How to Define a Private Static Constant `std::string` in C ?

Defining a Static Data Member of Type const std::string

In C , defining a private static constant for a class can be challenging when using the standard const std::string type. The error messages encountered indicate that the method being used is not compliant with the ISO C standard.

To define a private literal constant without using a #define directive, consider the following two approaches:

Inline Variables (C 17 and later)

Since C 17, inline variables can be used to declare static data members with constant initializers. This is done by adding the inline keyword before the static declaration within the class definition:

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

Definition Outside Class Definition (Prior to C 17)

Prior to C 17, static members must be defined outside the class definition and initialized separately. The static declaration within the class definition is made without an initializer:

class A {
private:
  static const std::string RECTANGLE;
};

// In an implementation file
const std::string A::RECTANGLE = "rectangle";

Note that the syntax of directly initializing static members with non-integral types within class definitions is only allowed with integral and enum types.

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