When Should I Use `const` vs. `constexpr` for Variables?

const vs constexpr on Variables

At first glance, it may seem that the following definitions are equivalent:

However, there is a subtle but important difference.

Compile-Time and Run-Time Constants

Variables declared as const can be initialized either at compile time or run time. Variables declared as constexpr must be initialized at compile time.

Therefore, PI1 is a run-time constant, while PI2 is a compile-time constant. This distinction matters because only compile-time constants can be used in contexts that require a known value at compile time, such as array sizes and template parameters.

Usage Comparison

The following examples illustrate the difference:

Выбор

The choice between const and constexpr depends on your specific requirements. Use constexpr if you need a compile-time constant, such as for array sizes or template parameters. Use const if you need a constant that can be initialized at run time, such as for user-entered data.

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