X Macro

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X Macro is a technique in the C programming language for generating repeating code structures at compile time. It is used when the same operation has to be executed for a list of items, but regular for loops cannot be used.

Usage of X Macros dates back to 1960's.[1] It remains useful also in modern-day C, but is nevertheless relatively unknown.[2] [3]


Implementation[edit]

An X-macro consists of two parts: the list, and the execution of the list. The list is just a #define construct which generates no code by itself. The execution of the list provides another #define macro, which is expanded for each entry in the list.

Example[edit]

This example defines a list of variables, and automatically generates their declarations and a function to print them out.

First the list definition. The list entries could contain multiple arguments, but here only the name of the variable is used.

#define LIST_OF_VARIABLES \
    X(value1) \
    X(value2) \
    X(value3)

Then we execute this list to generate the variable declarations:

#define X(name) int name;
LIST_OF_VARIABLES
#undef X

In a similar way, we can generate a function that prints the variables and their names:

void print_variables()
{
#define X(name) printf(#name " = %d\n", name);
LIST_OF_VARIABLES
#undef X
}

When run through the C preprocessor, the following code is generated. Line breaks have been added for ease of reading, even though they are not actually generated by the preprocessor:

int value1;
int value2;
int value3;
 
void print_variables()
{
    printf("value1" " = %d\n", value1);
    printf("value2" " = %d\n", value2);
    printf("value3" " = %d\n", value3);
}

Further reading[edit]

References[edit]

  1. ^ Meyers, Randy. The New C: X Macros. Dr.Dobb's 2001.
  2. ^ Bright, Walter. The X Macro. Dr.Dobb's 2010
  3. ^ Lucas, Andrew. Reduce C-language coding errors with X macros. Embedded.com 2013.