C ++模板变量和参数包扩展

Question

In libstdc++ variant source, it defines the following template variable (taken out from the struct _Traits),

template<typename... _Types>
  static constexpr bool _S_copy_ctor = 
    (is_copy_constructible_v<_Types> && ...);

What does the '&&' do here?

I tried to take out the '&&' it failed to compile, so, what's difference of the two?

  static constexpr bool _S_copy_ctor = (is_copy_constructible_v<_Types> && ...);
  static constexpr bool _S_copy_ctor = (is_copy_constructible_v<_Types>    ...);

Answer 1

In this very context, the && is a simple logical AND operator called fold operator .

It's used to unfold an expression based on typename... _Types . Example:

let _Types be deduced to int, double, float , then the expression:

(is_copy_constructible_v<_Types> && ...)

will be expanded ( unfolded ) to:

(is_copy_constructible_v<int> &&
 is_copy_constructible_v<double> && 
 is_copy_constructible_v<float>)

You cannot simply erase the && . It has it's use case.