Constant Expressions

With the keyword constexpr, we 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 many advantages. A constant expression does the following:

• It can be evaluated at compile time.
• It gives the compiler deep insight into the code.
• It is implicitly thread-safe.
• It 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 them at compile time. This holds true both for built-in types and for instantiations of user-defined types. There are a few restrictions for objects in order to evaluate them at compile time.

To make this process easier, we will use built-in types like bool, char, int, and double. We call the remaining data types as user-defined data types, for example, std::string, types from the C++ library, and user-defined data types. User-defined types typically hold built-in types.

Variables

By using the keyword constexpr, the variable becomes a constant expression.

constexpr double myDouble=