How Can I Determine the Number of Arguments in a C Variadic Macro?

Determining the Number of Arguments in Variadic Macros Using C Preprocessor's VA_ARGS

Variadic macros in C allow the use of an arbitrary number of arguments. However, determining the number of arguments passed to a variadic macro can be challenging. This article explores a simple and efficient solution using the VA_ARGS preprocessor macro.

Understanding VA_ARGS

VA_ARGS is a built-in preprocessor macro that expands to the actual arguments passed to a variadic macro. However, it does not provide any information about the number of arguments.

Counting Arguments Using Integer Array Size

To determine the number of arguments in a variadic macro, we can exploit the behavior of array sizes in the C preprocessor. The following macro, NUMARGS(), uses this technique:

#define NUMARGS(...) (sizeof((int[]){__VA_ARGS__})/sizeof(int))

This macro expands to the size of an array containing all the arguments passed to it, divided by the size of an integer. The resulting value is the number of arguments.

Example Usage

Consider the following macro, SUM(), that calculates the sum of its arguments:

#define SUM(...) (sum(NUMARGS(__VA_ARGS__), __VA_ARGS__))

To use SUM(), we would call it with a variable number of arguments, like so:

SUM(1);
SUM(1, 2);
SUM(1, 2, 3);

The NUMARGS() macro would count the number of arguments and pass it along with the arguments to the sum() function.

Handling Empty Argument List

By default, the NUMARGS() macro will fail if called with an empty argument list. To address this, a variant of the macro can be defined using GNU C extensions:

#define NUMARGS(...) (sizeof((int[]){0, ##__VA_ARGS__})/sizeof(int)-1)

This variant allows for an empty argument list and correctly counts the number of arguments in all cases.

Conclusion

The VA_ARGS preprocessor macro can be leveraged to efficiently count the number of arguments in variadic macros. By combining this technique with integer array size manipulation, we can develop useful macros for handling variable numbers of arguments in C .

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