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nlohmann
86 lines
2.3 KiB
Markdown
86 lines
2.3 KiB
Markdown
# nlohmann::basic_json::emplace_back
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```
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template<class... Args>
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reference emplace_back(Args&& ... args);
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```
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Creates a JSON value from the passed parameters `args` to the end of the JSON value. If the function is called on a JSON `null` value, an empty array is created before appending the value created from `args`.
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## Template parameters
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`Args` : compatible types to create a `basic_json` object
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## Iterator invalidation
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By adding an element to the end of the array, a reallocation can happen, in which case all iterators (including the [`end()`](https://json.nlohmann.me/api/basic_json/end/index.md) iterator) and all references to the elements are invalidated. Otherwise, only the [`end()`](https://json.nlohmann.me/api/basic_json/end/index.md) iterator is invalidated.
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## Parameters
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`args` (in) : arguments to forward to a constructor of `basic_json`
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## Return value
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reference to the inserted element
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## Exceptions
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Throws [`type_error.311`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error311) when called on a type other than JSON array or `null`; example: `"cannot use emplace_back() with number"`
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## Complexity
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Amortized constant.
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## Examples
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Example
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The example shows how `emplace_back()` can be used to add elements to a JSON array. Note how the `null` value was silently converted to a JSON array.
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```
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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 JSON values
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json array = {1, 2, 3, 4, 5};
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json null;
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// print values
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std::cout << array << '\n';
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std::cout << null << '\n';
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// add values
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array.emplace_back(6);
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null.emplace_back("first");
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null.emplace_back(3, "second");
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// print values
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std::cout << array << '\n';
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std::cout << null << '\n';
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}
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```
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Output:
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```
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[1,2,3,4,5]
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null
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[1,2,3,4,5,6]
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["first",["second","second","second"]]
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```
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## See also
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- [operator+=](https://json.nlohmann.me/api/basic_json/operator%2B%3D/index.md) add a value to an array/object
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- [push_back](https://json.nlohmann.me/api/basic_json/push_back/index.md) add a value to an array/object
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- [Modifying values](https://json.nlohmann.me/features/modifying_values/index.md) - the article on modifying values
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## Version history
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- Since version 2.0.8.
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- Returns reference since 3.7.0.
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