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constexpr-exceptions.md

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execute false

What It Does

With this, exceptions may be thrown and caught during constant expression evaluation. If an exception is not caught within the constant expression context, the result is a compile-time error. If caught within the evaluation context, evaluation continues.

Why It Matters

constexpr functions previously required distinct error-handling mechanisms for compile-time versus runtime evaluation contexts. Exception support in constant expressions unifies error handling, permitting identical code to operate in both compile-time and runtime contexts.

Example

#include <stdexcept>

constexpr int safe_divide(int a, int b) {
    if (b == 0) {
        throw std::invalid_argument("division by zero");
    }

    return a / b;
}

constexpr int safe_compute(int x, int y) {
    try {
        return safe_divide(x, y);
    } catch (const std::invalid_argument&) {
        return 0;  // fallback for division by zero
    }
}

// Works at compile time
static_assert(safe_compute(10, 2) == 5);
static_assert(safe_compute(10, 0) == 0);  // caught and handled

// This would fail to compile (uncaught exception):
// constexpr int bad = safe_divide(1, 0);