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295 lines
9.3 KiB
Markdown
295 lines
9.3 KiB
Markdown
# <small>nlohmann::basic_json::</small>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() noexcept;
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template<typename PointerType>
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const PointerType get() const noexcept; // constexpr since C++14
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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>`](json_serializer.md) `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 overload 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 overload 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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## Exception safety
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Depends on what `json_serializer<ValueType>` `from_json()` method throws for overloads (1) and (2); the JSON value
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itself is never modified, since `get()` is a `#!cpp const` member function. No-throw guarantee for overload (3): this
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function never throws exceptions.
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## Exceptions
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Depends on what `json_serializer<ValueType>` `from_json()` method throws
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## Complexity
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Depends on the `json_serializer<ValueType>::from_json()` implementation for overloads (1) and (2); constant for
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overload (3).
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## Notes
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!!! danger "Undefined behavior for pointers"
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Writing data to the pointee (overload 3) of the result yields an undefined state.
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!!! danger "Undefined behavior for numeric conversions"
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Conversions between numeric types are performed by the corresponding
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`from_json()` implementation using the target C++ type. When converting
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between numeric types, the library does not check whether the source
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value is representable by the target type.
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If the source value is outside the range of the target type, the behavior
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is the same as the corresponding C++ conversion. In particular, converting
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a floating-point value to an integer type that cannot represent the value
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results in undefined behavior.
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See [Number conversion](../../features/types/number_handling.md#number-conversion)
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for more information.
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!!! note "`std::optional` conversions"
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Prior to version 3.13.0, `#!cpp get<std::optional<T>>()` (and other conversions to `std::optional<T>`) failed to
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compile in every configuration, due to an internal implementation bug that made the `from_json` overload for
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`std::optional` unreachable regardless of the [`JSON_USE_IMPLICIT_CONVERSIONS`](../macros/json_use_implicit_conversions.md)
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setting. This has been fixed.
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## Examples
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??? example "Example: (1) explicit conversion to compatible types"
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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::map<std::string, json>`.
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```cpp
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#include <iostream>
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#include <map>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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{
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// create a JSON value with different types
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json json_types =
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{
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{"boolean", true},
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{
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"number", {
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{"integer", 42},
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{"floating-point", 17.23}
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}
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},
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{"string", "Hello, world!"},
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{"array", {1, 2, 3, 4, 5}},
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{"null", nullptr}
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};
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// use explicit conversions
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auto v1 = json_types["boolean"].get<bool>();
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auto v2 = json_types["number"]["integer"].get<int>();
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auto v3 = json_types["number"]["integer"].get<short>();
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auto v4 = json_types["number"]["floating-point"].get<float>();
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auto v5 = json_types["number"]["floating-point"].get<int>();
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auto v6 = json_types["string"].get<std::string>();
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auto v7 = json_types["array"].get<std::vector<short>>();
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auto v8 = json_types.get<std::map<std::string, json>>();
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// print the conversion results
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std::cout << v1 << '\n';
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std::cout << v2 << ' ' << v3 << '\n';
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std::cout << v4 << ' ' << v5 << '\n';
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std::cout << v6 << '\n';
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for (auto i : v7)
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{
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std::cout << i << ' ';
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}
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std::cout << "\n\n";
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for (auto i : v8)
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{
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std::cout << i.first << ": " << i.second << '\n';
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}
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}
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```
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Output:
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```json
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1
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42 42
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17.23 17
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Hello, world!
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1 2 3 4 5
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array: [1,2,3,4,5]
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boolean: true
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null: null
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number: {"floating-point":17.23,"integer":42}
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string: "Hello, world!"
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```
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??? example "Example: (2) explicit conversion to another `basic_json` specialization"
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The example below shows how a `json` value is converted to an `ordered_json` value using `get<BasicJsonType>()`.
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```cpp
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#include <iostream>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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using ordered_json = nlohmann::ordered_json;
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int main()
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{
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// create a JSON value
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json j = {{"one", 1}, {"two", 2}, {"three", 3}};
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// convert to a different basic_json specialization
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ordered_json oj = j.get<ordered_json>();
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std::cout << j << '\n';
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std::cout << oj << '\n';
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}
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```
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Output:
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```json
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{"one":1,"three":3,"two":2}
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{"one":1,"three":3,"two":2}
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```
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??? example "Example: (3) explicit pointer access to the stored value"
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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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#include <iostream>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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{
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// create a JSON number
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json value = 17;
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// explicitly getting pointers
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auto p1 = value.get<const json::number_integer_t*>();
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auto p2 = value.get<json::number_integer_t*>();
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auto p3 = value.get<json::number_integer_t* const>();
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auto p4 = value.get<const json::number_integer_t* const>();
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auto p5 = value.get<json::number_float_t*>();
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// print the pointees
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std::cout << *p1 << ' ' << *p2 << ' ' << *p3 << ' ' << *p4 << '\n';
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std::cout << std::boolalpha << (p5 == nullptr) << '\n';
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}
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```
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Output:
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```json
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17 17 17 17
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true
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```
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## See also
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- [get_to](get_to.md) convert and write into a passed value
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- [get_ptr](get_ptr.md) get a pointer to the stored value
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- [get_ref](get_ref.md) get a reference to the stored value
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- [operator ValueType](operator_ValueType.md) get a value via implicit conversion
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- [Converting values](../../features/conversions.md) - the type conversions article
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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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