push_back和emplace_back的区别

/*
    vector push_back 源码实现
*/
void push_back(const value_type &__x)
{
    if (this->_M_impl._M_finish != this->_M_impl._M_end_of_storage)
    {
        //复制一份__x并构造
        _Alloc_traits::construct(this->_M_impl, this->_M_impl._M_finish,
                                 __x);
        ++this->_M_impl._M_finish;
    }
}

/*
    vector emplace_back 源码实现
*/
template <typename _Tp, typename _Alloc>
template <typename... _Args>
void vector<_Tp, _Alloc>::
    emplace_back(_Args &&... __args)
{
    if (this->_M_impl._M_finish != this->_M_impl._M_end_of_storage)
    {
        /*
            std::forward<_Args>(__args)...) 直接构造一个新的对象
            class A
            {
                A(int i1,int i2,int i3);
            }
            vector<A> vec;
            vec.emplace_back(1,2,3)等价于vec.push_back(A(1,2,3))
            少了一个临时对象和copy过程，效率会高
        */
        
        _Alloc_traits::construct(this->_M_impl, this->_M_impl._M_finish,
                                 std::forward<_Args>(__args)...);
        ++this->_M_impl._M_finish;
    }
    else
        _M_emplace_back_aux(std::forward<_Args>(__args)...);
}