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std::vector::emplace_back

template< class... Args > void emplace_back( Args&&... args );		(since C++11) (until C++17)
template< class... Args > reference emplace_back( Args&&... args );		(since C++17)

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 the new size() is greater than capacity() then all iterators and references (including the past-the-end iterator) are invalidated. Otherwise only the past-the-end iterator is invalidated.

Parameters

args	-	arguments to forward to the constructor of the element
Type requirements
-`T (the container's element type)` must meet the requirements of `MoveInsertable` and `EmplaceConstructible`.

Return value

(none)	(until C++17)
A reference to the inserted element.	(since C++17)

Complexity

Amortized constant.

Exceptions

If an exception is thrown, this function has no effect (strong exception guarantee). If T's move constructor 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.

Notes

The specialization std::vector<bool> did not have emplace_back() member until C++14.

Example

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 <string>
#include <iostream>
 
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";
    elections.emplace_back("Nelson Mandela", "South Africa", 1994);
 
    std::vector<President> reElections;
    std::cout << "\npush_back:\n";
    reElections.push_back(President("Franklin Delano Roosevelt", "the USA", 1936));
 
    std::cout << "\nContents:\n";
    for (President const& president: elections) {
        std::cout << president.name << " was elected president of "
                  << president.country << " in " << president.year << ".\n";
    }
    for (President const& 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.