NAME¶

std::vector::emplace_back - std::vector::emplace_back

Synopsis¶

template< class... Args > (since C++11)
void emplace_back( Args&&... args ); (until C++17)
template< class... Args > (since C++17)
reference emplace_back( Args&&... args ); (until C++20)
template< class... Args > (since C++20)
constexpr 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 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¶

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

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.

// Run this code

#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.

See also¶

push_back adds an element to the end
(public member function)
emplace constructs element in-place
(C++11) (public member function)