C++ 浅谈emplace_back及使用误区

//Base.h
#include<string>
class Base
{
public:
Base() = default;
Base(std::string t,int m):s(t),i(m){}
Base(const Base& b):s(b.s),i(b.i){}
Base(Base&&) = delete;
private:
std::string s;
int i;
};

template<typename T>
template<class ...Args>
void
Vec<T>::emplace_back(Args&& ...args)
{
chk_n_alloc();
alloc.construct(first_free++, std::forward<Args>(args)...);
}

#include<bits/stdc++.h>
using namespace std;
int i=0,j=0;
class A {
public:
  A(int i){
    str = to_string(i);
    cout << "构造函数" <<++i<< endl; 
  }
  ~A(){}
  A(const A& a): str(a.str){
    cout << "拷贝构造" <<++j<< endl;
  }
 
public:
  string str;
};
 
int main(){
  vector<A> vec;
  vec.reserve(10);//开辟capacity
  for(int i=0;i<10;i++){
    vec.push_back(i); //调用了10次构造函数和10次拷贝构造函数,
//    vec.emplace_back(i); //调用了10次构造函数一次拷贝构造函数都没有调用过
  }
}