How to Iterate over a Packed Variadic Template Argument List Efficiently?

How to Iterate Over a Packed Variadic Template Argument List

Iterating over a packed variadic template argument list requires addressing two challenges: extracting data from the list and determining the number of arguments.

Extracting Data

One method involves wrapping data into a custom type, expanding it into a vector, and iterating over that. However, this approach requires complex function calls and limits argument types.

Counting Arguments

Traditional methods like recursion or loops are not feasible within a macro-generated function. Instead, lambda functions can be employed.

Lambda Function Solution

Using fold expressions (C 17), we can iterate over arguments using a lambda function:

<code class="cpp">template <class ... Ts>
void Foo(Ts &&... inputs) {
    int i = 0;

    ([&amp;] {
        ++i;
        std::cout << "input " << i << " = " << inputs << std::endl;
    }(), ...);
}</code>

This lambda can execute actions, such as printing arguments, during iteration.

Handling Returns and Breaks

For complex loops, one can utilize:

• try/throw: Throw an exception to end the loop prematurely. However, this can cause performance issues.
• variable/if switches: Create a variable/if statement for each action. This approach is less elegant but offers more control.

By leveraging fold expressions and lambda functions, we can effectively iterate over a packed variadic template argument list, extracting data and handling breaks/returns as needed.

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