CODEWITHSUNDEEP
c++templatesmetaprogrammingtemplate-auto
v.oddou
v.oddou

Reputation: 6775

C++ how do you "overload" a template to work for both value and type?

Consider this simple meta query to check integral:

#include <type_traits>

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

// partial specialization if type destructuring matches an integral_constant
template <typename T, auto N>
struct IsIntegralConstant<std::integral_constant<T, N>>
{
    static constexpr bool value = true;
};
template <typename T> constexpr auto isIntegralConstant_v = IsIntegralConstant<T>::value;

// Now I can do queries such as:
static_assert( isIntegralConstant_v<int> );
static_assert( isIntegralConstant_v<std::integral_constant<int, 2>> );

// it'd be nice if it could work for direct values too 
// static_assert( isIntegralConstant_v<2> ); ?

// let's see...
template <auto N>
struct IsIntegralConstant<std::integral_constant<decltype(N), N>>
{
    static constexpr bool value = true;
};
// gcc:
 //error: partial specialization of 'struct IsIntegralConstant<std::integral_constant<decltype (N), N> >' after instantiation of 'struct IsIntegralConstant<std::integral_constant<int, 2> >' [-fpermissive]
 //struct IsIntegralConstant<std::integral_constant<decltype(N), N>>
// what ?

// clang says nothing, until we force instanciation:
static_assert( isIntegralConstant_v<2> );

//#1 with x86-64 clang 8.0.0
//<source>:35:38: error: template argument for template type parameter must be a type

play with it: https://godbolt.org/z/dosK7G

It seems when your primary template has made a decision between type or value, then you're simply stuck with that for all specializations.

Is there no way to have a more generic thing such as:

template <autotypename TorV> ... ?

Upvotes: 1

Views: 158

Answers (1)

StoryTeller - Unslander Monica
StoryTeller - Unslander Monica

Reputation: 170269

Is there no way to have a more generic thing such as

None whatsoever in any version of C++ so far. And it's also unlikely to change. Despite the loaded name IsIntegralConstant, we are obviously not going to be testing things like IsIntegralConstant_v<2>. The utility of templates is in generic code, where nothing is lacking:

template<auto wut>
struct bar {
     // static_assert(IsIntegralConstant_v<wut>);
     // ill-formed, but we only really care and need
     static_assert(IsIntegralConstant_v<decltype(wut)>);
};

Upvotes: 1

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