Why are `constexpr` function declarations essential in C ?

Why the Need for constexpr Function Declarations?

In C 11, the constexpr specifier allows functions to be used in constant expressions. However, it comes with stringent requirements, restricting functions to encapsulating a single subexpression. Beyond revealing intent, it raises the question: Why is this keyword necessary?

Preventing Unwanted Dependency

The constexpr keyword helps prevent client code from relying on mutable aspects of a function that should remain constant. Consider a function f() that currently returns a fixed value:

inline int f() { return 4; }

Without constexpr, client code might use f() as a compile-time constant, such as in template arguments or array dimensions. However, if f() were to later become a non-constant function, it could break client code without warning.

Compiler Enforcement

constexpr forces programmers to explicitly indicate functions that are suitable for constant expressions, ensuring that client code can rely on them as such. The compiler then enforces this declaration, prohibiting the use of non-constant functions in constant expressions. This provides a stronger guarantee than documentation alone.

Comparison with Non-const Member Functions

Much like the use of const with member functions, constexpr prevents unwanted usage. However, unlike const, constexpr does not enforce a compile-time constant result. This is a practical limitation of compilers, allowing functions to return runtime results for runtime-known arguments while providing compile-time results when possible.

Conclusion

The constexpr keyword is essential for guaranteeing that functions can be used as compile-time constants. By explicitly declaring functions as constexpr, programmers can prevent client code from depending on mutable aspects and ensure that the function's constant nature is enforced by the compiler.

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