C Variadic Templates: Invoking a Function on All Template Arguments

In C , it's often desirable to iterate through variadic template arguments and perform a specific operation, such as calling a function. This can be achieved using either:

C 17 Fold Expression

(f(args), ...);

However, if the called function potentially returns an object with an overloaded comma operator, you should use:

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

Pre-C 17 Solution

A common approach is to leverage list-initialization and perform the expansion within it:

{ print(Args)... }

Since print() returns void, you can workaround the issue by returning int:

{ (print(Args), 0)... }

To ensure this works with any number of arguments, you can make the pack always have at least one element:

{ 0, (print(Args), 0)... }

You can encapsulate this pattern into a reusable macro:

namespace so {
    using expand_type = int[];
}

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

To handle overloaded comma operators, you can modify the macro:

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

If you're concerned about unnecessary memory allocation, you can define a custom type that supports list-initialization but doesn't store data:

namespace so {
    struct expand_type {
        template <typename... T>
        expand_type(T&&...) {}
    };
}