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Showing content from https://en.cppreference.com/w/cpp/algorithm/ranges/../../container/vector/emplace_back.html below:

std::vector<T,Allocator>::emplace_back - cppreference.com

template< class... Args >
void emplace_back( Args&&... args );

template< class... Args >
reference emplace_back( Args&&... args );

Appends a new element to the end of the container. The element is constructed through std::allocator_traits::construct, which typically uses placement new to construct the element in-place at the location provided by the container. The arguments args... are forwarded to the constructor as std::forward<Args>(args)....

If after the operation the new size() is greater than old capacity() a reallocation takes place, in which case all iterators (including the end() iterator) and all references to the elements are invalidated. Otherwise only the end() iterator is invalidated.

• T is not EmplaceConstructible into vector from args....
• T is not MoveInsertable into vector.

(none)

A reference to the inserted element.

Amortized constant.

If an exception is thrown for any reason, this function has no effect (strong exception safety guarantee). If the move constructor of T is not noexcept and is not CopyInsertable into *this, vector will use the throwing move constructor. If it throws, the guarantee is waived and the effects are unspecified.

Since reallocation may take place, emplace_back requires the element type to be MoveInsertable for vector.

The following code uses emplace_back to append an object of type President to a std::vector. It demonstrates how emplace_back forwards parameters to the President constructor and shows how using emplace_back avoids the extra copy or move operation required when using push_back.

#include <vector>
#include <cassert>
#include <iostream>
#include <string>
 
struct President
{
    std::string name;
    std::string country;
    int year;
 
    President(std::string p_name, std::string p_country, int p_year)
        : name(std::move(p_name)), country(std::move(p_country)), year(p_year)
    {
        std::cout << "I am being constructed.\n";
    }
 
    President(President&& other)
        : name(std::move(other.name)), country(std::move(other.country)), year(other.year)
    {
        std::cout << "I am being moved.\n";
    }
 
    President& operator=(const President& other) = default;
};
 
int main()
{
    std::vector<President> elections;
    std::cout << "emplace_back:\n";
    auto& ref = elections.emplace_back("Nelson Mandela", "South Africa", 1994);
    assert(ref.year == 1994 && "uses a reference to the created object (C++17)");
 
    std::vector<President> reElections;
    std::cout << "\npush_back:\n";
    reElections.push_back(President("Franklin Delano Roosevelt", "the USA", 1936));
 
    std::cout << "\nContents:\n";
    for (const President& president: elections)
        std::cout << president.name << " was elected president of "
                  << president.country << " in " << president.year << ".\n";
 
    for (const President& president: reElections)
        std::cout << president.name << " was re-elected president of "
                  << president.country << " in " << president.year << ".\n";
}

Output:

emplace_back:
I am being constructed.
 
push_back:
I am being constructed.
I am being moved.
 
Contents:
Nelson Mandela was elected president of South Africa in 1994.
Franklin Delano Roosevelt was re-elected president of the USA in 1936.

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