Yes, you can use both `constexpr`

and `consteval`

with recursive functions. This is a powerful feature that allows you to perform complex compile-time computations. Let's explore this with someÂ examples:

`constexpr`

FunctionHere's a classic example of a recursive factorial function using `constexpr`

. This function will be evaluated at compile-time when used in a constantÂ expression.

```
#include <iostream>
constexpr int factorial(int n) {
if (n <= 1) return 1;
return n * factorial(n - 1);
}
int main() {
constexpr int result = factorial(5);
std::cout << "5! = " << result;
}
```

`5! = 120`

`consteval`

FunctionYou can also use `consteval`

to ensure the function is always evaluated atÂ compile-time:

```
#include <iostream>
consteval int fibonacci(int n) {
if (n <= 1) return n;
return fibonacci(n - 1) + fibonacci(n - 2);
}
int main() {
constexpr int result = fibonacci(10);
std::cout << "10th Fibonacci number: "
<< result << '\n';
}
```

`10th Fibonacci number: 55`

While you can use recursion in `constexpr`

and `consteval`

functions, be aware of compiler limitations. Most compilers have a limit on the depth of constexprÂ recursion:

This might result in a compiler error or warning about exceeding recursionÂ limits.

```
constexpr long long very_deep_recursion(int n) {
if (n == 0) return 0;
return 1 + very_deep_recursion(n - 1);
}
int main() {
// Might exceed limits
constexpr int result = very_deep_recursion(1000);
}
```

```
error: expression did not evaluate to a constant
note: failure was caused by evaluation exceeding call depth limit of 512
```

Here's a more practical example using recursion to compute a string hash atÂ compile-time:

This recursive function computes the FNV-1a hash of a string at compile-time, which could be useful for switch statements or other compile-time decisions based onÂ strings.

```
#include <iostream>
#include <string_view>
constexpr uint32_t fnv1a_32(
std::string_view sv, size_t idx = 0) {
constexpr uint32_t FNV_offset_basis = 2166136261U;
constexpr uint32_t FNV_prime = 16777619U;
return idx == sv.size() ? FNV_offset_basis
: (fnv1a_32(sv, idx + 1) ^ sv[idx]) * FNV_prime;
}
int main() {
constexpr auto hash = fnv1a_32("Hello, World!");
std::cout << "Hash of 'Hello, World!': " << hash;
}
```

`Hash of 'Hello, World!': 3644920852`

Remember, when using recursion in `constexpr`

or `consteval`

functions, ensure that there's a clear base case to prevent infinite recursion. Also, be mindful of the potential for long compilation times with complex recursiveÂ computations.

Despite these considerations, recursive `constexpr`

and `consteval`

functions are powerful tools for compile-time metaprogramming inÂ C++.

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

This Question is from the Lesson:### Compile-Time Evaluation

Learn how to implement functionality at compile-time using `constexpr`

and `consteval`