Re: [PATCH 1/4] Add history graph API

On Sun, 6 Apr 2008, Johannes Schindelin wrote:

AFAICT you do not even need them then.  Using "struct strbuf *" without 
ever declaring struct strbuf before that is perfectly valid.

In traditional C, and inside structure declarations etc, yes.

In modern C, in other contexts, no.

Modern C considers a function declaration to be its own scope (it's the 
scope of the function definition, which in a declaration is obviously 
just the declaration). So if you use a "struct xyzzy *" in a function 
declaration, it will be a *different* "struct xyzzy *" from one declared 
later.

Try to compile something like this:

	int fn(struct xyzzy *);
	int fn(struct xyzzy *);

with a modern C compiler, and it will actually say something along the 
lines of "conflicting types for ‘fn’", because while the two declarations 
look identical, they actually have two different (private) declarations of 
"struct xyzzy" going on.

But to make it even more interesting, you don't actually need a full 
declaration of "struct xyzzy" to make the compiler happy, you only need an 
implicit one ahead of time. You can do that with the incomplete 
declaration, of course (like the --graph patch did), ie just a simple

	struct xyzzy;

before those declarations is sufficient, but so is the implicit 
declaration of just using the pointer to it in some non-private scope, ie 
it's equally valid to do

	struct foobar {
		struct xyzzy *ptr;
	};

and this will already be enough to declare "struct xyzzy" in scope for the 
function declarations afterwards.

Is this illogical? Somewhat. Why is it a private scope in a function 
declaration but not in a struct declaration? Why isn't the function scope 
limited to the stuff *inside* the function? Somebody probably knows, but 
for the rest of us the answer is just "that's how it is, deal with it".

			Linus