Alias for preprocessor methods in C++

Question

I am writing a library that will use openmp however I want to give the user the ability to disable/enable my libraries openmp with a preprocessor definition (in case they want to use openmp but avoid not have my lib use it)

The most obvious solution would be something like

void foo() {
#ifndef disable
#pragma omp parallel for
#endif
    for (int i = 0; i < 100; ++i) {
        std::cout << i << " " << std::endl;
    }
#ifndef disable
#pragma omp barrier
#endif

However I have a multitude of methods that utilize openmp. Is there any way I can create an alias for openmp?

IE:

#if OpenMpSupported 
#define AliasParallel = #pragma omp parallel for
#define AliasBarrier  = #pramgma omp barrier
#else 
#define AliasParallel = //some thing that doesn't matter
#define AliasBarrier = //some thing that that doesn't matter
#endif

void foo() {
#AliasParallel
    for (int i = 0; i < 100; ++i) {
        std::cout << i << " " << std::endl;
    }
#AliasBarrier

Simply put is there anyway I could create a shorthand for this.

Answer 1

In general, there is no way to do this. The result of macro expansion is not rescanned for # directives.

However, in the particular case of #pragma there is an alternative (since C++11):

_Pragma("foo bar")

is equivalent to

#pragma foo bar

and the former can be produced by macros.

In your case that would be:

#if OpenMpSupported 
#define AliasParallel _Pragma("omp parallel for")
#define AliasBarrier  _Pragma("omp barrier")
#else 
#define AliasParallel
#define AliasBarrier
#endif

void foo() {
AliasParallel
    for (int i = 0; i < 100; ++i) {
        std::cout << i << " " << std::endl;
    }
AliasBarrier

(I don't know if this works with openmp, however.)