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

在容器末尾追加一个新元素。该元素通过 std::allocator_traits::construct 构造，通常使用 placement new 在容器提供的位置就地构造元素。参数 args... 会作为 std:: forward < Args > ( args ) ... 转发给构造函数。

如果操作后新的 size() 大于原来的 capacity() ，则会发生重新分配，这种情况下所有迭代器（包括 end() 迭代器）和所有元素引用都会失效。否则仅 end() 迭代器会失效。

参数

返回值

复杂度

摊销常数。

异常

若因任何原因抛出异常，此函数不产生任何效果（ 强异常安全保证 ）。 若 T 的移动构造函数非 noexcept 且不满足 可复制插入 到 * this 的要求， vector 将使用可能抛出异常的移动构造函数。若此时发生异常，则不再保证安全性且效果未定义。

注释

由于可能发生重新分配， emplace_back 要求元素类型对于 vector 必须是 MoveInsertable 的。

示例

#include <vector>
#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::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 (const President& president: elections)
        std::cout << president.name << " was elected president of "
                  << president.country << " in " << president.year << ".\n";
    for (const President& president: reElections)
        std::cout << president.name << " was re-elected president of "
                  << president.country << " in " << president.year << ".\n";
}

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.

参见