Georg Hager's Blog

Random thoughts on High Performance Computing


Restricting member function calls by numeric template parameters

Thanks to Johannes for this interesting problem.

Let’s say you have a class template B where T is an arbitrary type and C is an integer template argument:

template  <class T, int C> B;

B has two member functions with the same name:

template  <class T, int C> void B::member(T _t);
template  <class T, int C> void B::member(T _t, int _i);

How do you make sure that the first member (the one with just a single argument) can only be called for instances of the class template with C==1? This is supposed to happen at compile time (runtime would be easy, of course).

One could (partially) specialize the whole class for C=1, which generates a whole lot of code bloat. Another solution would be to have a base class with only the two-argument member and a derived class (inheriting from B) implementing the single-argument member. This is also unsatisfactory because the derived class must have a name different from the first.

A more elegant solution is to have a private template class declaration encapsulated into the base class which gets instantiated when calling the single-argument member:

template  class B {
  template  <int U> class BX {
    BX(B<T,U>* p) {}
  // no problem here
  void member(T _x, int _i);
  // only valid if C==1
  void member(T _x) {
    BX<2-C> bx(this);

Only if C==1 will calling the single-argument member not fail, because BX<2-C> gets instantiated using a pointer to class B. If C!=1, the compiler spits out an error message saying that it can’t find a constructor with the appropriate argument. In this special case we could just have used BX<1>, but I wanted to show that any simple integer expression will do (see below).

Admittedly, the error message is a little clumsy, but it does the job. This example can easily be generalized – remember that the ternary operator will also be evaluated at compile time. All you have to do is provide a function of C that is equal to C for all permitted values of C, and different from C otherwise.

I’ve checked that this trick works using the Intel 10.1 and GNU 4.1.3 compilers.