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

 

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std::deque

成员类型
成员函数
deque::deque deque::~deque deque::operator= deque::assign deque::assign_range (C++23)   deque::get_allocator 元素访问 deque::at deque::operator[] deque::front deque::back 迭代器 deque::begindeque::cbegin (C++11) deque::enddeque::cend (C++11) deque::rbegindeque::crbegin (C++11) deque::renddeque::crend (C++11)	容量 deque::empty deque::size deque::max_size deque::shrink_to_fit (DR*) 修改器 deque::clear deque::insert deque::insert_range (C++23) deque::emplace deque::erase deque::push_front deque::emplace_front (C++11) deque::prepend_range (C++23) deque::pop_front deque::push_back deque::emplace_back (C++11) deque::append_range (C++23) deque::pop_back deque::resize deque::swap
非成员函数
operator==operator<=> (C++20 起) swap(std::deque) erase(std::deque)erase_if(std::deque) (C++20)(C++20)	operator!=operator<operator>operator<=operator>= (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)
推导指南(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)

在容器末尾添加一个新元素。该元素通过 std::allocator_traits::construct 构造，它通常使用 placement-new 在容器提供的位置原地构造元素。参数 args... 被转发给构造函数作为 std::forward<Args>(args)...。

所有迭代器（包括 end() 迭代器）都将失效。无引用失效。

参数

args	-	转发给元素构造函数的参数
类型要求
- T (容器的元素类型) 必须满足 EmplaceConstructible 的要求。

返回值

(无)	(C++17 前)
对插入元素的引用。	(C++17 起)

复杂度

常数时间。

异常

如果由于任何原因抛出异常，此函数无效果（强异常安全保证）。

示例

以下代码使用 emplace_back 将 President 类型的对象附加到 std::deque。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 时所需的额外拷贝或移动操作。

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 (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	添加元素到结尾 (公共成员函数)
emplace (C++11)	就地构造元素 (公共成员函数)