std::vector emplace_back() 方法

自 C++20 起

// prism-push-types:reference
// Non const version only
template< class... Args >
constexpr reference emplace_back( Args&&... args );

自 C++17 起

// Non const version only
template< class... Args >
void emplace_back( Args&&... args );

自 C++11 起

// Non const version only
template< class... Args >
void emplace_back( Args&&... args );

将一个新元素添加到容器的末尾。该元素通过 std::allocator_traits<Alloc>::construct() 构造，该方法通常使用 placement-new 在容器提供的位置就地构造元素。参数 args... 以 std::forward<Args>(args)... 的形式转发给构造函数。

失效

如果新的 size() 大于 capacity()，则所有迭代器和引用（包括尾后迭代器）都将失效。

否则，只有尾后迭代器会失效。

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 MoveInsertable 和 EmplaceConstructible 的要求。

返回值

自 C++17 起

对插入元素的引用。

C++17 之前

(无)

复杂度

分摊常数 - O(1)。

异常

如果抛出异常，此函数不产生任何效果（强异常保证）。

如果 T 的移动构造函数不是 noexcept 且 T 无法 CopyInsertable 到 *this 中，则 vector 将使用抛出异常的移动构造函数。如果它抛出异常，则保证失效，并且效果未指定。

备注

由于可能发生重新分配，emplace_back() 要求 vector 的元素类型为 MoveInsertable。

示例

Main.cpp

#include <vector>
#include <string>
#include <cassert>
#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";
    auto& ref = elections.emplace_back("Nelson Mandela", "South Africa", 1994);
    assert(ref.year == 1994 && "uses a reference to the created object (C++17)");
 
    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.