Creating Strings from Macro Arguments: # and ##

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Creating Strings from Macro Arguments: The # Operator

Here's a function-like macro:

#define PSQR(X)  printf("The square of X is %d.\n", ((X)*(X)));

Suppose you used the macro like this:

PSQR(8);

Here's the output:

The square of X is 64.

Note that the X in the quoted string is treated as ordinary text, not as a token that can be replaced.

Suppose you do want to include the macro argument in a string. ANSI C enables you to do that. Within the replacement part of a function-like macro, the # symbol becomes a preprocessing operator that converts tokens into strings. For example, say that x is a macro parameter, and then #x is that parameter name converted to the string "x". This process is called stringizing. [Listing 16.3]illustrates how this process works.

Listing 16.3. The subst.c Program

/* subst.c -- substitute in string */

#include 

#define PSQR(x) printf("The square of " #x " is %d.\n",((x)*(x)))



int main(void)

{

    int y = 5;



    PSQR(y);

    PSQR(2 + 4);



    return 0;

}

Here's the output:

The square of y is 25.

The square of 2 + 4 is 36.

In the first call to the macro, #x was replaced by "y", and in the second call #x was replaced by "2 + 4". ANSI C string concatenation then combined these strings with the other strings in the printf() statement to produce the final strings that were used. For example, the first invocation becomes this:

printf("The square of " "y" " is %d.\n",((y)*(y)));

Then string concatenation converts the three adjacent strings to one string:

"The square of y is %d.\n"

Preprocessor Glue: The ## Operator

Like the # operator, the ## operator can be used in the replacement section of a function-like macro. Additionally, it can be used in the replacement section of an object-like macro. The ## operator combines two tokens into a single token. For example, you could do this:

#define XNAME(n) x ## n

Then the macro

XNAME(4)

would expand to the following:

x4

[Listing 16.4] uses this and another macro using ## to do a bit of token gluing.

Listing 16.4. The glue.c Program

// glue.c -- use the ## operator

#include 

#define XNAME(n) x ## n

#define PRINT_XN(n) printf("x" #n " = %d\n", x ## n);



int main(void)

{

    int XNAME(1) = 14;  // becomes int x1 = 14;

    int XNAME(2) = 20;  // becomes int x2 = 20;

    PRINT_XN(1);        // becomes printf("x1 = %d\n", x1);

    PRINT_XN(2);        // becomes printf("x2 = %d\n", x2);



    return 0;

}

Here's the output:

x1 = 14

x2 = 20

Note how the PRINT_XN() macro uses the # operator to combine strings and the ## operator to combine tokens into a new identifier.

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