Yes, `fold_left()`

and `fold_right()`

can be used with custom data types, provided that the custom data type supports the required binaryÂ operation.

- The custom data type must support the operation used in the fold function. This means that if you are using
`std::plus<>`

, your custom type must have an`operator+`

. - The initial value and the elements in the range must be compatible with the binary operation.

Letâ€™s create a custom data type `Accumulator`

that supports the addition operation. Weâ€™ll use this type with `fold_left()`

to demonstrate how folding works with custom dataÂ types.

```
#include <algorithm>
#include <iostream>
#include <vector>
struct Accumulator {
int sum;
int count;
Accumulator operator+(int n) const {
return Accumulator{sum + n, count + 1}; }
void log() const {
std::cout << "Count: " << count
<< "\nSum: " << sum << "\n";
}
};
int main() {
std::vector<int> numbers{1, 2, 3, 4, 5};
Accumulator result = std::ranges::fold_left(
numbers, Accumulator{}, std::plus<>());
result.log();
}
```

```
Count: 5
Sum: 15
```

- The
`Accumulator`

struct has two members:`sum`

and`count`

. - We define the
`operator+`

to allow adding an integer to an`Accumulator`

object, which updates the`sum`

and`count`

. - The
`log()`

function is used to print the current state of the`Accumulator`

.

In the `main()`

function, we use `std::ranges::fold_left()`

to combine elements of the `numbers`

vector using an `Accumulator`

as the initial value. The custom `operator+`

ensures that each element of the vector is added to the `Accumulator`

, updating the `sum`

and `count`

Â accordingly.

You can also define other operations for your custom data types. For example, if you have a custom type that represents a complex number, you can define an `operator*`

for multiplication and use it with `fold_left()`

.

```
#include <algorithm>
#include <iostream>
#include <vector>
struct Complex {
double real;
double imag;
Complex operator*(const Complex& other) const {
return Complex{
real * other.real - imag * other.imag,
real * other.imag + imag * other.real
};
}
};
int main() {
std::vector<Complex> numbers{
{1, 2}, {3, 4}, {5, 6}};
Complex initial{1, 0};
Complex result = std::ranges::fold_left(
numbers, initial, std::multiplies<>());
std::cout << "Result: (" << result.real
<< ", " << result.imag << ")";
}
```

`Result: (-85, 20)`

In summary, `fold_left()`

and `fold_right()`

can be used with custom data types as long as the required operations are defined. This allows for flexible and powerful ways to process collections of customÂ objects.

Answers to questions are automatically generated and may not have been reviewed.

This Question is from the Lesson:### C++23 Fold Algorithms

An introduction to the 6 new folding algorithms added in C++23, providing alternatives to `std::reduce`

and `std::accumulate`