std::vector::emplace_back

来自cppreference.com
< cpp‎ | container‎ | vector
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...std::forward<Args>(args)... 转发到构造函数。

若新的 size() 大于 capacity() ,则所有迭代器和引用(包含尾后迭代器)都被非法化。否则仅尾后迭代器被非法化。

目录

[编辑] 参数

args - 转发到元素构造函数的参数
类型要求
-
T (容器元素类型) 必须满足 MoveInsertable EmplaceConstructible 的要求。

[编辑] 返回值

(无) (C++17 前)
到被插入元素的引用。 (C++17 起)

[编辑] 复杂度

均摊常数。

[编辑] 异常

若抛出异常,则此函数无效果(强异常保证)。 若 T 的移动构造函数非 noexcept 且非可复制插入 (CopyInsertable) 到 *this ,则 vector 将使用抛出的移动构造函数。若它抛出,则保证被舍弃,且效果未指定。

注意

特化 std::vector<bool> 在 C++14 前无 emplace_back() 成员。

[编辑] 示例

下列代码用 emplace_back 后附 President 类型对象到 std::vector 。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 <vector>
#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::vector<President> elections;
    std::cout << "emplace_back:\n";
    elections.emplace_back("Nelson Mandela", "South Africa", 1994);
 
    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";
    }
}

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.

[编辑] 参阅

将元素添加到容器末尾
(公开成员函数) [编辑]