Why is the `constexpr` Keyword Necessary for Function Declarations in C 11?

The Rationale Behind Constexpr Function Declarations

C 11 introduces the constexpr specifier for functions that allows their usage in constant expressions like template arguments. However, it raises the question of why this keyword is required and what it offers.

Revealing Intent and Preventing Over-Reliance

Requiring the constexpr keyword aids in showcasing the designer's intent for a function's usage. It signifies that the function aims to encapsulate a constant expression. However, this semantic constraint isn't always validated, leaving it to the programmer to ensure:

• The function is utilized in a constant expression.
• Documentation is provided to define valid parameter values in this context.

Guaranteeing Client Code Integrity

By flagging functions as constexpr, library authors convey that client code can rely on them in such contexts. This prevents clients from inadvertently using the function in ways that would break compilation should the function's implementation change.

For example, without constexpr, a function returning a constant may be assumed immutable. However, a later implementation that retrieves its value from a configuration file could disrupt clients depending on its constancy. Constexpr ensures that client code adheres to the function's intended usage and prevents unforeseen breakages.

Avoiding Unwanted Dependencies

Constexpr also helps prevent client code from depending on non-constexpr functions. As with non-const member functions, constexpr ensures that client code doesn't introduce unexpected dependencies or usages.

Since the compiler doesn't enforce compile-time constant results with constexpr, the onus is on the programmer to design functions that meet this requirement.

Comparison to Non-const Member Functions

Analogous to non-const member functions, constexpr:

• Prevents inappropriate usage (e.g., calling member functions on const objects or using values in template parameters that may change at runtime).
• Differs in that it doesn't enforce compile-time constant results due to compiler limitations.

In essence, constexpr is not strictly optional as it serves the purpose of clarifying intent, preventing misuse, and safeguarding client code from unintended dependencies.

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