Templates for unique id generation for types : a beginning

After reading one of the comments on this blog post, I decided to publish here a proof of concept for the generation of unique ids for types.

Basically, it’s just about one template structure, using very basical templates techniques : specialization, metaprogramming-level recursion and nested enum for the computation. Either a familiarity with template metaprogramming or with functional programming is enough to understand the following code.

If you’re familiar with any scheme of functional recursion, or recursion in its mathematical sense, you’ll get it quite easily. Mathematically, we’re somehow defining a recursive function generate_id this way :

Which, in C++, becomes :

template <typename T>
struct id_generator;

template <typename T>
struct id_generator<T*>
{
    enum { result = id_generator<T>::result*2 };
};

template <typename T>
struct id_generator<T&>
{
    enum { result = id_generator<T>::result*3 };
};

template <typename T>
struct id_generator< std::vector<T> >
{
    enum { result = id_generator<T>::result*11 };
};

template <>
struct id_generator<int>
{
    enum { result = 5 };
};

template <>
struct id_generator<char>
{
    enum { result = 7 };
};

And a minimal example of use :

int main()
{
    std::cout << "int : " << id_generator< int >::result << std::endl
              << "char**** : " << id_generator< char**** >::result << std::endl
              << "vector<int> : " << id_generator< std::vector<int> >::result << std::endl;
    return 0;
}
/* Output :
int : 5
char**** : 112
vector<int> : 55
*/

To make it really usable, you’ll have to add much (much much much) more :

• base types, like int and char here, which help for terminating the recursive call
• composition rules, like vector, references and pointers here, which help for propagating the recursion

To ensure the uniqueness here, I use prime numbers, simply [1]. Indeed, for the base types, I give a value being a prime number. For the different compositions, I multiply by a different prime number for each. However, most of you may have noticed that the id only lets you know about what type compositions and base types are used, but gives nothing about the order. If we add a rule for std::list, multiplying by 13 for example, then a list of vectors of ints will have the same id as a vector of lists of int. It’d need additional code and workarounds to take the order of the compositions in account — I guess it is possible though. If any reader here comes up with a solution, let us know. A possible solution would be to drop prime numbers, since they rely on a commutative (abelian) ring [2].

[1] http://en.wikipedia.org/wiki/Fundamental_theorem_of_arithmetic
[2] http://en.wikipedia.org/wiki/Commutative_ring