Constant Expressions: constexpr

Constant expressions #

With constexpr, we can define an expression that can be evaluated at compile time. constexpr can be used for variables, functions, and user-defined types. An expression that is evaluated at compile time has a lot of advantages.

A constant expression:

• can be evaluated at compile time
• gives the compiler deep insight into the code
• is implicitly thread-safe
• can be constructed in the read-only memory (ROM-able)

constexpr - variables and objects #

If we declare a variable as constexpr, the compiler will evaluate it at compile time. This holds true not only for built-in types but also for instantiations of user-defined types. There are a few serious restrictions for objects in order to evaluate them at compile time.

To make our lives easier, we will use types built into the C++ library like bool, char, int, and double. We will call the remaining data types user-defined data types. These are all std::string types. User-defined types typically hold built-in types.

Variables #

By using the keyword, constexpr, a variable can be made a constant expression.

constexpr double myDouble= 5.2;

Therefore, we can use the variable in contexts that require a constant expression, e.g., defining the size of an array. This has to be done at compile time.

For the declaration of a constexpr variable, we have to keep a few rules in mind.

The variable: ...