How do constexpr functions, like const members, protect client code from unexpected dependency changes?

Why Const Marks Enhance the Benefits of Constexpr Functions

C 11 introduced the constexpr specifier, enabling functions to be utilized in constant expressions. Despite imposing strict requirements on function behavior, it presents a crucial benefit.

Preventing Unexpected Dependency

Consider a scenario where a library function initially returns a constant, as illustrated in awesome_lib.hpp:

inline int f() { return 4; }

In the absence of constexpr, client code could use this function for purposes requiring compile-time constants, such as template arguments or array dimensions.

#include <awesome_lib.hpp>
#include <array>

std::array<int f>;   // needs CT template arg
int my_c_array[f()];             // needs CT array dimension</int></array></awesome_lib.hpp>

However, if the implementation of f() changed to retrieve its value from a configuration file, this client code would break. This change would effectively alter the usage of the interface without the client coder's knowledge.

By marking the function as constexpr, the programmer acknowledges that client code can reasonably expect its constant nature. This ensures that the library function's usage is not compromised by implementation changes. The compiler enforces this expectation, guaranteeing that client code does not depend on non-constant functions.

Comparison with Member Function const Specifier

Similar to the const specifier for member functions, the constexpr specifier prevents client code from relying on features that may not be available in the future. However, they differ in their enforcement mechanisms.

The const specifier ensures that other member functions within a constant member function are also constant, whereas constexpr does not enforce compile-time constant results for all its uses. This flexibility allows for hybrid functions that can return compile-time results when possible but revert to runtime calculations under non-constexpr contexts.

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