X Macro

X Macro is a technique in the C and C++ programming languages 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. It remains useful also in modern-day C and C++, but is nevertheless relatively unknown.

Implementation

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

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 and indentation 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

• Wikibooks on X Macros