Static constant string (class member)

Question

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.

Answer 1

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.

Answer 2

In C++11 you can do now:

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