Why Enforce `constexpr` Functions in C : Just Intent or True Protection?

Constexpr Function Declarations: Beyond Mere Intent

C 11 introduced the constexpr specifier, allowing functions to participate in constant expressions. While it reinforces the intended use of these functions, the question arises: why enforce the marker at all, given the programmer still bears responsibility for ensuring the function's applicability in constant expressions?

Safeguarding Against Unintended Client Dependency

Enforcing the constexpr keyword serves a crucial purpose: safeguarding client code from unintentional dependencies. Without it, a client may assume a constant function remains constant, only to discover breakage upon changes to the implementation. By marking the function as constexpr, the compiler guarantees it can be safely employed in constant expressions, protecting client code from such pitfalls.

Example:

Consider a function f that originally returns a constant, but is later modified to return a value from a config file.

// Original implementation
inline int f() { return 4; }

In the absence of constexpr, client code could unknowingly use f as follows:

int my_array[f()]; // Non-constant array dimension

Upon changing f to retrieve its value from a config file, the client code would fail to compile due to the runtime nature of the function. However, with constexpr enforced, the compiler would prevent such misuse from the outset.

Addressing Concerns

Critics argue that constexpr may provide a false sense of security, as it merely checks syntactic constraints without guaranteeing the function's actual usability in constant expressions. While it is true that the programmer retains the burden of validation, the compiler's enforcement of constexpr remains valuable in ensuring the intended use is adhered to.

Comparison with Non-Const Member Functions

constexpr functions resemble non-const member functions in their role as safeguards. Both prevent client code from relying on assumptions about the function's behavior that may change in the future. However, constexpr differs by allowing for the compilation of non-constant results when the function is used in dynamic contexts.

Conclusion

constexpr declarations are vital in C for several reasons. They prevent erroneous client dependencies, align with the language's move away from preprocessor macros, and offer a reliable mechanism for distinguishing between const and non-const interfaces. While the compiler cannot automatically determine a function's const-ness, enforcing constexpr syntax provides a level of protection for client code and ensures consistent usage across the program.

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