FreeBSD Manual Pages
std::vector::emplace_back(3) C++ Standard Libary std::vector::emplace_back(3) 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 con- tainer. The arguments args... are forwarded to the constructor as std::for- ward<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 ex- ception 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.coun- try)), 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) http://cppreference.com 2022.07.31 std::vector::emplace_back(3)
NAME | Synopsis | Parameters | Type requirements | Return value | Complexity | Exceptions | Notes | Example | Output: | See also
Want to link to this manual page? Use this URL:
<https://man.freebsd.org/cgi/man.cgi?query=std::vector::emplace_back&sektion=3&manpath=FreeBSD+Ports+15.0>