Thread: Using classes in another .cpp file

http://faq.cprogramming.com/cgi-bin/...&id=1043284392

But using this method, is it possible to do that with an anonymous class? How would I reference it using the scope operator, if possible?

Without a name, how would one would refer to the class? Just give the class a name.

Originally Posted by lastdestined

Define your class in head file.

I would declare it in your header and define it in your source file(s).

No, lastdestined's terminology is correct.

This is a class declaration:

Code:

class Foo;

This is a class definition:

Code:

class Foo : public Bar
{
  int m_i;

public:
  void func();
};

What remains is the class implementation, i.e. the definition of its member functions and static member variables. That's the part that should go into a source file.

On what grounds do you base this on?
It's hard to exactly say what's what, but it was discussed sometime before: http://cboard.cprogramming.com/showt...ht=declaration

declaration - an introduction of a name into a scope. The type of the name must be specified. If the declaration also specifies the entity to which the name refers, the declaration is also a definition.
definition - a declaration that specifies the entity to which the declared name refers.

Declaration is the introduction of name into a scope, but it also must have a type. So does that means that your "definition" is also a declaration since it actually defines the type or is it a definition since it specifies the entity to which the declared name applies to?

Um, my knowledge?

But also the C++ standard, section 9. In particular, 9/2:

A class-specifier is commonly referred to as a class definition.

The standard subsequently uses "class definition" the same way as I do. Class-specifier is a reference to the grammar in 9/1:

Code:

          class-specifier:
                  class-head { member-specificationopt }
          class-head:
                  class-key identifieropt base-clauseopt
                  class-key nested-name-specifier identifier base-clauseopt
          class-key:
                  class
                  struct
                  union

As for class declaration, see 9.1/2:

A declaration consisting solely of class-key identifier ; is either a redeclaration of the name in the current scope or a forward declaration of the identifier as a class name.

The standard goes on to refer to this construct as a "class declaration", e.g. in 9.1/3.

So you see, I'm backed by good authority.

Note that the cplusplus.com tutorial, on the other hand, refers to class definitions as class declarations. I consider this an error. Declarations introduce the name to the compiler, nothing else. It's that way with functions and with variables. Why should it be different with classes?

Originally Posted by CornedBee

Um, my knowledge?

Yes, but I was wondering where you got that knowledge from

Code:

class MyClass;

Class declaration.

Code:

class MyClass { /* Stuff here */ };

Class definition.

Code:

void MyClass::foo() { /* Function body here */ }

Class implementation.

I believe this is what the standard says, then?

The first two; the third I don't think is given a name by the standard.

Avoid global variables if you can.
Anyway, to solve this problem, define the variable in one .cpp file, then add "extern type yourvariable" to the header and include it and it will work fine.
The problem is that each .cpp file is treated separately by the compiler but the linker will combine them all into one big file and if you define your variable in a header, all .cpp files which includes this header gets this variable in the .cpp file and thus the linker will find more than one global variable with the same name, which is bad, since it cannot determine which variable your program's separate parts are using, hence it is also not allowed.

Code:

// .h file
extern int giDum1;

// .cpp file
int giDum1;