使用 std::forward 进行参数包扩展的 '...' 的语法

Question

In C++, it seems that a parameter may be normally expanded with ... directly after the parameter pack's name. For example,

template <class... Tys>
void function(Tys... params) {
    function(params...);
}

However, when using std::forward , the ... goes seemingly after the parameter pack's name. For example,

template <class... Tys>
void function(Tys... params) {
    function(std::forward<Tys>(params)...); // The '...' is outside the parenthesis of 'params'
}

My question is why is there a difference? Why is the following code incorrect?

template<class... Tys>
void function(Tys... params) {
    function(std::forward<Tys>(params...));
}

At what point in the parameter pack expanded? I probably am not fully understanding how std::forward works or how it forwards arguments perfectly.

I appreciate any answers!

Answer 1

The ... applies to the thing on its left.

• function(params...)

  expands to

  function(param1, param2, ..., paramN) .

• function(std::forward<Tys>(params)...)

  expands to

  function(std::forward<Tys1>(param1), std::forward<Tys2>(param2), ..., std::forward<TysN>(paramN))

• function(std::forward<Tys>(params...))

  expands to

  function(std::forward<Tys>(param1, param2, ..., paramN))