Conditional compilation depending on function argument?

Question

I searched far and wide and information on the net seems to suggest that conditional compilation using the preprocessor works exclusively on environment variables.

Basically, I would like to have an inline function or macro perform different operations based in its input arguments. Example of what I want to achieve:

inline void foo(int x) {

    #if (x < 32)

        register0 |= (1 << x);

    #else

        register1 |= (1 << (x - 32));

    #endif

}

The main goal here is that the resulting inline code will not contain conditional code for constant inputs.

Basically, I currently program for a microcontroller (lpc213x) and would like to have an inline function or macro to do pin configuration setup. Since pin configurations are split across multiple registers (e.g. 0 and 1 above), I would like to perform some conditional checks to decide which register is supposed to be written to for a given pin constant.

However, the pin configurations are all constant at compile time, so I would like to eliminate the conditional checks from compile code. I know that optimization would likely get rid of unnecessary conditionals anyway, but I'm wondering whether there is a way to achieve this behavior explicitly, because I might need to disable optimization in the future.

Thanks,

FRob

Answer 1

With C++, you could use template functions, like this:

template <bool B>
void doSetRegister(int x);

template<>
inline void doSetRegister<true>(int x) {
    register0 |= (1 << x);
}

template<>
inline void doSetRegister<false>(int x) {
    register1 |= (1 << (x - 32));
}

template <int X>
inline void setRegister() {
    doSetRegister<X <= 32>(X);
}

int main() {
    setRegister<1>();
    setRegister<33>();
}