std::vector::emplace_back

 

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std::vector

成员函数 Original: Member functions The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.

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元素的访问 Original: Element access The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.

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迭代器 Original: Iterators The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.

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容量 Original: Capacity The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.

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修饰符 Original: Modifiers The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.

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template< class... Args > void emplace_back( Args&&... args );		（C++11 起）

添加一个新元素到结束的容器。该元件是构成在就地，即没有复制或移动操作进行。提供的功能完全一样的参数的构造函数的元素被称为.

Original:

Appends a new element to the end of the container. The element is constructed in-place, i.e. no copy or move operations are performed. The constructor of the element is called with exactly the same arguments that are supplied to the function.

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If the new size() is greater than capacity(), all iterators and references are invalidated. Otherwise no iterators and references are invalidated.

参数

args

-

元素的构造函数的参数转发 Original: arguments to forward to the constructor of the element The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.

返回值

（无）

Original:

(none)

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复杂性

恒定

Original:

Constant.

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为例

下面的代码使用emplace_back追加到Presidentstd::vector类型的对象。它演示了如何emplace_back转发的President构造函数的参数，并显示如何使用emplace_back避免了额外的拷贝或移动操作时，需要使用push_back.

Original:

The following code uses emplace_back to append an object of type President to a std::vector. It demonstrates how emplace_back forwards parameters to the President constructor and shows how using emplace_back avoids the extra copy or move operation required when using push_back.

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#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";
    }
}

Output:

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.

另请参阅

push_back	将元素添加到年底 Original: adds elements to the end The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. （公共成员函数）