在引入右值引用，轉(zhuǎn)移構(gòu)造函數(shù)，轉(zhuǎn)移復(fù)制運(yùn)算符之前，通常使用push_back()向容器中加入一個(gè)右值元素（臨時(shí)對(duì)象）的時(shí)候，首先會(huì)調(diào)用構(gòu)造函數(shù)構(gòu)造這個(gè)臨時(shí)對(duì)象，然后需要調(diào)用拷貝構(gòu)造函數(shù)將這個(gè)臨時(shí)對(duì)象放入容器中。原來(lái)的臨時(shí)變量釋放。這樣造成的問(wèn)題是臨時(shí)變量申請(qǐng)的資源就浪費(fèi)。
引入了右值引用，轉(zhuǎn)移構(gòu)造函數(shù)后，push_back()右值時(shí)就會(huì)調(diào)用構(gòu)造函數(shù)和轉(zhuǎn)移構(gòu)造函數(shù)。
在這上面有進(jìn)一步優(yōu)化的空間就是使用emplace_back，相較而言，emplace_back整體耗時(shí)減少17%。

emplace_back函數(shù)原型：

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

在容器尾部添加一個(gè)元素，這個(gè)元素原地構(gòu)造，不需要觸發(fā)拷貝構(gòu)造和轉(zhuǎn)移構(gòu)造。而且調(diào)用形式更加簡(jiǎn)潔，直接根據(jù)參數(shù)初始化臨時(shí)對(duì)象的成員。
給出一個(gè)示例，這個(gè)示例很有用。

#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(méi)有類(lèi)的創(chuàng)建 ?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"; ?}}

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.
Franklin Delano Roosevelt was re-elected president of the USA in 1936.

Q：為什么push_back 調(diào)用了構(gòu)造函數(shù)之后，調(diào)用移動(dòng)構(gòu)造函數(shù)，而不是調(diào)用拷貝構(gòu)造函數(shù)？

A：構(gòu)造函數(shù)生成的對(duì)象是一個(gè)右值對(duì)象，所以使用 push_back(&&) 這個(gè)重載函數(shù)。

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