What's the Difference Between `const` and `constexpr` Variables in C ?

Difference Between const and constexpr Variables

In C , the keywords const and constexpr are both used to define constants. However, there are subtle differences between the two in terms of how they are applied and their impact on code behavior.

Definitions:

• const: Declares a constant variable that cannot be modified after initialization. It can be initialized at compile time or runtime.
• constexpr: Declares a compile-time constant variable that must be initialized with a constant expression evaluated at compile time.

Usage:

• PI1 (const): May be initialized at compile time or runtime, and cannot be modified thereafter.
• PI2 (constexpr): Must be initialized at compile time with a constant expression, and cannot be modified. It can be used in contexts that require compile-time constants.

Examples:

Consider the following definitions:

const     double PI1 = 3.141592653589793;
constexpr double PI2 = 3.141592653589793;

• PI1: Can be used in contexts where a const variable is required, but cannot be used in contexts that require a compile-time constant.
• PI2: Can be used in contexts where both const and compile-time constants are required.

Preferred Usage in C 11:

The preferred style in C 11 depends on the specific requirements of the application:

• If you need a constant variable that can be initialized at runtime and does not need to be used in contexts that require compile-time constants, use const.
• If you need a compile-time constant that must be initialized with a constant expression and can be used in contexts that require compile-time constants, use constexpr.

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