Check if the class has any base class in C++ in compile time

Question

I want to check whether the class X has ANY base class using type utilities in C++.

For example:

class X : public Y
{
}
static_assert(std::has_base_class<X>::value, "") // OK

but:

class X
{
}
static_assert(std::has_base_class<X>::value, "") // Failed

Does anything like my imaginary has_base_class exist in the standard library? Thanks!

This seems like XY problem, why do you need that info? BTW, what you want is not currently possible — bartop, May 21 '20 at 11:20
You can check for a specific base class, but not if the class has any in general. — super, May 21 '20 at 11:23
You can check the can-be-converted-to-single-type relationship, if that's close enough. — lorro, May 21 '20 at 11:24
@lorro [gcc](https://godbolt.org/z/euXbCT) seems to disagree. — super, May 21 '20 at 11:29
@super : my fault, indeed, overload resolution is before accessibility check. — lorro, May 21 '20 at 11:34

score 4 · Accepted Answer · answered May 21 '20 at 12:31

As mentioned in comments you can't do exactly this in standard C++. The closest you get from std library is std::is_base_of, but it is for testing a specific base class.

But as mentioned here GCC has std::tr2::bases (and std::tr2::direct_bases) that solves your question for a generic "has any base" assertion. These came from the N2965 proposal and unfortunately was rejected for std C++.

Here's a sample code showing how you can use this GCC extension to assert just what you want:

#include <tr2/type_traits>

class B {};

class X : public B {};
static_assert(std::tr2::bases<X>::type::empty(),"X");
// doesn't compile because X bases tuple returned is not empty

class Y {};
static_assert(std::tr2::bases<Y>::type::empty(),"Y");

#include <iostream>
using namespace std;

int main() {
  return 0;
}

Check if the class has any base class in C++ in compile time

1 Answers1