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

 

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推导指引(C++17)

 

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。

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。

参数

args	-	转发到元素构造函数的参数
类型要求
- value_type 必须满足可移动插入 (MoveInsertable) 和 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

均摊常数。

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

因为可能发生再分配， emplace_back 对 vector 要求元素类型为可移动插入 (MoveInsertable) 。

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

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。

运行此代码

#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.

参阅

push_back

将元素添加到容器末尾 (公开成员函数)