检查模板类型T是否是C ++ 17中可变参数包的一部分

Question

I want to check that a type T is also part of a parameter pack Ts . There are solutions that do that in C++14, but I'm wandering if this can be simplified in C++17. If T is not found in Ts the compiler should stop (static_assertion should fail).

template<typename... Ts>
class A
{
  template<typename T>
  void action() {
    // check that T is also in Ts (static_assertion)
  }
}

Answer 1

我听说折叠表达是新的热点：

static_assert((std::is_same_v<T, Ts> || ...));

Answer 2

If you prefer a library trait:

static_assert(std::disjunction_v<std::is_same<T, Ts>...>);

Note that this performs short circuiting (perhaps not exceedingly beneficial here, but something to keep in mind). Fold expressions are equally viable:

static_assert((std::is_same_v<T, Ts> || ...));

(Stolen from @Barry.)

Answer 3

Easy enough in C++ with fold expressions:

template<typename... Ts>
class A
{
  template<typename T>
  void action() {
    static_assert((... || std::is_same_v<T, Ts>)), "Not!")
  }
}