Constexpr Functions or Constants: When Should I Use Which?

Constexpr Functions vs. Constants: When to Use Which

When encountering functions that consistently return the same value, such as "return 5," one may wonder why such a feature exists in C 11 and if it should replace declaring constants instead.

Constants vs. Constexpr Functions

Consider the following examples of handling a constant value:

#define MEANING_OF_LIFE 42
const int MeaningOfLife = 42;
constexpr int MeaningOfLife() { return 42; }

Traditionally, one would simply declare a constant value. However, constexpr functions offer certain advantages.

When Constexpr Functions are Useful

Constexpr functions are valuable when the returned value requires more complex calculations, such as:

constexpr int MeaningOfLife(int a, int b) { return a * b; }
const int meaningOfLife = MeaningOfLife(6, 7);

This allows for more readable code while still facilitating compile-time evaluation.

Compile-Time Calculations

Constexpr functions enable explicit compile-time calculation of constants, as seen in:

template<typename Type> constexpr Type max(Type a, Type b) { return a < b ? b : a; }

This allows for efficient handling of constant values.

Increased Readability

For functions like DegreesToRadians, using constexpr functions enhances readability, making it clearer that the value is being calculated at compile time:

const float oneeighty = DegreesToRadians(180.0f);

Conclusion

While declaring constants remains a valuable practice, constexpr functions offer advantages when working with more complex or readable constant values. They facilitate compile-time evaluation and enhance code maintainability. For more information on these topics, refer to http://en.cppreference.com/w/cpp/language/constexpr.

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