基本类型情况西，两者几乎没什么区别
但是再自定义类型的时候？emplace——back更高效，但是emplace_back 没有类型检查的安全；只有运行时候才会报错。

#include <vector> 
#include <iostream> 
using namespace std;
class testDemo
{
public:explicit testDemo(int num) :num(num) {std::cout << "call constructor function" << endl;}testDemo(const testDemo& other) :num(other.num) {std::cout << "call copy constructor function" << endl;}testDemo(testDemo&& other) :num(other.num) {std::cout << "call move constructor function" << endl;}int getnum(){return num;}
private:int num;
};int main()
{cout << "emplace_back:" << endl;std::vector<testDemo> demo1;demo1.emplace_back(2);cout << demo1[0].getnum() << std::endl;cout << "push_back:" << endl;std::vector<testDemo> demo2;testDemo test(2);demo2.push_back(test);
}

#-----------------------------------------------------------------------------------------------------------------

#include <vector> 
#include <iostream> 
using namespace std;
class testDemo
{
public:explicit testDemo(int num) :num(num) {std::cout << "call constructor function" << endl;}testDemo(const testDemo& other) :num(other.num) {std::cout << "call copy constructor function" << endl;}testDemo(testDemo&& other) :num(other.num) {std::cout << "call move constructor function" << endl;}int getnum(){return num;}
private:int num;
};int main()
{cout << "emplace_back:" << endl;std::vector<testDemo> demo1;demo1.emplace_back("asdasd");cout << demo1[0].getnum() << std::endl;cout << "push_back:" << endl;std::vector<testDemo> demo2;testDemo test(2);demo2.push_back(test);
}

报错

为什么会高效？

emplace_back:
call constructor function
11
push_back:
call constructor function
call copy constructor function

原理是？ 直接再最后构造对象 ，然后push_back 先构造 ，再转移。