Is Compile-Time Evaluation of `constexpr` Functions Guaranteed in C ?

Compile-Time String Length Computation: A Caveat for C Programmers

Determining the length of a string at compile time can be a valuable optimization for efficient string handling. In C , programmers may utilize the constexpr keyword to achieve this. However, a common misconception exists regarding the guaranteed evaluation of constexpr functions at compile time.

Consider the following code snippet:

<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"));
}</cstdio></code>

In this code, we define a constexpr function length that recursively computes the length of a C-style string. The assembly code generated shows that the results are calculated during compilation.

So, is compile-time evaluation of length guaranteed by the C standard?

No. While it is possible that many compilers will evaluate constexpr functions at compile time, the standard does not mandate it. According to the draft C standard section 5.19, constant expressions can be evaluated during translation (i.e., compile time), but this is merely a non-normative note.

To ensure compile-time evaluation, programmers can adopt two strategies:

1. Require compile-time evaluation: Use constexpr in a context that necessitates constant expressions, such as array bounds or case labels.
2. Initialize a constexpr variable: Assign the result of the constexpr function to a constexpr variable.

For example:

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

Conclusion:

While constexpr functions offer the potential for compile-time computation, programmers should be aware of the limitations of their guaranteed evaluation. By adhering to the aforementioned strategies, developers can harness the power of constexpr to optimize string-handling tasks at compile time.

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