How Can We Guarantee Compile-Time Evaluation of String Length?

Compile-Time Computation of String Length

To compute a string literal's length at compile time, the code snippet below utilizes a recursive function:

<code class="cpp">#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 function successfully computes the lengths as expected, as evidenced by the generated assembly code from clang, which shows the results being computed at compile time.

Standard Guarantee for Compile-Time Evaluation

However, it is crucial to note that the evaluation of constant expressions at compile time is not explicitly guaranteed by the standard. While the draft C standard section 5.19 does include a non-normative quote stating that constant expressions can be evaluated during translation, this does not provide a definitive guarantee.

Ensuring Compile-Time Evaluation

To ensure that a function is evaluated at compile time, Bjarne Stroustrup recommends assigning its result to a constexpr variable. This can be seen in the following example:

<code class="cpp">constexpr int len1 = length("abcd");</code>

Additionally, Bjarne Stroustrup outlines two specific cases where compile-time evaluation is guaranteed:

1. When a function is used where a constant expression is required, such as an array bound.
2. When a function is used to initialize a constexpr.

Therefore, for reliable compile-time evaluation, it is advisable to follow either of these two approaches.

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