Difference between revisions of "cpp/container/vector/emplace back"
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Latest revision as of 02:44, 15 December 2020
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) (constexpr since C++20) |
|
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 after the operation the new size() is greater than old capacity() a reallocation takes place, in which case all iterators (including the end() iterator) and all references to the elements are invalidated. Otherwise only the end() iterator is invalidated.
Contents |
[edit] 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.
| ||
[edit] Return value
|
(none) |
(until C++17) |
|
A reference to the inserted element. |
(since C++17) |
[edit] Complexity
Amortized constant.
[edit] Exceptions
If an exception is thrown for any reason, this function has no effect (strong exception safety guarantee).
If the move constructor of T 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.
[edit] 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 <cassert> #include <iostream> #include <string> 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.
[edit] See also
| adds an element to the end (public member function) | |
| (C++11) |
constructs element in-place (public member function) |
