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

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 to figure out how CRTP is relevant here (well, I never used CRTP before, so I am not used to thinking in these terms). So, how would a CRTP-based solution look like?
Here is the cited question:
Is it possible to write a template function that would possess type information about the base class of the template argument? (assuming that the template argument derives from one class only)
So, I am looking for something like this:
template <class T>
auto f(T t) -> decltype(...) { // ... is some SFINAE magic that
// catches B, the base of T
std::cout << (B)t << std::endl;
}
Some relevant background: I am writing a generic implementation of the A* algorithm. The template argument is a Node structure. So, the user might define:
struct NodeBase {
REFLECTABLE((double)g, (double)f)
// Using the REFLECTABLE macro as described here:
// https://stackoverflow.com/a/11744832/2725810
};
struct NodeData : public NodeBase {
using Base1 = NodeBase;
REFLECTABLE((double)F)
};
I would like to write a function that prints the contents of the node structure. REFLECTABLE does all the hard work of extracting the fields of the struct. However, when the user gives me a NodeData instance, my function needs to print the contents of the NodeBase component as well. I would like to later add overloads of my function for two and three base classes.

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 <iostream>
#include <type_traits>
// 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<class T, class = void>
struct report_impl
{
static void apply(const T&) {};
};
// apply the case where an object T is derived from node_base
template<class T>
struct report_impl<T, std::enable_if_t< std::is_base_of<node_base, T>::value > >
{
static void apply(const T& t) {
std::cout << static_cast<const node_base&>(t) << std::endl;
};
};
// the general form of the report function defers to the partially
// specialised application class
template<class T>
void report(const T& t)
{
report_impl<T>::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

Answer 2

Here is my own first solution. It is not CRTP though and it suffers from a huge drawback as explained at the end of the answer:
template <class Base1_ = void, class Base2_ = void, class Base3_ = void,
class Base4_ = void>
struct ManagedNode;
// For classes that do not derive
template <> struct ManagedNode<void, void, void, void> {
using Base1 = void; using Base2 = void; using Base3 = void;
using Base4 = void;
};
// To avoid inaccessible base
// See http://stackoverflow.com/q/34255802/2725810
struct Inter0: public ManagedNode<>{};
// For classes that derive from a single base class
template <class Base1_>
struct ManagedNode<Base1_, void, void, void> : public Inter0,
public Base1_ {
using Base1 = Base1_;
};
// To avoid inaccessible base
template <class Base1_>
struct Inter1: public ManagedNode<Base1_>{};
// For classes that derive from two base classes
template <class Base1_, class Base2_>
struct ManagedNode<Base1_, Base2_, void, void> : public Inter1<Base1_>,
public Base2_ {
using Base2 = Base2_;
};
// Some user classes for testing the concept
struct A : public ManagedNode<> {
int data1;
};
struct B : public ManagedNode<> {};
struct C : public ManagedNode<A, B> {};
int main() {
C c;
std::cout << sizeof(c) << std::endl;
return 0;
}
This code produces the output of 12, which means that c contains the data1 member three times! For my purposes this drawback over-weighs the benefits of the reflection that this approach provides. So, does anyone have a suggestion for a better approach?