How to Call Functions on Variadic Template Arguments in C ?

Calling Functions on Variadic Template Arguments

The goal is to implement a print() function that takes a variable number of arguments and executes an arbitrary function on each argument. This is desired as a compact alternative to the recursive implementation with separate specializations for each argument type.

C 17 Solution

In C 17, this can be achieved using a fold expression:

(f(args), ...);

This expression expands the arguments in args and calls f() on each one. If f() returns an object with an overloaded comma operator, use:

((void)f(args), ...);

Pre-C 17 Solution

For versions of C prior to C 17, a common approach is to use a list initializer to expand the arguments:

{ print(Args)... }

However, print() returns void, so to avoid a compile error, the code can be modified as shown below:

using expand_type = int[];
expand_type{ (print(Args), 0)... };

This works for variadic arguments, but there are some caveats to consider. To make this pattern reusable, a macro can be created:

#define SO_EXPAND_SIDE_EFFECTS(PATTERN) ::so::expand_type{ 0, ((PATTERN), 0)... }

However, to prevent potential issues with overloaded comma operators, the macro can be updated as follows:

#define SO_EXPAND_SIDE_EFFECTS(PATTERN) ::so::expand_type{ 0, ((PATTERN), void(), 0)... }

Pre-C 17 solutions may also require additional considerations to avoid unnecessary memory allocation.

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