Defining Static Data Members of Type const std::string

In C , defining a private static const member of type std::string within a class using in-class initialization, as shown below, is not compliant with the C standard:

class A {
   private:
      static const string RECTANGLE = "rectangle";
}

However, C provides alternative approaches to achieve this functionality.

C 17 Inline Variables

Since C 17, you can use inline variables for this purpose. An inline variable is a C 17 feature that allows for the declaration of a static variable directly within the class definition, with the inline keyword. For example:

// In a header file (if necessary)
class A {
private:
  inline static const string RECTANGLE = "rectangle";
};

Pre-C 17 Approach

Prior to C 17, you must define the static member outside the class definition and provide the initializer there. Here's an example:

// In a header file (if necessary)
class A {
private:
  static const string RECTANGLE;
};

// In one of the implementation files
const string A::RECTANGLE = "rectangle";

Note: The initialization syntax you attempted (within the class definition) is only supported for integral and enum types.