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Preprocessor: Concatenate words to single identifiers

The preprocessor directive ## (two hashes or number signs, aka the token pasting operator) allows to concatenate two symbols into one.

#include <stdio.h>

//  CONCAT(foo, bar) -> foobar
#define CONCAT(id1, id2) id1##id2


int main() {

 // Declare a variable named FooBar, after running the
 // preprocessor, the line will read:
 //
 //    int FooBar
 //
    int CONCAT(Foo, Bar);

    FooBar = 42;

    printf("FooBar = %d\n", FooBar);
}

Github repository about-preprocessor, path: /hash/concatenate.c

The program prints:

FooBar = 42

It doesn't matter if spaces are to the left or right side of the hashes (ident##other is equal to ident ## other):

#include <stdio.h>

#define DECLARE_NUM_A(id1, id2) int id1##id2
#define DECLARE_NUM_B(id1, id2) int id1    ##id2
#define DECLARE_NUM_C(id1, id2) int id1    ##     id2

int main() {

 //
 // The following three lines are expanded by the preprocessor to
 //     int numOne;
 //     int numTwo
 //     int numThree
 //
    DECLARE_NUM_A( num, One   ); 
    DECLARE_NUM_B( num, Two   );
    DECLARE_NUM_C( num, Three );

    numOne   = 42;
    numTwo   = 99;
    numThree = -1;

    printf("The numbers are %d %d %d\n", numOne, numTwo, numThree);

}

Github repository about-preprocessor, path: /hash/concatenate-with-spaces-around-hashes.c

Preprocessor: Concatenate words to single identifiers

See also