C++ classes use of double colon [duplicate]

Question

I am learning C++. Now I don't fully understand what this does

Some_Class::Some_Class {
    etc...
}

I would do some research for myself, but I'm not sure where to begin or what's it called. Help would be appreciated.

Answer 1

The :: resolves either a class or namespace.

For example

namespace test1 { int i = 0; }
cout << test1::i << endl;

or

class test2 { 
     public: 
     static int i = 0; 
 };
 // after in
cout << test2::i << endl;

You can also add this:

using namespace test1;
cout << i << endl;

Answer 2

There's no way to say what it is, since the "code" you posted is invalid and ambiguous.

• It could be a nested class definition made in out-of-class fashion. When you define nested classes, you can immediately define the inner class inside, as in

  class Some_Class {   // <- definition of the outer class
    ...
    class SomeClass {  // <- definition of the inner class
      ...
    };
    ...
  };
  

  Or, if you prefer, you can only declare the nested class inside, and move the actual definition outside

  class Some_Class {   // <- definition of the outer class
    ...
    class SomeClass;   // <- declaration of the inner class
    ...
  }; 
  
  class Some_Class::SomeClass { // <- definition of the inner class
    ...
  };
  

  However, for that it has to begin with class/struct, which is not present in what you posted.

• Or it could be a definition of member function SomeClass of class Some_Class.

  class Some_Class {
    ...
    void SomeClass(int i);          // <- declaration of member function
    ...
  };
  
  void Some_Class::SomeClass(int i) // <- definition of member function
  {
    ...
  }
  

  But for that it has to include return type and parameter list.

• Or it could be a definition of a static member with {}-enclosed initializer

  class Some_Class {
    ...
    static int SomeClass;
    ...
  };
  
  int Some_Class::SomeClass { 42 };
  

  But for that it has to include static member's type.

In other words, there's no way to say what it is you posted and what your question is really about.