C++ preprocessor conditional parameter

Question

Please note C++03! any C++11 solutions are not good for me, but do post them just for knowledge sake.

I know the preprocessor can do things like:

#de         


        

Answer 1

You can do this with the preprocessor if the domain of values for the conditional parameter is well-known (and preferably small). For example, supposing the parameter can only have the values 0 and 1:

#define DOIT_0(X)
#define DOIT_1(X) X
#define CONCAT_(X, Y) X ## Y
#define MAYBE(X) CONCAT_(DOIT_, X)

#define BAR(X) MAYBE(X)(  cout<<"hi"<

Live on coliru.

Beware of unprotected commas in the argument to BAR.

For equality checks, again over a small range:

#define CONCAT3_(X,Y,Z) X ## Y ## Z
#define EQUAL_0_0(X) X
#define EQUAL_1_1(X) X
#define EQUAL_1_1(X) X
#define EQUAL_0_1(X)
#define EQUAL_0_2(X)
#define EQUAL_1_0(X)
#define EQUAL_1_2(X)
#define EQUAL_2_0(X)
#define EQUAL_2_1(X)
#define DO_IF_EQUAL(X, Y) CONCAT3_(EQUAL_, X, Y)

#define BAR(X) DO_IF_EQUAL(X, 2) ( std::cout << "hi\n"; )