Checking that a template type T is part of a variadic parameter pack in C++17

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 Tsthe 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)
  }
}

The solution you linked to is 5 lines long. How much simplification are you looking for? — Sneftel, Aug 09 '17 at 15:18
My question [here](https://stackoverflow.com/questions/45578484/is-it-possible-to-get-the-first-type-of-a-parameter-pack-in-a-one-liner/45578533#45578533) had four working lines, the answer had only one. Solutions you find here for making a `tuple` of `vector`s of some types in a pack are longer than 10 lines. Its now doable in one. — dani, Aug 09 '17 at 15:23

score 6 · Accepted Answer · answered Aug 09 '17 at 15:27

6

I hear fold-expressions are the new hotness:

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

answered Aug 09 '17 at 15:27

Barry

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score 5 · Answer 2 · answered Aug 09 '17 at 15:28

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.)

score 0 · Answer 3 · answered Aug 09 '17 at 15:28

0

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!")
  }
}

answered Aug 09 '17 at 15:28

SergeyA

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Checking that a template type T is part of a variadic parameter pack in C++17

3 Answers3