Fold expressions handle empty parameter packs differently depending on whether they are unary folds or binary folds. Here's how empty parameter packs are treated in eachÂ case:

Unary folds expect the parameter pack to have at least one element. If the parameter pack is empty, it results in a compilationÂ error.

For example, in this case, callingÂ `Product()`

Â with an empty parameter pack triggers a compilation error because a unary fold over theÂ `*`

operator requires a non-empty parameterÂ pack.

To handle empty parameter packs with unary folds, you need to provide an explicit base case or use a binary foldÂ instead.

```
template <typename... Types>
auto Product(Types... Args) {
return (... * Args);
}
int main() {
// Compilation error
Product();
}
```

`error: a unary fold expression over '*' must have a non-empty expansion`

Binary folds handle empty parameter packs by using the identity value of the operator as the result of the fold expression.Â For example:In this case, callingÂ `Sum()`

Â with an empty parameter pack results in the identity value of theÂ `+`

Â operator, which isÂ `0`

.

```
#include <iostream>
template <typename... Types>
auto Sum(Types... Args) {
return (0 + ... + Args);
}
int main() {
int EmptySum = Sum();
std::cout << EmptySum;
}
```

`0`

The binary fold expressionÂ `(0 + ... + Args)`

Â evaluates toÂ `0`

Â whenÂ `Args`

Â isÂ empty.

The identity value depends on the operator used in the binaryÂ fold

- For addition (
`+`

), the identity value isÂ`0`

. - For multiplication (
`*`

), the identity value isÂ`1`

. - For logical AND (
`&&`

), the identity value isÂ`true`

. - For logical OR (
`||`

), the identity value isÂ`false`

. - For comma (
`,`

), the identity value isÂ`void()`

.

To ensure that your fold expressions handle empty parameter packs correctly, youÂ can:

- Use binary folds with an appropriate identity value when you need to support empty parameter packs.
- Provide an explicit base case or specialization for empty parameter packs if using unary folds.
- Be aware of the operators that have an identity value and choose them appropriately based on your requirements.

By understanding how fold expressions handle empty parameter packs, you can write more robust and flexible code that works correctly in both empty and non-emptyÂ scenarios.

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This Question is from the Lesson:### Fold Expression

An introduction to C++17 fold expressions, which allow us to work more efficiently with parameter packs