背景和区别
emplace_back() 是 C++11 之后，vector容器中添加的新方法，和 push_back()一样，都是在容器末尾添加一个新的元素，相对于push_back函数，它减少了一次类的构造。不同的是emplace_back() 在效率上相比较于 push_back() 有了一定的提升。
废话不多说，上经典代码：
#include <vector> #include <string> #include <cassert> #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"; 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 (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"; } }
看运行结果：
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.
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。
emplace_back() 函数在原理上比 push_back() 有了一定的改进，包括在内存优化方面和运行效率方面。内存优化主要体现在使用了就地构造（直接在容器内构造对象，不用拷贝一个复制品再使用）+强制类型转换的方法来实现，在运行效率方面，由于省去了拷贝构造过程，因此也有一定的提升。
emplace_back能完全代替push_back吗？
std::vector<std::vector<int>> v; v.push_back({1,2,3}); // OK v.emplace_back({1,2,3}); // error v.emplace_back(std::vector<int>{1,2,3}); // OK v.emplace_back<std::vector<int>>({1,2,3}); // OK
std::vector<std::regex> v; v.push_back(nullptr); // 编译出错 v.emplace_back(nullptr); // 通过编译，但运行时抛出异常并且难以定位
