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c++templatescompile-time
Roman2452809
Roman2452809

Reputation: 448

compile-time function for checking type equality

I need to implement self contained compile-time function for checking type equality (function template without arguments bool eqTypes<T,S>()).

self contained means not relying on library.

I'm not good in all this. That's what I tried, but it's not what I need.

template<typename T>
bool eq_types(T const&, T const&) { 
return true;
}

template<typename T, typename U> 
bool eq_types(T const&, U const&) { 
return false; 
}

Upvotes: 16

Views: 23640

Answers (3)

Sam Petrocelli
Sam Petrocelli

Reputation: 39

#define CHECK_TYPE_EQUAL(type_a, type_b) { \
    type_a type_a##_var __attribute__((unused)); \
    type_b *type_b##_ptr __attribute__((unused)); \
_Pragma("GCC diagnostic push") \
_Pragma("GCC diagnostic error "-Wincompatible-pointer-types\"") \
    type_b##_ptr = &type_a##_var; \
_Pragma("GCC diagnostic pop") \
}

Upvotes: 0

Igor Skochinsky
Igor Skochinsky

Reputation: 25268

Here's how you can do it in C, without any magic GCC extensions:

#define CHECKED_TYPE(original_type, p) ((conversion_type*) (1 ? p : (original_type*) 0))

E.g: 

void *q = CHECKED_TYPE(int, &y);

Will trigger a compile error if y is not int.
For explanation, see here.

Upvotes: 4

Andy Prowl
Andy Prowl

Reputation: 126412

It's quite simple. Just define a type trait and a helper function:

template<typename T, typename U>
struct is_same
{
    static const bool value = false;
};

template<typename T>
struct is_same<T, T>
{
    static const bool value = true;
};

template<typename T, typename U>
bool eqTypes() { return is_same<T, U>::value; }

Here is a live example.

In C++11, if you are allowed to use std::false_type and std::true_type, you would rewrite the above this way:

#include <type_traits>

template<typename T, typename U>
struct is_same : std::false_type { };

template<typename T>
struct is_same<T, T> : std::true_type { };

template<typename T, typename U>
constexpr bool eqTypes() { return is_same<T, U>::value; }

Notice, that the type trait std::is_same, which does pretty much the same thing, is available as part of the Standard Library.

Upvotes: 32

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