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

< cpp‎ | container‎ | deque

 
C++
 
容器库
 
std::deque
 
template< class... Args >
void emplace_back( Args&&... args );
 (C++11 起)
(C++17 前)
template< class... Args >
reference emplace_back( Args&&... args );
 (C++17 起)

添加新元素到容器尾。元素通过 std::allocator_traits::construct 构造，它典型地用布置 new 于容器所提供的位置原位构造元素。参数 args... 以 std::forward<Args>(args)... 转发到构造函数。

所有迭代器，包含尾后迭代器，都被非法化。没有引用被非法化。

参数

args - 转发到元素构造函数的参数
类型要求
-
value_type 必须满足可就位构造 (EmplaceConstructible) 的要求。

返回值

（无）

(C++17 前)

到被插入元素的引用。

(C++17 起)

复杂度

常数。

异常

若抛出异常，则此函数无效果（强异常保证）。


示例

下列代码用 emplace_back 后附 President 类型对象到 std::deque 。它演示 emplace_back 如何转发参数到 President 的构造函数，并展示如何用 emplace_back 避免用 push_back 时的额外复制或移动操作。

运行此代码
#include <deque>
#include <string>
#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::deque<President> elections;
    std::cout << "emplace_back:\n";
    elections.emplace_back("Nelson Mandela", "South Africa", 1994);
 
    std::deque<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";
    }
}

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.

参阅

 将元素添加到容器末尾
(公开成员函数)