C++ – Static constant string (class member)

c++classconstantsstaticstring

I'd like to have a private static constant for a class (in this case a shape-factory).

I'd like to have something of the sort.

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

Unfortunately I get all sorts of error from the C++ (g++) compiler, such as:

ISO C++ forbids initialization of
member ‘RECTANGLE’

invalid in-class initialization of static data member of non-integral type ‘std::string’

error: making ‘RECTANGLE’ static

This tells me that this sort of member design is not compliant with the standard. How do you have a private literal constant (or perhaps public) without having to use a #define directive (I want to avoid the uglyness of data globality!)

Any help is appreciated.

Best Solution

You have to define your static member outside the class definition and provide the initializer there.

First

// In a header file (if it is in a header file in your case)
class A {   
private:      
  static const string RECTANGLE;
};

and then

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

The syntax you were originally trying to use (initializer inside class definition) is only allowed with integral and enum types.


Starting from C++17 you have another option, which is quite similar to your original declaration: inline variables

// In a header file (if it is in a header file in your case)
class A {   
private:      
  inline static const string RECTANGLE = "rectangle";
};

No additional definition is needed.

Starting from C++20 instead of const you can declare it constexpr in this variant. Explicit inline would no longer be necessary, since constexpr implies inline.