emplace_back出现两次构造。。原来是vector扩容了

#include <iostream>
#include <vector>

//emplace_back可以通过传入的参数直接构造对象，其余emplace同理

struct A {
    int val_int;
    double val_double;
    A(int v, double v2):val_int(v), val_double(v2) {
        std::cout << "construct" << std::endl;
    }

    //创建拷贝构造和移动构造
    A(const A& other) {
        std::cout << "copy construct" << std::endl;
        val_int = other.val_int;
        val_double = other.val_double;
    }

    A(A && r_val) {
        std::cout << "move construct" << std::endl;
        val_int = r_val.val_int;
        val_double = r_val.val_double;
    }
};

/**
 * @brief 测试emplace_back搭配move
 * 
 */
void demo1(){
    A a(10,10.2);
    std::vector<A> vec;
    std::cout << "----push_back----" << std::endl;
    vec.push_back(a);//新特性，如果push_back传入右值，等同于emplaceback，可以看源码
    std::cout << "---emplace---" << std::endl;
    vec.emplace_back(a);
    std::cout << "---over---" << std::endl;
}

int main () {
    demo1();
}
           

输出

construct

----push_back----

copy construct

—emplace—

copy construct

copy construct

—over—

为什么emplace_back输出两次copy构造？

原来是vector在第二次插入数据的时候扩容了…