[pjmlp]

C++ was designed as "Typescript for C" for its time, because sometimes that is exactly the kind of code one needs to write, even if we discourage many of the classical patterns when better alternatives exist.

The same C example compiled in C++23 mode, https://godbolt.org/z/MWa7qqrK7

As for possible alternatives, here is a basic one without taking into consideration virtual mechanics, only to show the principles.

    #include <concepts>

    template <class T>
    concept has_mmap = requires (T obj)
    {
        { obj.mmap() } -> std::convertible_to<int>;

    };

    class VFS {
    public:
        VFS() = default;
        virtual ~VFS() = default;
    };

    class ExampleFS : public VFS {
        // mmap not available
    };

    class ExampleWithMMAP : public VFS {
    public:

        int mmap() {
            return 0;
        } 
    };

    int main() {
        ExampleFS fs;
        ExampleWithMMAP fsWithMMAP;

        /*
        <source>: In function 'int main()':
        <source>:33:19: error: 'class ExampleFS' has no member named 'mmap'
         40 |         return fs.mmap();
            |    
        */
        if constexpr (has_mmap<ExampleFS>) {
            return fs.mmap();
        }

        // ExampleWithMMAP has mmap(), just call it without issues
        if constexpr (has_mmap<ExampleWithMMAP>) {
            return fsWithMMAP.mmap();
        }

        // want to use the variable name instead of the type?
        if constexpr (has_mmap<decltype(fsWithMMAP)>) {
            return fsWithMMAP.mmap();
        }
    }

-- https://godbolt.org/z/cjcbrzT3z

Naturally it is possible to be a bit even more creative, and moreso with C++26 reflection.

[ryao]

  The same C example compiled in C++23 mode, https://godbolt.org/z/MWa7qqrK7

Everyone knows this. The original comment was saying not to do this (even in C++) and use C++ classes instead. I was making the point that is a bad idea. You seem to have not understood that.