How Can I Correctly Iterate Over a C Parameter Pack to Avoid Compiler Errors?

Looping over Parameter Packs with Pack Expansion

In variadic templates, parameter packs allow for a flexible number of arguments to be passed to a function. While trying to iterate over a parameter pack, a compiler error may arise if the pack is not properly expanded.

Consider the following code:

template<typename... Args>
static void foo2(Args... args)
{
    (bar(args)...); // Error: parameter pack must be expanded
}

This code fails to compile with the error "Error C3520: 'args': parameter pack must be expanded in this context." The reason for this error is that the parameter pack args needs to be expanded before it can be used within the bar function call.

One way to achieve this expansion is by utilizing a braced-init-list (initializer list enclosed in braces). By placing the expansion inside the initializer list of a dummy array, the compiler can perform the necessary expansion.

template<typename... Args>
static void foo2(Args &&&... args)
{
    int dummy[] = { 0, ( (void) bar(std::forward<Args>(args)), 0) ... };
}

The initializer list allows the entire pack to be expanded, and the cast to void ensures that the comma operator is used, regardless of any overloaded operators for the return type of bar.

An alternative approach, introduced in C 17, is the use of fold expressions:

((void) bar(std::forward<Args>(args)), ...);

This code provides a concise and efficient way to loop over the parameter pack.

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