std::vector emplace_back() method

od C++20

// prism-push-types:reference
// Non const version only
template< class... Args >
constexpr reference emplace_back( Args&&... args );

od C++17

// Non const version only
template< class... Args >
void emplace_back( Args&&... args );

od C++11

// Non const version only
template< class... Args >
void emplace_back( Args&&... args );

Appends a new element to the end of the container. The element is constructed through std::allocator_traits<Alloc>::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)....

Invalidation

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

od C++17

A reference to the inserted element.

do C++17

(none)

Complexity

Amortized constant - O(1).

Exceptions

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

If T's move constructor is not noexcept and T 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

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

Example

Main.cpp

#include <vector>
#include <string>
#include <cassert>
#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";
    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 (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.