CODEWITHSUNDEEP
c++templatesc++11
roger.james
roger.james

Reputation: 1498

Do static_assert on every other type in a template

How can I do static_asserts (or other checks) on every other type in a template?

template<typename... Ts> //T1,T2,T3,...
struct foo {
  //How can I
  //for T1,T3,T5,T7,...
  //do some checks, for example:
  //static_assert(std::is_default_constructible<Tn>::value,"invalid type");
  //static_assert(std::is_copy_constructible<Tn>::value,"invalid type");
};

Upvotes: 4

Views: 200

Answers (2)

David G
David G

Reputation: 96800

Please try this:

#include <type_traits>

template <typename... Ts>
struct default_constructible;

template <typename T>
struct default_constructible<T>
{
    static constexpr bool value = std::is_default_constructible<T>::value;
};

template <>
struct default_constructible<>
{
    static constexpr bool value = true;
};

template <typename T, typename U, typename... Ts>
struct default_constructible<T, U, Ts...>
{
    static constexpr bool value = std::is_default_constructible<T>::value && default_constructible<Ts...>::value;
};

template <typename... Ts>
struct foo
{
    static_assert(default_constructible<Ts...>::value, "");
};

class A { A() = delete; };

template class foo<int, bool>;      // Compiles
template class foo<int, bool, A>; // Does not compile

Demo

Upvotes: 6

Kerrek SB
Kerrek SB

Reputation: 476990

You could try this:

template <template <typename...> class Pred, typename ...Args> struct check_odd;

template <template <typename...> class Pred>
struct check_odd<Pred> : std::true_type { };

template <template <typename...> class Pred, typename T>
struct check_odd<Pred, T> : Pred<T> { };

template <template <typename...> class Pred, typename T1, typename T2, typename ...Args>
struct check_odd<Pred, T1, T2, Args...>
{
    static constexpr bool value = Pred<T1>::value && check_odd<Pred, Args...>::value;
};

Usage:

static_assert(check_odds<std::is_arithmetic, T1, T2, T3, T4, T5, T6>::value,
              "Not all odd types are arithmetic.");

Obvious routes of generalization are parametrizing the combiner (currently hardcoded as "AND", or &&), and giving the predicate additional parameters.

Upvotes: 2

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