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How Can I Correctly Loop Through a C Parameter Pack Using Pack Expansion?
How Can I Correctly Loop Through a C Parameter Pack Using Pack Expansion?
Jan 04, 2025 am 05:41 AM
Looping over Parameter Packs with Pack Expansion
In the provided code snippet, you intend to loop through a variable-length parameter pack using the pack expansion syntax. However, the code fails to compile with the error "parameter pack must be expanded in this context."
To resolve this issue, you need to place the pack expansion inside a context where it's permitted. One suitable location is within a braced-init-list. Consider the following modified code:
template<typename... Args>
static void foo2(Args &&... args)
{
int dummy[] = { 0, ( (void) bar(std::forward<Args>(args)), 0) ... };
}
Here's how it works:
- The code inside the initializer list of the dummy array is enclosed in {}.
- The pack expansion syntax is used within the initializer list. Specifically, ( (void) bar(std::forward
(args)), 0) is expanded for each parameter in the pack. - The comma operator is used to separate each expression, ensuring that the bar() function is called for each parameter.
- The parentheses around bar(std::forward
(args)) are used to cast the result to void. This ensures that the built-in comma operator is used and not an overloaded one. - The dummy array is created with at least one element to avoid creating an illegal 0-length array when the parameter pack is empty.
With this modification, the code will successfully compile and loop over the parameter pack.
C 17 Fold Expressions
In C 17, you can simplify the code using fold expressions:
((void) bar(std::forward<Args>(args)), ...);
This expression expands the pack and applies the specified operation (in this case, calling bar()) left-to-right.
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