Can constexpr Functions Guarantee Compile-Time String Length Computation?

Compile-Time String Length Computation: Is It Truly constexpr?

In an attempt to compute string length at compile time, a portion of code utilizes a recursive function (length) to achieve this. However, questions arise regarding the guaranteed evaluation of this function at compile time.

The standard does not explicitly mandate that constexpr functions be evaluated at compile time. Nevertheless, a non-normative note in the draft C standard section 5.19 suggests that constant expressions may undergo evaluation during translation.

To ensure compile-time evaluation, one can assign the result to a constexpr variable. As stated by Bjarne Stroustrup:

"In addition to be able to evaluate expressions at compile time, we want to be able to require expressions to be evaluated at compile time; constexpr in front of a variable definition does that (and implies const)."

For instance:

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

Moreover, Stroustrup outlines specific conditions for guaranteeing compile-time evaluation:

• Using the function where a constant expression is required, such as in an array bound.
• Initializing a constexpr variable with the function's result.

Therefore, while the mere declaration of a constexpr function does not guarantee compile-time evaluation, assigning its result to a constexpr variable or using it in an appropriate context ensures this.

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