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nlohmann
72 lines
2.0 KiB
Markdown
72 lines
2.0 KiB
Markdown
# <small>nlohmann::basic_json::</small>emplace
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```cpp
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template<class... Args>
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std::pair<iterator, bool> emplace(Args&& ... args);
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```
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Inserts a new element into a JSON object constructed in-place with the given `args` if there is no element with the key
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in the container. If the function is called on a JSON null value, an empty object is created before appending the value
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created from `args`.
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## Template parameters
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`Args`
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: compatible types to create a `basic_json` object
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## Iterator invalidation
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For [`ordered_json`](../ordered_json.md), adding a value to an object can yield a reallocation, in which case all
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iterators (including the `end()` iterator) and all references to the elements are invalidated.
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## Parameters
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`args` (in)
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: arguments to forward to a constructor of `basic_json`
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## Return value
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a pair consisting of an iterator to the inserted element, or the already-existing element if no insertion happened, and
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a `#!cpp bool` denoting whether the insertion took place.
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## Exception safety
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Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
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## Exceptions
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Throws [`type_error.311`](../../home/exceptions.md#jsonexceptiontype_error311) when called on a type other than JSON
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object or `#!json null`; example: `"cannot use emplace() with number"`
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## Complexity
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Logarithmic in the size of the container, O(log(`size()`)).
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## Examples
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??? example
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The example shows how `emplace()` can be used to add elements to a JSON object. Note how the `#!json null` value was
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silently converted to a JSON object. Further note how no value is added if there was already one value stored with
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the same key.
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```cpp
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--8<-- "examples/emplace.cpp"
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```
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Output:
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```json
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--8<-- "examples/emplace.output"
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```
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## See also
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- [emplace_back](emplace_back.md) add a value to an array
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- [insert](insert.md) add values to an array/object
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- [Modifying values](../../features/modifying_values.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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