Static constant string (class member)

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.

In C++11 you can do now:

class A {
 private:
  static constexpr const char* STRING = "some useful string constant";
};

Inside class definitions you can only declare static members. They have to be defined outside of the class. For compile-time integral constants the standard makes the exception that you can "initialize" members. It's still not a definition, though. Taking the address would not work without definition, for example.

I'd like to mention that I don't see the benefit of using std::string over const char[] for constants. std::string is nice and all but it requires dynamic initialization. So, if you write something like

const std::string foo = "hello";

at namespace scope the constructor of foo will be run right before execution of main starts and this constructor will create a copy of the constant "hello" in the heap memory. Unless you really need RECTANGLE to be a std::string you could just as well write

// class definition with incomplete static member could be in a header file
class A {
    static const char RECTANGLE[];
};

// this needs to be placed in a single translation unit only
const char A::RECTANGLE[] = "rectangle";

There! No heap allocation, no copying, no dynamic initialization.

Cheers, s.

In C++ 17 you can use inline variables:

class A {
 private:
  static inline const std::string my_string = "some useful string constant";
};

Note that this is different from abyss.7's answer: This one defines an actual std::string object, not a const char*

Static constant string (class member)

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