【C/C++開發】emplace_back() 和 push_back 的區別

在引入右值引用，轉移構造函數，轉移複製運算符以前，一般使用push_back()向容器中加入一個右值元素（臨時對象）的時候，首先會調用構造函數構造這個臨時對象，而後須要調用拷貝構造函數將這個臨時對象放入容器中。原來的臨時變量釋放。這樣形成的問題是臨時變量申請的資源就浪費。 
引入了右值引用，轉移構造函數（請看這裏）後，push_back()右值時就會調用構造函數和轉移構造函數。 
在這上面有進一步優化的空間就是使用emplace_back

emplace_back

函數原型：

template <class... Args>
  void emplace_back (Args&&... args);

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在容器尾部添加一個元素，這個元素原地構造，不須要觸發拷貝構造和轉移構造。並且調用形式更加簡潔，直接根據參數初始化臨時對象的成員。 
給出一個示例，這個示例頗有用。

#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(const President& other)
        : name(std::move(other.name)), country(std::move(other.country)), year(other.year)
    {
        std::cout << "I am being copy 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);  
};  

int main()  
{  
    std::vector<President> elections;  
    std::cout << "emplace_back:\n";  
    elections.emplace_back("Nelson Mandela", "South Africa", 1994); //沒有類的建立 

    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";  
    }

}

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輸出

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.