When Are Constexpr Functions Evaluated: Compile Time or Runtime?

Evaluating constexpr Functions at Compile Time

A constexpr function can be evaluated at compile time or during runtime. The compiler determines the evaluation time based on specific criteria.

Criteria for Compile-Time Evaluation:

A constexpr function will be evaluated at compile time if:

• All its arguments are constant expressions.
• The result is used in a constant expression.

Constant expressions include literals, non-type template arguments, enum element declarations, and other constexpr variables.

Runtime Evaluation:

If any of the function's arguments or the result is not a constant expression, or if the function is called at runtime, it will be evaluated at runtime.

Implications and Pitfalls:

The dynamic behavior of constexpr functions can have some implications. For example, a compiler may treat a constexpr function as a regular function even if it could be evaluated at compile time.

A common pitfall is using non-constexpr variables or expressions as arguments to a constexpr function. This can result in the function being evaluated at runtime even if the arguments and result are otherwise constant.

To ensure compile-time evaluation, carefully verify that all arguments and the result of the constexpr function are constant expressions. If non-constant expressions are used, consider using a non-constexpr function instead.

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