Is there a way to get type of template class from its complete type?

问题

I need a meta-function that for given complete class type returns its template (e.g. f<foo<bar>>::type or f<foo<baz>>::type results in foo).

Or it may return true on f<foo<bar>, foo<baz>>::value and false on f<foo<bar>, not_foo<baz>>::value

P.S: this was meant to be used with many chrono::duration like classes (but for weight units, mass units and so on). I needed different units not to convert one to another.

回答1:

f<foo<bar>>::type or f<foo<baz>>::type results in foo

Not exactly (see is-an-alias-template-considered-equal-to-the-same-template), you can do something like:

template <typename T> struct template_class;

template <template <typename> class C, typename T>
struct template_class<C<T>>
{
    template <typename U>
    using type = C<U>;
};

Or it may return true on f<foo<bar>, foo<baz>>::value and false on f<foo<bar>, not_foo<baz>>::value

It is easier, even if limited, specialization mostly as is_same:

template <typename, typename> struct has_same_template_class : std::false_type{};

template <template<typename> class C, typename T1, typename T2>
struct has_same_template_class<C<T1>, C<T2>> : std::true_type{};

回答2:

Probably, you want something like this:

#include <type_traits>

template<class> struct foo;
template<class> struct not_foo;

struct bar;
struct baz;

template<class, class>
struct trait : std::false_type {};

template<template<class> class T, class S1, class S2>
struct trait<T<S1>, T<S2>> : std::true_type {};

static_assert( trait<foo<bar>,     foo<baz>    >::value);
static_assert( trait<not_foo<bar>, not_foo<baz>>::value);
static_assert(!trait<foo<bar>,     not_foo<baz>>::value);
static_assert(!trait<foo<bar>,     not_foo<bar>>::value);

Demo

来源：https://stackoverflow.com/questions/61278669/is-there-a-way-to-get-type-of-template-class-from-its-complete-type