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Showing content from https://en.cppreference.com/w/cpp/language/../error/error_code/../../container/map/emplace.html below:

std::map<Key,T,Compare,Allocator>::emplace - cppreference.com

Inserts a new element into the container constructed in-place with the given args, if there is no element with the key in the container.

The constructor of the new element (i.e. std::pair<const Key, T>) is called with exactly the same arguments as supplied to emplace, forwarded via std::forward<Args>(args).... The element may be constructed even if there already is an element with the key in the container, in which case the newly constructed element will be destroyed immediately (see try_emplace() if this behavior is undesirable).

If value_type is not EmplaceConstructible into map from args, the behavior is undefined.

No iterators or references are invalidated.

A pair consisting of an iterator to the inserted element (or to the element that prevented the insertion) and a bool value set to true if and only if the insertion took place.

If an exception is thrown for any reason, this function has no effect (strong exception safety guarantee).

Logarithmic in the size of the container.

Careful use of emplace allows the new element to be constructed while avoiding unnecessary copy or move operations.

#include <iostream>
#include <string>
#include <utility>
#include <map>
 
int main()
{
    std::map<std::string, std::string> m;
 
    // uses pair's move constructor
    m.emplace(std::make_pair(std::string("a"), std::string("a")));
 
    // uses pair's converting move constructor
    m.emplace(std::make_pair("b", "abcd"));
 
    // uses pair's template constructor
    m.emplace("d", "ddd");
 
    // emplace with duplicate key has no effect
    m.emplace("d", "DDD");
 
    // uses pair's piecewise constructor
    m.emplace(std::piecewise_construct,
              std::forward_as_tuple("c"),
              std::forward_as_tuple(10, 'c'));
    // an alternative is: m.try_emplace("c", 10, 'c');
 
    for (const auto& p : m)
        std::cout << p.first << " => " << p.second << '\n';
}

Output:

a => a
b => abcd
c => cccccccccc
d => ddd

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