C++ 14 – relaxations for constexpr

You can use constexpr to define a constant expression since C++ 11. The expression is evaluated at compile-time and hence boosts performance.

C++ 14 introduced more features in terms of relaxations for constexpr. Let us see them one by one.

constexpr allows control structures

In C++ 11, only single expressions were allowed in constexpr, but in C++ 14, we can use if, switch, and loops such as for and while. Below is a table showing the difference across factorial calculation code across C++ 11 and C++ 14.

//C++ 11 code snippet
constexpr int factorial(int n) {
    return (n <= 1) ? 1 : (n * factorial(n - 1));
}

//C++ 14 code snippet
constexpr int factorial(int n) {
    int result = 1;
    for (int i = 1; i <= n; ++i) {
        result *= i;
    }
    return result;
}

constexpr allows mutable variables

From C++ 14 onwards, non-const local variables are allowed inside constexpr. The only rule is that the values of the variables should not change after initialization.

//C++ 11 code snippet
// Not allowed: mutable variable inside constexpr function
constexpr int func(int n) {
    int x = n; // Error
    return x;
}

//C++ 14 code snippet
// Allowed: x should not change values after initialization
constexpr int func(int n) {
    int x = n; // Allowed
    return x;
}

Multiple return statements are allowed

In C++ 11, only a single return statement was allowed. It mainly was nested ternary conditional statements. From C++ 14 onwards, nested if and else statements are allowed inside constexpr. See below the differences in terms of code snippets.

// C++ 11 code snippet
constexpr int func(int n) {
    return (n > 0) ? 1 : ((n < 0) ? -1 : 0);
}

// C++ 14 code snippet
constexpr int func(int n) {
    if (n > 0) {
        return 1;
    } else if (n < 0) {
        return -1;
    } else {
        return 0;
    }
}