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112 lines
3.3 KiB
Markdown
112 lines
3.3 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. As an exception, a `#!json null`
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value is converted to an empty object before the element is added and keeps that type if adding the element throws.
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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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#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 object = {{"one", 1}, {"two", 2}};
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json null;
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// print values
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std::cout << object << '\n';
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std::cout << null << '\n';
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// add values
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auto res1 = object.emplace("three", 3);
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null.emplace("A", "a");
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null.emplace("B", "b");
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// the following call will not add an object, because there is already
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// a value stored at key "B"
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auto res2 = null.emplace("B", "c");
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// print values
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std::cout << object << '\n';
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std::cout << *res1.first << " " << std::boolalpha << res1.second << '\n';
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std::cout << null << '\n';
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std::cout << *res2.first << " " << std::boolalpha << res2.second << '\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,"two":2}
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null
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{"one":1,"three":3,"two":2}
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3 true
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{"A":"a","B":"b"}
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"b" false
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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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- Fixed in version 3.13.0: for [`ordered_json`](../ordered_json.md), the value could previously only be passed as an
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rvalue; it can now also be passed as an lvalue or a `#!cpp const` lvalue, matching the behavior of `json`.
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