Does `constexpr` Implicitly Make a Function `inline` in C ?

Constexpr Function Inline Implementation

Question:

During C code compilation, does the constexpr specifier implicitly trigger the inline specifier for a function, forcing the compiler to inline the function if a non-constant argument is passed to it?

Answer:

Yes. According to the C 11 standard ([dcl.constexpr], §7.1.5/2), "constexpr functions and constexpr constructors are implicitly inline (7.1.2). "

Implications:

While the inline specifier minimally impacts the likelihood of function inlining, its presence, when combined with constexpr, has the following implications:

• The compiler must follow the same rules for constexpr functions as for inline functions, ensuring that they both adhere to the one definition rule.
• In earlier C 11 versions, constexpr functions were restricted to be simple enough for easy inline expansion. However, recent standard revisions have relaxed these restrictions, allowing for the use of constexpr with larger, more complex functions.

Impact on Code Optimization:

While the constexpr specifier does not forcibly inline functions, it suggests that they are suitable for inlining due to their expected simplicity and limited complexity. This leads compilers to prioritize optimizing such functions for inline expansion, although the actual decision remains at the compiler's discretion.

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