# std::list<T,Allocator>::emplace_back

< cpp‎ | container‎ | list

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。

没有引用和迭代器被非法化。

### 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::list 。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。

    #include <list>
    #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::list<President> elections;
        std::cout << "emplace_back:\n";
        elections.emplace_back("Nelson Mandela", "South Africa", 1994);

        std::list<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

push_back | 将元素添加到容器末尾 (公开成员函数)