How to Correctly Loop Over a Parameter Pack in Variadic Templates?

How to Loop Over a Parameter Pack Using a Pack Expansion

When using variadic templates and functions, it's essential to understand the use of pack expansions. Consider the following code:

template<typename T>
static void bar(T t) {}

template<typename... Args>
static void foo2(Args... args)
{
    (bar(args)...);
}

Upon compilation, this code fails with the error:

Error C3520: 'args': parameter pack must be expanded in this context

The Solution: Pack Expansion in Braced-Init-List

To address this issue, you can use a pack expansion within a braced-init-list:

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

Understanding the Braced-Init-List

The initializer list inside the array definition expands the pack expansion into a comma-separated list. The cast to void ensures that the comma operator used is the built-in one rather than any overloaded versions. This guarantees left-to-right evaluation of the pack expansion.

Additional Options with C 17

In C 17, you can simplify the code using fold expressions:

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

Conclusion

By utilizing a pack expansion within a braced-init-list or a fold expression in C 17, you can effectively loop over a parameter pack and avoid compilation errors. Understanding this technique is crucial for working with variadic templates and functions in C .

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