Fold Expressions with Side Effects

When using fold expressions with operations that have side effects, there are a few caveats to keep in mind:

Order of evaluation

The order in which the elements of the parameter pack are evaluated in a fold expression is unspecified. This means that if the operations have side effects that depend on a specific evaluation order, the behavior of the program may be undefined.

For example, consider the following fold expression that modifies elements of a parameter pack:In this case, the order in which the elements of Args are incremented is unspecified. If the order of increments matters for the correctness of the program, the behavior is undefined.

1template <typename... Types>
2void ModifyValues(Types&... Args) {
3  (++Args, ...);
4}

Short-circuit evaluation

Fold expressions do not perform short-circuit evaluation. This means that even if the result of the fold expression can be determined before evaluating all the elements, the remaining elements will still be evaluated.

For example, consider the following fold expression that performs a logical OR operation:In this case, even if one of the elements of Args evaluates to true, the remaining elements will still be evaluated. If the elements have side effects, those side effects will occur regardless of the final result.

1template <typename... Types>
2bool AnyTrue(Types... Args) {
3  return (... || Args);
4}

Overloaded operators

When using fold expressions with overloaded operators, be cautious of any unexpected behavior introduced by the overloaded operators.

For example, if you have a type that overloads the comma operator (operator,) in a way that modifies the behavior of the fold expression, it can lead to surprising results:In this case, the overloaded comma operator modifies the behavior of the fold expression, resulting in unexpected output.

1struct MyType {
2  int Value;
3  MyType operator,(const MyType&) {
4    return {Value + 1};
5  }
6};
7
8template <typename... Types>
9void PrintValues(Types... Args) {
10  (std::cout << Args.Value << ' ', ...);
11}
12
13int main() {
14  PrintValues(MyType{1}, MyType{2}, MyType{3});
15}

To mitigate these caveats, it's important to:

• Be mindful of the potential undefined behavior when using fold expressions with operations that have side effects and depend on evaluation order.
• Understand that fold expressions do not perform short-circuit evaluation and all elements will be evaluated regardless of the final result.
• Be cautious of overloaded operators that may introduce unexpected behavior in fold expressions.

If you encounter situations where these caveats are problematic, you may need to consider alternative approaches or use traditional variadic template techniques for more fine-grained control over the evaluation and side effects.