emplace()

template<template< typename U, typename V, typename... Args > class ObjectType = std::map, template< typename U, typename... Args > class ArrayType = std::vector, class StringType = std::string, class BooleanType = bool, class NumberIntegerType = std::int64_t, class NumberUnsignedType = std::uint64_t, class NumberFloatType = double, template< typename U > class AllocatorType = std::allocator, template< typename T, typename SFINAE=void > class JSONSerializer = adl_serializer, class BinaryType = std::vector<std::uint8_t>>

template<class... Args>

std::pair< iterator, bool > nlohmann::basic_json< ObjectType, ArrayType, StringType, BooleanType, NumberIntegerType, NumberUnsignedType, NumberFloatType, AllocatorType, JSONSerializer, BinaryType >::emplace ( Args &&...  args )

inline

add an object to an object if key does not exist

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

[in]

args

arguments to forward to a constructor of basic_json

Template Parameters

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

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())).

@liveexample{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.,emplace}

Since

version 2.0.8

Definition at line 23392 of file json.hpp.

    {
        // emplace only works for null objects or arrays
        if (JSON_HEDLEY_UNLIKELY(!(is_null() || is_object())))
        {
            JSON_THROW(type_error::create(311, "cannot use emplace() with " + std::string(type_name()), *this));
        }
 
        // transform null object into an object
        if (is_null())
        {
            m_type = value_t::object;
            m_value = value_t::object;
            assert_invariant();
        }
 
        // add element to array (perfect forwarding)
        auto res = m_value.object->emplace(std::forward<Args>(args)...);
        set_parent(res.first->second);
        // create result iterator and set iterator to the result of emplace
        auto it = begin();
        it.m_it.object_iterator = res.first;
        // return pair of iterator and boolean
        return {it, res.second};
    }