Class definition vs class declaration

[soft breach]

I’m confused on this terminology. I understand function definition vs declaration but it seems like a class declaration is a definition so what is the difference? Is a class declaration just the class without any method bodies?

[tropic hull]

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[turbid jungle]

;compile

class C;
int main(){}

[summer falcon]

Compilation successful

j.a.r.v.i.s8285 | 24ms | c++ | x86-64 gcc 15.2 | godbolt.org

[turbid jungle]

there is declaration but no definition

Is a class declaration just the class without any method bodies?
so no

[soft breach]

Okay so it’s just a forward declaration then

One more thing

So in classes when you have methods or friend functions you don’t need to forward declare them outside the class before implementing their bodies else where. So does this mean that the declaration is inside the class

[gleaming temple]

this terminology isn't exactly used consistently by different sources/people
if you come across a source that uses specifically declaration and definition, then yes a declaration would be a "forward declaration", just that "forward declaration" isn't really the standard term iirc, it's just widespread and I guess some people like it better and find it less ambiguous/overloaded

[gleaming temple]

a friend declaration is a declaration

and yes member declarations are also declarations

[neon horizon]

Class declarations just Informs the compiler it exists

[soft breach]

So if you put it in a class you don’t need another declaration which I’ve seen. So I should just know this works as if it’s already declared outside the class?

Ie. Friend functions

[gleaming temple]

It's complicated

[soft breach]

So it would be useless to know why it works? Just know that it works? Lol

I was just confused at first when doing friend functions because I realize putting it inside the class as a friend function can double as a declaration

[gleaming temple]

I don't understand how you reached that conclusion

[soft breach]

Wdym? I tried it out

I put a friend function inside a class definition and define the function else where and it worked fine

[gleaming temple]

I don't even fully understand that sentence, but whether or not you need a different declaration than the friend declaration depends on how you intend to use the function in the first place

[gleaming temple]

This is a reply to a message where I say
"I don't understand how you reached the conclusion that it's useless to know why it works"

[soft breach]

If I have a header with MyClass Definrion and inside the MyClass defitniom I put friend void foo();

I can define foo in a separate file fine. I wouldn’t need to also do void foo() in the header

[gleaming temple]

Implied there's no relation to "trying things out", since this is all about knowledge being useful or useless

[soft breach]

Im confused on what’s going on

[soft breach]

I was just asking why this works how it does and you said it’s complicated and I asked if whether or not It’s just good enough to know that it works then (if it’s complicated) or should I know why it works like this.

[gleaming temple]

You said "it's useless to know why it works", I've asked why you would say that

On a different topic, there's the whole "when or why would you declare a function" which I haven't addressed since the moment I asked about why you think it's useless to know stuff

[gleaming temple]

Part of the reason I said "it's complicated" is because having just the friend declaration for a function may lead to surprising results as to what function exactly is being declared

In the first place, a declaration for a function just informs that this name/identifier names a function with a specific type

The issue is then generally about what the exact name is, and how it can be reached/seen

This can lead to somewhat surprising results when you have namespaces

Specifically there are restrictions if the class is in some namespace but the function should be in another namespace

This is how cppreference puts it

A name first declared in a friend declaration within a class or class template X becomes a member of the innermost enclosing namespace of X, but is not visible for lookup (except argument-dependent lookup that considers X) unless a matching declaration at namespace scope is provided

Anyway the general point I'm trying to make, is that having "only" a friend declaration is not strictly equivalent to having a non-friend declaration

[gleaming temple]

Also this isn't clear to me what you've tried to test
There has never been a need to declare a function before defining it, so I assume you did something else in your test, to actually test something

[soft breach]

Misunderstanding then, I asked a question I didn’t state that it was lol

[gleaming temple]

But then there is the question of how you dealt with function name lookup, unless you were in a situation where ADL applied

And this is where it really gets "complicated"

For a beginner

Arguably if you want to really understand and get proficient with c++ there is a point where you'll need to look at ADL, imo

Or at least remember that "operator overloads" and other "common operation customisation point" should be members or friends of the type

Right, and if you generic code, the topic of ADL also comes up when you need to consider options to let your user provide custom behaviours

;compile

struct Foo 
{
  friend void fun();
};

int main()
{
  fun();
}

[summer falcon]

Compiler Output

<source>: In function 'int main()':
<source>:8:3: error: 'fun' was not declared in this scope
    8 |   fun();
      |   ^~~
Build failed

slybach | c++ | x86-64 gcc 15.2 | godbolt.org

[gleaming temple]

One consequence of that, is that the above does not work

So even if a function should be a friend to get access to whatever private member of a class, you should consider "who" needs to call the function

;compile

struct Foo
{
  operator int() const { return 0; }

  friend void fun(int value) {}
};

int main()
{
  // ok
  fun(Foo{});

  // not ok
  fun(0);
}

[summer falcon]

Compiler Output

<source>: In function 'int main()':
<source>:14:3: error: 'fun' was not declared in this scope
   14 |   fun(0);
      |   ^~~
Build failed

slybach | c++ | x86-64 gcc 15.2 | godbolt.org

[gleaming temple]

Random example with ADL, which is where it gets "weird"

[fair pulsar]

Oh I love ADL just for that