Compile-Time Evaluation

Exceptions in constexpr and consteval

How do I handle exceptions in constexpr and consteval functions?

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Handling exceptions in constexpr and consteval functions is a bit tricky because these functions are designed for compile-time evaluation. Let's explore the rules and best practices:

Basic Rules

  • Prior to C++20, constexpr functions were not allowed to throw exceptions.
  • In C++20 and later, constexpr functions can throw exceptions, but only during runtime execution.
  • consteval functions (introduced in C++20) cannot throw exceptions at all.

Exception Handling in C++20 constexpr Functions

In C++20, you can use try-catch blocks in constexpr functions, but they will only have an effect during runtime execution:

#include <iostream>
#include <stdexcept>

constexpr int divide(int a, int b) {
  if (b == 0) {
    throw std::runtime_error("Division by zero");
  }
  return a / b;
}

int main() {
  constexpr int result1 = divide(10, 2);
  std::cout << "Result1: " << result1 << '\n';

  try {
    // Runtime error that will be caught:
    int result2 = divide(10, 0);
    std::cout << "Result2: " << result2 << '\n';
  } catch (const std::exception& e) {
    std::cout << "Caught exception: "
      << e.what() << '\n';
  }

  // This would cause a compile-time error:
  // constexpr int result3 = divide(10, 0);
}
Result1: 5
Caught exception: Division by zero

Compile-Time Error Handling

For true compile-time error handling, you should use techniques that result in compilation failures rather than runtime exceptions:

#include <iostream>
#include <type_traits>

template <int A, int B>
struct SafeDivide {
  static_assert(B != 0, "Division by zero");
  static constexpr int value = A / B;
};

int main() {
  constexpr int result1 = SafeDivide<10, 2>::value;
  std::cout << "Result1: " << result1 << '\n';

  // This would cause a compile-time error:
  // constexpr int result2 = SafeDivide<10, 0>;
}
Result1: 5

Using std::is_constant_evaluated()

You can use std::is_constant_evaluated() to have different behavior at compile-time and runtime:

#include <iostream>
#include <stdexcept>
#include <type_traits>

constexpr int safeDivide(int a, int b) {
  if (std::is_constant_evaluated()) {
    // Avoid exception at compile-time
    return b != 0 ? a / b : 0;
  } else {
    if (b == 0) {
      throw std::runtime_error("Division by zero");
    }
    return a / b;
  }
}

int main() {
  constexpr int result1 = safeDivide(10, 2);
  std::cout << "Result1: " << result1 << '\n';

  // This is 0, not an error
  constexpr int result2 = safeDivide(10, 0);
  std::cout << "Result2: " << result2 << '\n';

  try {
    // This throws at runtime
    int result3 = safeDivide(10, 0);
    std::cout << "Result3: " << result3 << '\n';
  } catch (const std::exception& e) {
    std::cout << "Caught exception: " << e.what();
  }
}
Result1: 5
Result2: 0
Caught exception: Division by zero

consteval Functions

Remember, consteval functions must be evaluated at compile-time and cannot throw exceptions:

consteval int alwaysCompileTime(int x) {
  // This function can't use try-catch
  // or throw exceptions
  return x * 2;
}

In summary, while C++20 allows exceptions in constexpr functions during runtime, it's often better to design your compile-time code to avoid exceptions altogether.

Use techniques like static assertions, SFINAE, or compile-time if-constexpr statements to handle errors at compile-time. For consteval functions, focus on designs that don't require exception handling.

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

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