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std::deque<T,Allocator>:: emplace_back

From cppreference.net

C++
Containers library
std::deque
Member types
Member functions
Non-member functions
(until C++20) (until C++20) (until C++20) (until C++20) (until C++20)
Deduction guides (C++17)
template < class ... Args >
void emplace_back ( Args && ... args ) ;
(C++17 前)
template < class ... Args >
reference emplace_back ( Args && ... args ) ;
(C++17 起)
(C++26 起为 constexpr)

在容器末尾追加一个新元素。该元素通过 std::allocator_traits::construct 构造，通常使用 placement new 在容器提供的位置就地构造元素。参数 args... 会作为 std:: forward < Args > ( args ) ... 转发给构造函数。

所有迭代器（包括 end() 迭代器）都将失效。所有引用保持有效。

目录

参数

args - 要转发给元素构造函数的参数
类型要求
-
如果 T 不能通过 args... 在 deque 中实现 EmplaceConstructible ，则行为未定义。

返回值

（无）

(C++17 前)

指向被插入元素的引用。

(C++17 起)

复杂度

常量。

异常

若因任何原因抛出异常，此函数不产生任何效果（ 强异常安全保证 ）。

示例

以下代码使用 emplace_back 将 President 类型的对象追加到 std::deque 中。它演示了 emplace_back 如何将参数转发给 President 构造函数，并展示了使用 emplace_back 如何避免使用 push_back 时所需的额外复制或移动操作。

运行此代码 
#include <deque>
#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::deque<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::deque<President> reElections;
    std::cout << "\npush_back:\n";
    reElections.push_back(President("Franklin Delano Roosevelt", "the USA", 1936));
    std::cout << "\nContents:\n";
    for (const President& president: elections)
        std::cout << president.name << " was elected president of "
                  << president.country << " in " << president.year << ".\n";
    for (const President& president: reElections)
        std::cout << president.name << " was re-elected president of "
                  << president.country << " in " << president.year << ".\n";
}

输出： 

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.

参见

在末尾添加元素
(公开成员函数)
(C++11)
原位构造元素
(公开成员函数)