Static assertion if possible, dynamic assertion otherwise?

Question

Let's say I have a template function that takes an integer and a const reference to an instance of type T. Now depending on the integer, only some T's are acceptible, otherwise an exception is thrown at runtime.

If all uses of this function would use constant integers, it would be possible to make the int a template parameter and use a static assertion to check if it is acceptable. So instead of func(1,c) one would use func<1>(c) and would gain compile-time type checking. Is there any way to write func(1,c) and still keep the compile-time check, while also being able to write func(i,c) and use a dynamic assertion? The goal is to make it transparent to the developer. It would simply be great to add this safety without bothering the developers about things like compile-time constants. They'd probably only remember that func(1,c) always works and use that, avoiding the check.

How can I define a function with a static assertion whenever possible and a dynamic assertion otherwise?

---

The following code shows the solution for GCC by Ivan Shcherbakov:

#include <iostream>
#include <cassert>

template<typename T>
void __attribute__((always_inline)) func(const int& i, const T& t);

void compile_time_error_() __attribute__((__error__ ("assertion failed")));

template<>
  void __attribute__((always_inline))
  func(const int& i, const float& t)
{
    do {
        if (i != 0) {
            if (__builtin_constant_p(i)) compile_time_error_();
            std::cerr << "assertion xzy failed" << std::endl;
            exit(1);
        }
    } while (0);
    func_impl<float>(i,t);
}

This will only allow the combination of i=0 and T=float. For other combinations a good way would be creating a Macro that produces the code of template<> func(const int& i, const T& t) with T and i != 0 replaced.

Answer 1

Well, if you're using GCC, you can use a dirty hack, but it will only work when function inlining is enabled (-O1 or more):

void my_error() __attribute__((__error__ ("Your message here")));

template <typename T1, typename T2> struct compare_types 
{
    enum {Equals = 0};
};

template <typename T1> struct compare_types<T1,T1> 
{
    enum {Equals = 1};
};

template <typename Type> __attribute__((always_inline)) void func(int a, Type &x)
{
    if (__builtin_constant_p(a))
    {
        if (a == 1 && compare_types<Type,char>::Equals)
            my_error();
    }
}

In this case when a == 1 and Type is char, you'll get an error. Here's an example that will trigger it:

int main()
{
    char x;
    func(1, x);
    return 0;
}

Note that this example heavily relies on the gcc-specific __builtin_constant_p() function and won't work with other compilers!