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nlohmann
134 lines
3.3 KiB
Markdown
134 lines
3.3 KiB
Markdown
# nlohmann::basic_json::get_to
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```
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template<typename ValueType>
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ValueType& get_to(ValueType& v) const noexcept(
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noexcept(JSONSerializer<ValueType>::from_json(
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std::declval<const basic_json_t&>(), v)));
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```
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Explicit type conversion between the JSON value and a compatible value. The value is filled into the input parameter by calling the `json_serializer<ValueType>` `from_json()` method.
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The function is equivalent to executing
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```
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ValueType v;
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JSONSerializer<ValueType>::from_json(*this, v);
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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 `void from_json(const basic_json&, ValueType&)`
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## Template parameters
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`ValueType` : the value type to return
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## Return value
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the input parameter, allowing chaining calls
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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.
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## Examples
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Example
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The example below shows several conversions from JSON values to other types. There a few things to note: (1) Floating-point numbers can be converted to integers, (2) A JSON array can be converted to a standard `std::vector<short>`, (3) A JSON object can be converted to C++ associative containers such as `#cpp std::unordered_map<std::string, json>`.
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```
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#include <iostream>
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#include <unordered_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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bool v1;
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int v2;
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short v3;
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float v4;
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int v5;
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std::string v6;
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std::vector<short> v7;
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std::unordered_map<std::string, json> v8;
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// use explicit conversions
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json_types["boolean"].get_to(v1);
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json_types["number"]["integer"].get_to(v2);
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json_types["number"]["integer"].get_to(v3);
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json_types["number"]["floating-point"].get_to(v4);
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json_types["number"]["floating-point"].get_to(v5);
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json_types["string"].get_to(v6);
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json_types["array"].get_to(v7);
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json_types.get_to(v8);
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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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```
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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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string: "Hello, world!"
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number: {"floating-point":17.23,"integer":42}
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null: null
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boolean: true
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array: [1,2,3,4,5]
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```
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## See also
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- [get](https://json.nlohmann.me/api/basic_json/get/index.md) get a value (explicit conversion)
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- [get_ref](https://json.nlohmann.me/api/basic_json/get_ref/index.md) get a reference to the stored value
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- [get_ptr](https://json.nlohmann.me/api/basic_json/get_ptr/index.md) get a pointer to the stored value
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- [Converting values](https://json.nlohmann.me/features/conversions/index.md) - the type conversions article
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## Version history
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- Since version 3.3.0.
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