Use a template parameter in a preprocessor directive?

Question

Is it possible to use a non-type constant template parameter in a preprocessor directive? Here's what I have in mind:

template <int DING>
struct Foo
{
    enum { DOO = DING };
};

template <typename T>
struct Blah
{
    void DoIt()
    {
        #if (T::DOO & 0x010)

        // some code here

        #endif
    }
};

When I try this with something like Blah<Foo<0xFFFF>>, VC++ 2010 complains something about unmatched parentheses in the line where we are trying to use "#if". I am guessing the preprocessor doesn't really know anything about templates and this sort of thing just isn't in its domain. What say? Thanks!

Answer 1

No, this is not possible. The preprocessor is pretty dumb, and it has no knowledge of the structure of your program. If T::Doo is not defined in the preprocessor (and it can't be, because of the ::), it cannot evaluate that expression and will fail.

However, you can rely on the compiler to do the smart thing for you:

        if (T::Doo & 0x010) {
            // some code here
        }

Constant expressions and dead branches are optimized away even at the lower optimization settings, so you can safely do this without any runtime overhead.

Answer 2

what members are available in T depends on which bits are set in T::DOO

It sounds to me like T::DOO is acting like a subclass identifier. So I'm thinking that your Foo and related classes should be subclasses of a class that guarantees that DOO is defined.

The key is: why must you use a bit field?

Answer 3

Not sure if this applies to your situation, but it is possible to isolate different cases with template classes. For example: (using a modified version of your code from above)

template <typename T, int N>
struct Blah
{
    void DoIt()
    {
        // normal DoIt() code
    }
};

template <typename T>
struct Blah<T,5>
{
    void DoIt()
    {
        // special DoIt() code for only when N==5
    }
};