Is the Evaluation of a Recursive String Length Function Guaranteed at Compile Time in C ?

Computing Length of a C String at Compile Time

In C , it is possible to perform computations on string literals during compilation. Consider the following code snippet:

<code class="c++">#include <cstdio>

int constexpr length(const char* str)
{
    return *str ? 1 + length(str + 1) : 0;
}

int main()
{
    printf("%d %d", length("abcd"), length("abcdefgh"));
}</code>

This code utilizes a recursive function to determine the length of a given string literal at compile time. By printing the results, we can confirm that the function correctly calculates the string lengths, i.e., 4 and 8. The resulting assembly code also demonstrates that these computations are performed during compilation.

However, the critical question remains: is the evaluation of this length function guaranteed to occur at compile time according to the C standard?

The answer is not as straightforward as one might assume. According to the draft C standard (section 5.19), constant expressions "can be evaluated during translation." However, this is merely a non-normative note, not a steadfast guarantee.

To ensure compile-time evaluation, we can utilize two strategies:

1. Utilize constant expressions: Certain expressions must be constant according to the standard, such as array bounds or case labels. Employing the length function in these contexts guarantees its compile-time evaluation.
2. Initialize a constexpr variable: Assigning the result of the length function to a constexpr variable compels the compiler to execute the function at compile time.

Therefore, while the standard does not explicitly mandate that the length function be evaluated at compile time, we can enforce such evaluation by utilizing it as a constant expression or initializing a constexpr variable. This practice opens up the possibility of performing sophisticated computations on string literals during compilation, such as calculating hashes or performing complex string manipulations.

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