What would a CRTP-based solution to this look like?

Question

I asked the following question in this post (pasted below for convenience). One of the comments suggested that there is a CRTP-based solution to the problem. I am not able t

Answer 1

to know whether a class derives from a base class we have the std::is_base_of<> template structure, which can be used in conjunction with partial specialisation, or std::enable_if.

Here is a demonstration of using a partially specialised structure to apply a an operation depending on whether it's derived from node_base or not (in this case, it just prints the base object but you could do any other operation)

#include 
#include 

// base class
struct node_base
{

};

std::ostream& operator<<(std::ostream& os, const node_base& nb)
{
    os << "node_base_stuff";
    return os;
}

// a class derived from node_base
struct node : public node_base
{

};

// a class not derived from node_base    
struct not_node
{

};

// apply the general case - do nothing
template
struct report_impl
{
    static void apply(const T&) {};
};

// apply the case where an object T is derived from node_base    
template
struct report_impl::value > >
{
    static void apply(const T& t) {
        std::cout << static_cast(t) << std::endl;
    };
};

// the general form of the report function defers to the partially
// specialised application class
template
void report(const T& t)
{
    report_impl::apply(t);
}

using namespace std;

// a quick test    
auto main() -> int
{
    node n;
    not_node nn;
    report(n);
    report(nn);

    return 0;
}

expected output:

node_base_stuff