在引入右值引用，转移构造函数，转移复制运算符之前，通常使用push_back()向容器中加入一个右值元素（临时对象）的时候，首先会调用构造函数构造这个临时对象，然后需要调用拷贝构造函数将这个临时对象放入容器中。原来的临时变量释放。这样造成的问题是临时变量申请的资源就浪费。
引入了右值引用，转移构造函数（）后，push_back()右值时就会调用构造函数和转移构造函数。 在这上面有进一步优化的空间就是使用emplace_backemplace_back
函数原型：
1 template2 void emplace_back (Args&&... args);
在容器尾部添加一个元素，这个元素原地构造，不需要触发拷贝构造和转移构造。而且调用形式更加简洁，直接根据参数初始化临时对象的成员。
给出一个示例，这个示例很有用。1 #include2 #include 3 #include 4 5 struct President 6 { 7 std::string name; 8 std::string country; 9 int year; 10 11 President(std::string p_name, std::string p_country, int p_year) 12 : name(std::move(p_name)), country(std::move(p_country)), year(p_year) 13 { 14 std::cout << "I am being constructed.\n"; 15 }16 President(const President& other)17 : name(std::move(other.name)), country(std::move(other.country)), year(other.year)18 {19 std::cout << "I am being copy constructed.\n";20 }21 President(President&& other) 22 : name(std::move(other.name)), country(std::move(other.country)), year(other.year) 23 { 24 std::cout << "I am being moved.\n"; 25 } 26 President& operator=(const President& other); 27 }; 28 29 int main() 30 { 31 std::vector elections; 32 std::cout << "emplace_back:\n"; 33 elections.emplace_back("Nelson Mandela", "South Africa", 1994); //没有类的创建 34 35 std::vector reElections; 36 std::cout << "\npush_back:\n"; 37 reElections.push_back(President("Franklin Delano Roosevelt", "the USA", 1936)); 38 39 std::cout << "\nContents:\n"; 40 for (President const& president: elections) { 41 std::cout << president.name << " was elected president of " 42 << president.country << " in " << president.year << ".\n"; 43 } 44 for (President const& president: reElections) { 45 std::cout << president.name << " was re-elected president of " 46 << president.country << " in " << president.year << ".\n"; 47 }48 49 }
输出
1 emplace_back:2 I am being constructed.3 4 push_back:5 I am being constructed.6 I am being moved.7 8 Contents:9 Nelson Mandela was elected president of South Africa in 1994.
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