std::vector emplace_back() method
- since C++20
- since C++17
- since C++11
// prism-push-types:reference
// Non const version only
template< class... Args >
constexpr reference emplace_back( Args&&... args );
// Non const version only
template< class... Args >
void emplace_back( Args&&... args );
// 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)...
.
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 ofMoveInsertable
andEmplaceConstructible
.
Return value
- since C++17
- until C++17
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
#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";
}
}
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.
Hover to see the original license.