`const` vs. `constexpr` in C 11: When to Use Which for Variables?

const vs constexpr on Variables: Determining the Difference and Preferred Style in C 11

In C programming, the usage of const and constexpr when defining variables can lead to confusion regarding their differences. To clarify this, let's consider the following code samples:

const     double PI = 3.141592653589793;
constexpr double PI2 = 3.141592653589793;

1. Runtime vs Compile-Time Initialization:

const variables can be initialized either at compile time or runtime. However, constexpr variables must be initialized at compile time only. This restriction ensures that the value of the variable is known and unchangeable throughout the program's execution.

2. Use in Compile-Time Contexts:

constexpr variables have the advantage of being usable in contexts that require compile-time constants. This includes situations such as array dimensions, case label values, and as static constexpr member variables. In contrast, const variables cannot be used in these contexts because their values may not be known until runtime.

For example:

// Valid: compile-time constant
constexpr double PI3 = PI2;

// Invalid: PI1 is not a compile-time constant
const double PI3 = PI1;

Preferred Style in C 11:

The choice between const and constexpr depends on the specific requirements of the application. If you need a variable that cannot be modified and can be initialized at either compile time or runtime, const is appropriate. On the other hand, if you require a compile-time constant that can be used in contexts like array dimensions, constexpr is preferred.

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