How to count the number of CRTP subclasses of a template class?

问题

Does anyone know of a method to use CRTP to count the number of subclasses of an object?

Suppose we had a setup similar to the following one:

template <typename T>    
class Object
{
    ....  
};

const unsigned int ObjectSubClassCount = ...; 

class Subobject : public Object<SubObject>
{
    ....
};

class Second : public Object<Second>
{
    ....
};

and so on, such that, using TMP, we might have a constant (ObjectSubClassCount) that represents the total number of subclasses?

Does anyone know a way to do this?

Edit: I am wanting to use the result as a template parameter later on, so I need it to be done with TMP...

回答1:

Without the requirement to use the result as a template parameter later I would try it doing like this:

// Class which increments a given counter at instanciation
struct Increment {
  Increment(std::size_t& counter)
  {
    counter++;
  }
};

// This is your template base
template <typename T>    
class Object
{
  private:
    // For every instanciation of the template (which is done for a subclass)
    // the class counter should get incremented
    static Increment incrementCounter;
};

// This is the global object counter
static std::size_t classCounter;

// Static Member Variable
template<typename T>
Object<T>::incrementCounter(classCounter);

Haven't tried it but should do. To have the result available as a template parameter again (MPL) I don't have enough experience in MPL but I doubt this is possible.

回答2:

Ok, so I've come upon a... somewhat acceptable answer. I figured that it would not work out if the subclasses had absolutely not knowledge of eachother (I mean, we're talking somewhat functional programming...).

Here's a solution for this. It's definitely not the solution I'd wish for; however, it is a start. I've forced all objects to use a form of CRTP, but one that uses more of a linked list format. In this way, our subclasses must be derived from an Object<> templated from:

A: itself and B: the most recent previously defined subclass

here is my code for this (I use a template from <type_traits> once, just a note)

template <typename T>   //SFINAE check for the existance of subclasscount static member
struct has_subclasscount
{

    template <typename U>
    static typename std::enable_if<sizeof(U::subclasscount) != 0, int>::type test(int);

    template <typename U>
    static char test(...);

    static const bool result = (sizeof(test<T>(0)) == sizeof(int))?(true):(false);
};


template <bool res, typename T>
struct return_subclasscount //the value to return is 0 if false
{
    static const int result = 0;
};


template <typename T>       
struct return_subclasscount<true, T>    //returns subclasscount only if the first parameter is true
{
    static const int result = T::subclasscount;
};


template <typename T>               //combines return_subclasscount and has_subclasscount
struct get_subclasscount
{
    static const int result = return_subclasscount<has_subclasscount<T>::result, T>::result;
};


template <typename This, typename Prev>
class Object
{
public:

    static const int subclasscount = 1 + get_subclasscount<Prev>::result;   //the subclass count
};


class sub1 : public Object<sub1, int>
{

};


class sub2 : public Object<sub2, sub1>
{

};


class sub3 : public Object<sub3, sub2>
{

};

These last 3 empty classes are the subclasses that we're counting. This is our header file. In our main .cpp file, we have:

int main() {

std::cout << sub3::subclasscount;

char c;
std::cin >> c;
}

Running it, we get a simple output of:

3

Which confirms that it has worked. Now, some of the downsides to this solution is:

We must know what our last defined subclass was, before we add on.
We must keep up with anywhere we use the subclass counter, always modifying it to be from the last subclass in the list (this can be averted by using a consistant "endoflist" subclass, which would need to be maintained instead)

Upsides, though, include the fact that we do not need to upkeep any of our previously defined subclasses. However, I consider this answer more of a "starting point" than a "final solution"; perhaps something that can be expanded upon?

(also, this can easily be abused to make a form of tree structure, where subclasscount would actually represent the depth of any given node in the tree)

Anyone have any ideas from here?

来源：https://stackoverflow.com/questions/11113702/how-to-count-the-number-of-crtp-subclasses-of-a-template-class

标签

c++

counter

template-meta-programming

crtp

subclass