nlohmann::basic_json::emplace¶
template<class... Args>
std::pair<iterator, bool> emplace(Args&& ... args);
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.
Template parameters¶
Args- compatible types to create a
basic_jsonobject
Parameters¶
args(in)- arguments to forward to a constructor of
basic_json
Return value¶
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¶
Throws 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())).
Examples¶
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.
#include <iostream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
int main()
{
// create JSON values
json object = {{"one", 1}, {"two", 2}};
json null;
// print values
std::cout << object << '\n';
std::cout << null << '\n';
// add values
auto res1 = object.emplace("three", 3);
null.emplace("A", "a");
null.emplace("B", "b");
// the following call will not add an object, because there is already
// a value stored at key "B"
auto res2 = null.emplace("B", "c");
// print values
std::cout << object << '\n';
std::cout << *res1.first << " " << std::boolalpha << res1.second << '\n';
std::cout << null << '\n';
std::cout << *res2.first << " " << std::boolalpha << res2.second << '\n';
}
Output:
{"one":1,"two":2}
null
{"one":1,"three":3,"two":2}
3 true
{"A":"a","B":"b"}
"b" false
Version history¶
- Since version 2.0.8.