>(), std::declval<ValueType&>())));
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// (2)
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template<typename BasicJsonType>
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BasicJsonType get() const;
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// (3)
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template<typename PointerType>
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PointerType get_ptr();
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template<typename PointerType>
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constexpr const PointerType get_ptr() const noexcept;
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```
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1. Explicit type conversion between the JSON value and a compatible value which is
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[CopyConstructible](https://en.cppreference.com/w/cpp/named_req/CopyConstructible) and
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[DefaultConstructible](https://en.cppreference.com/w/cpp/named_req/DefaultConstructible). The value is converted by
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calling the `json_serializer<ValueType>` `from_json()` method.
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The function is equivalent to executing
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```cpp
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ValueType ret;
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JSONSerializer<ValueType>::from_json(*this, ret);
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return ret;
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```
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This overloads is chosen if:
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- `ValueType` is not `basic_json`,
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- `json_serializer<ValueType>` has a `from_json()` method of the form
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`void from_json(const basic_json&, ValueType&)`, and
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- `json_serializer<ValueType>` does not have a `from_json()` method of the form
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`ValueType from_json(const basic_json&)`
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If the type is **not** [CopyConstructible](https://en.cppreference.com/w/cpp/named_req/CopyConstructible) and
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**not** [DefaultConstructible](https://en.cppreference.com/w/cpp/named_req/DefaultConstructible), the value is
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converted by calling the `json_serializer<ValueType>` `from_json()` method.
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The function is then equivalent to executing
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```cpp
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return JSONSerializer<ValueTypeCV>::from_json(*this);
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```
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This overloads is chosen if:
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- `ValueType` is not `basic_json` and
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- `json_serializer<ValueType>` has a `from_json()` method of the form
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`ValueType from_json(const basic_json&)`
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If `json_serializer<ValueType>` has both overloads of `from_json()`, the latter one is chosen.
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2. Overload for `basic_json` specializations. The function is equivalent to executing
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```cpp
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return *this;
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```
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3. Explicit pointer access to the internally stored JSON value. No copies are made.
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## Template parameters
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`ValueType`
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: the value type to return
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`BasicJsonType`
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: a specialization of `basic_json`
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`PointerType`
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: pointer type; must be a pointer to [`array_t`](array_t.md), [`object_t`](object_t.md), [`string_t`](string_t.md),
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[`boolean_t`](boolean_t.md), [`number_integer_t`](number_integer_t.md), or
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[`number_unsigned_t`](number_unsigned_t.md), [`number_float_t`](number_float_t.md), or [`binary_t`](binary_t.md).
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Other types will not compile.
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## Return value
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1. copy of the JSON value, converted to `ValueType`
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2. a copy of `#!cpp *this`, converted into `BasicJsonType`
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3. pointer to the internally stored JSON value if the requested pointer type fits to the JSON value; `#!cpp nullptr`
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otherwise
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## Exceptions
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Depends on what `json_serializer<ValueType>` `from_json()` method throws
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## Notes
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!!! warning
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Writing data to the pointee (overload 3) of the result yields an undefined state.
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## Example
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??? example
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The example below shows several conversions from JSON values
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to other types. There a few things to note: (1) Floating-point numbers can
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be converted to integers, (2) A JSON array can be converted to a standard
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`std::vector<short>`, (3) A JSON object can be converted to C++
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associative containers such as `std::unordered_map<std::string, json>`.
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```cpp
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--8<-- "examples/get__ValueType_const.cpp"
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```
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Output:
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```json
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--8<-- "examples/get__ValueType_const.output"
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```
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??? example
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The example below shows how pointers to internal values of a JSON value can be requested. Note that no type
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conversions are made and a `#cpp nullptr` is returned if the value and the requested pointer type does not match.
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```cpp
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--8<-- "examples/get__PointerType.cpp"
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```
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Output:
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```json
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--8<-- "examples/get__PointerType.output"
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```
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## Version history
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1. Since version 2.1.0.
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2. Since version 2.1.0. Extended to work with other specializations of `basic_json` in version 3.2.0.
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3. Since version 1.0.0.
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