roger.james roger.james - 2 months ago 7
C++ Question

Doing a static_assert that a template type is another template

How do I

static_assert
like this? Maybe Boost supports it if not C++ or new features in C++11?

template<T>
struct foo {};

template<FooType>
struct bar {
static_assert(FooType is indeed foo<T> for some T,"failure"); //how?
};

Answer

You could do something along these lines. Given a trait that can verify whether a class is an instantiation of a class template:

#include <type_traits>

template<typename T, template<typename> class TT>
struct is_instantiation_of : std::false_type { };

template<typename T, template<typename> class TT>
struct is_instantiation_of<TT<T>, TT> : std::true_type { };

Use it as follows in your program:

template<typename T>
struct foo {};

template<typename FooType>
struct bar {
  static_assert(is_instantiation_of<FooType, foo>::value, "failure");
};

int main()
{
    bar<int> b; // ERROR!
    bar<foo<int>> b; // OK!
}

If you want, you could generalize this to detect whether a class is an instance of a template with any number of (type) parameters, like so:

#include <type_traits>

template<template<typename...> class TT, typename T>
struct is_instantiation_of : std::false_type { };

template<template<typename...> class TT, typename... Ts>
struct is_instantiation_of<TT, TT<Ts...>> : std::true_type { };

template<typename FooType>
struct bar {
  static_assert(is_instantiation_of<foo, FooType>::value, "failure");
};

You would then use it this way in your program:

template<typename FooType>
struct bar {
  static_assert(is_instantiation_of<foo, FooType>::value, "failure");
};

int main()
{
    bar<int> b; // ERROR!
    bar<foo<int>> b; // OK!
}

Here is a live example.