HP C/aC++ for PA-RISC Software manual Using Macros to Define Constants, Other Macros, Var1 is

Example 2

You can use the # and ## operators together:

#include <iostream.h>

#define show_me(arg) int var##arg=arg;\

cout << "var" #arg " is " << var##arg << "\n"; int main()

{

show_me(1);

}

Preprocessing this example yields the following code for the main procedure:

int main()

{

int var1=1; cout << "var" "1" " is " << var1 << "\n";

}

After compiling the code with aCC and running the resulting executable file, you get the following results:

var1 is 1

Spaces around the # and ## are optional.

In both the # and ## operations, the arguments are substituted as is, without any intermediate expansion. After these operations are completed, the entire replacement text is rescanned for further macro expansions.

NOTE: The result of the preprocessor concatenation operator ## must be a _single_ token. In particular, the use of ## to concatenate strings is redundant and not legal C or C++. For example:

#include

#define concat_token(a, b) a##b #define concat_string(a, b) a b int main() {

// Wrong:

printf("%s\n", concat_token("Hello,", " World!")); // Correct:

printf("%s\n", concat_string("Hello,", " World!"));

//Best: (macro not needed at all!): printf("%s\n", "Hello," " World!");

}

Using Macros to Define Constants

The most common use of the macro replacement is in defining a constant. In C++ you can also declare constants using the keyword const. Rather than explicitly putting constant values in a program, you can name the constants using macros, then use the names in place of the constants. By changing the definition of the macro, you can more easily change the program:

#define ARRAY_SIZE 1000float x[ARRAY_SIZE];

In this example, the array x is dimensioned using the macro ARRAY_SIZE rather than the constant 1000. Note that expressions that may use the array can also use the macro instead of the actual constant:

for (i=0; i<<ARRAY_SIZE; ++i) f+=x[i];

Changing the dimension of x means only changing the macro for ARRAY_SIZE. The dimension changes and so do all of the expressions that make use of the dimension.

Other Macros

Some other common macros used by C programmers include:

Overview of the Preprocessor 123