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◆ emplace()
template<class... Args>
std::pair<iterator, bool> nlohmann::basic_json::emplace |
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Args &&... |
args | ) |
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inline |
Inserts a new element into a JSON object constructed in-place with the given args if there is no element with the key in the container. If the function is called on a JSON null value, an empty object is created before appending the value created from args.
- Parameters
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[in] | args | arguments to forward to a constructor of basic_json |
- Template Parameters
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Args | compatible types to create a basic_json object |
- Returns
- a pair consisting of an iterator to the inserted element, or the already-existing element if no insertion happened, and a bool denoting whether the insertion took place.
- Exceptions
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type_error.311 | when called on a type other than JSON object or null; example: "cannot use emplace() with number" |
- Complexity
- Logarithmic in the size of the container, O(log(
size() )).
- Example
- The example shows how
emplace() can be used to add elements to a JSON object. Note how the null value was silently converted to a JSON object. Further note how no value is added if there was already one value stored with the same key.
2 #include <nlohmann/json.hpp>
9 json object = {{ "one", 1}, { "two", 2}};
13 std::cout << object << '\n';
14 std::cout << null << '\n';
17 auto res1 = object.emplace( "three", 3);
18 null.emplace( "A", "a");
19 null.emplace( "B", "b");
23 auto res2 = null.emplace( "B", "c");
26 std::cout << object << '\n';
27 std::cout << *res1.first << " " << std::boolalpha << res1.second << '\n';
29 std::cout << null << '\n';
30 std::cout << *res2.first << " " << std::boolalpha << res2.second << '\n';
Output (play with this example online): {"one":1,"two":2}
null
{"one":1,"three":3,"two":2}
3 true
{"A":"a","B":"b"}
"b" false
The example code above can be translated withg++ -std=c++11 -Isingle_include doc/examples/emplace.cpp -o emplace
- Since
- version 2.0.8
Definition at line 19659 of file json.hpp.
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