C++ template: How to put nontype constraints in compiling time

Question

Suppose I have following template

template<unsigned char I, unsigned char F>
class FOO
{
   ....
}

In fact, I require (I >= F). If someone misuses

FOO<1, 2> a;

I hope to raise a compiling error. How to do that?

Thanks

Answer 1

One way may be C++11's static_assert, which is similar to an assert, but checked at compile time:

template<unsigned char I, unsigned char F>
class FOO
{
    static_assert(I >= F, "I needs to be larger or equal to F");
    ...
};

Answer 2

If you don't have C++11, the good old array bounds trick works here too. Just put the following in the private section of your class:

static int const error_size = I >= F ? 1 : -1;
typedef char ERROR_I_must_not_be_less_than_F[error_size];

This will trigger a "negative array size" error whenever I is less than F.