How Can C 17 Fold Expressions Simplify Variadic Function Invocation?

Simplifying Variadic Function Invocation Using C 17 Folds

Variadic template arguments provide a convenient way to work with functions that accept a variable number of arguments. However, creating recursive chains to call functions on each variadic argument can lead to redundant code generation. Here's how you can overcome this issue using C 17 fold expressions or a pre-C 17 solution.

C 17 Fold Expression:

(f(args), ...);

This fold expression can be used to call a function on all variadic template arguments, resulting in a single N-arg function call.

Pre-C 17 Solution:

1. Wrap Function in Identifier:

Transform the function to return an int, which can be used as a dummy argument in an initializer list:

int print_wrap(const T& t) {
  print(t);
  return 0;
}

2. Create List Initializer:

Use a list initializer to expand the variadic arguments and call the modified function:

{ print_wrap(args)... }

3. Define Helper Namespace:

To make this approach reusable, define a helper namespace:

namespace so {
  using expand_type = int[];
}

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

4. Usage:

SO_EXPAND_SIDE_EFFECTS(f(args));

Advantages:

• Generates only a single function call with N arguments.
• Supports functions with overloaded comma operators.
• Ensures left-to-right evaluation of arguments.

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