c++ std::vector 使用push_back 和emplace_back的区别

首先贴函数的声明和定义部分源码：

template<typename _Tp, typename _Alloc = std::allocator<_Tp> >
class vector : protected _Vector_base<_Tp, _Alloc>
{
      typedef _Tp value_type;

      void  push_back(const value_type& __x)
      {
	    if (this->_M_impl._M_finish != this->_M_impl._M_end_of_storage)
	    {
	        _Alloc_traits::construct(this->_M_impl, this->_M_impl._M_finish,
				     __x);
	        ++this->_M_impl._M_finish;
	    }
	    else
	      _M_realloc_insert(end(), __x);
      }

#if __cplusplus >= 201103L
      void
      push_back(value_type&
                & __x)
      { emplace_back(std::move(__x)); }

      template<typename... _Args>
#if __cplusplus > 201402L
	reference
#else
	void
#endif
	emplace_back(_Args&&... __args);
#endif

     // ... 省略
};

 

说说区别：

使用emplace_back比push_back 更加高效。

怎么解释？上一段验证代码：

class Student{
public:

    Student(int age,string name):age_(age),name_(name){
        std::cout << "[" << __FUNCTION__ << ":" << __LINE__  << "    Student(int age,string name)]" << std::endl;
        std::cout << *this << std::endl;
    }

    Student(const Student& that){
        std::cout << "[" << __FUNCTION__ << ":" << __LINE__  << "  Student(const Student& that)]" << std::endl;
        std::cout << that << std::endl;
    }

    ~Student(){
        std::cout << "[" << __FUNCTION__ << ":" << __LINE__  << "  ~Student()]" << std::endl;
        std::cout << *this << std::endl;
    }

    friend ostream& operator <<(ostream& os,const Student& that){
        os << "[" <<__FUNCTION__ << ":" << __LINE__  << "] : " << that.name_ << "    " << that.age_ << endl;
        return os;
    }

private:
    int age_;
    string name_;
};

int main(int argc,char* argv[])
{
    vector<Student> v;

    {
        v.push_back(Student(12,string("zhangsan")));
    }

    {
        //v.emplace_back(12,string("lisi"));
    }
    return 0;
}

输出结果：

分别执行两种插入方式：



[Student:22    Student(int age,string name)]
[operator<<:37] : zhangsan    12

[Student:27  Student(const Student& that)]
[operator<<:37] : zhangsan    12

[~Student:32  ~Student()]
[operator<<:37] : zhangsan    12

[~Student:32  ~Student()]
[operator<<:37] :     7019200

-----------------------------------------------------------

[Student:22    Student(int age,string name)]
[operator<<:37] : lisi    12

[~Student:32  ~Student()]
[operator<<:37] : lisi    12

很明显可以看到，使用emplace_back 比使用push_back要少执行一次拷贝构造和析构