[itistoday]

In the never ending struggle to expand my knowledge of various programming languages, I have finally decided to learn C++. I am really familiar with Java and the way it does things, so I have quick question about the C++ scope operator (::). Why would you ever want to create a constructor or a method outside of the actual class declaration when you can create it inside of it? It just seems to add a layer of confusion...

Thanks!

[smeger]

Let's say that you've got two namespaces, and both of them contain a class with the same name. Without the scope operator, you have no way of specifying with one you want to use.

Let's say that your class contains a method named "strlen". If you want to call the standard library function named "strlen" from within a method of that class, you'd have no way of doing it without the scope operator.

And so on.

[parallax]

In general, you want to keep the interface and implementation of a class separate. Having a class declaration with method signatures and a bunch of comments explaining how to use the class enforces the idea that the client shouldn't really care about the details of implementation.

And another, more practical, reason. You generally keep class declarations in the header file, which is included by multiple other files. Not only would #including the implementation for that class in /each other file/ increase bloat, but it would be incorrect as the class definitions would not be linked to each other and would thus introduce namespace conflicts.

[itistoday]

Originally Posted by smeger

Let's say that you've got two namespaces, and both of them contain a class with the same name. Without the scope operator, you have no way of specifying with one you want to use.

Let's say that your class contains a method named "strlen". If you want to call the standard library function named "strlen" from within a method of that class, you'd have no way of doing it without the scope operator.

And so on.

Yes, I see that aspect of it, but I still don't understand why you can also use it to implement method functionality outside the class declaration.... In Java you always implement the class within the class... This is just weird. I see that the scope operator has its uses, but this particular one just doesn't seem to have a point.

[itistoday]

Originally Posted by parallax

In general, you want to keep the interface and implementation of a class separate. Having a class declaration with method signatures and a bunch of comments explaining how to use the class enforces the idea that the client shouldn't really care about the details of implementation.

And another, more practical, reason. You generally keep class declarations in the header file, which is included by multiple other files. Not only would #including the implementation for that class in /each other file/ increase bloat, but it would be incorrect as the class definitions would not be linked to each other and would thus introduce namespace conflicts.

Ahh! I see, now that makes sense. Thanks!

[itistoday]

Wait, one more question parallax, what do you mean when you say they "would not be linked to each other and thus introduce namespace conflicts"? (Sorry I come from a happy-go-lucky Java world)

[parallax]

This applies to functions as well, and it's a bit easier to imagine this using functions.

So if I have one file that contains two functions,

void foo() {
cout << "bar" << endl;
}

void foo() {
cout << "bar" << endl;
}

The compiler is going to complain because we obviously can't have two functions named "foo," even if they do the same thing.

So say we have two source files that we want to compile together. Imagine that a common header file contains the function,

void foo() {
cout << "bar" << endl;
}

and this is included by both source files. "#include" is actually just a directive that means "dump the contents of the file at this point", and so both source files will now contain definitions for foo. And of course, this is bad! We avoid this by only including the prototype of the function (void foo() in the header file and then leave the actual definition for a source file.

The same applies to classes and their methods.

[itistoday]

Ah, ok I think I get it now, thanks a bunch!