Specifying struct in function signature

Question

Say I have

struct mystruct
{
};

Is there a difference between:

void foo(struct mystruct x){}

and

void foo(mystruct x){}

?

score 11 · Answer 1 · answered Sep 08 '11 at 13:38

11

In C the latter isn't valid.

However in C++ they're almost the same: The first one would be valid if you haven't yet declared your struct at all, it would treat it as a forward declaration of the parameter all in one.

answered Sep 08 '11 at 13:38

Mark B

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1

Ahh, the implicit forward declaration is a nice subtlety! +1 – Kerrek SB Sep 08 '11 at 13:39

Steve Jessop · Accepted Answer · 2011-09-08T13:50:14.003

7

Not in the code you've written. The only difference I know of between using a defined class name with and without struct is the following:

struct mystruct
{
};

void mystruct() {}

void foo(struct mystruct x){} // compiles
void foo(mystruct x){} // doesn't - for compatibility with C "mystruct" means the function

So, don't define a function with the same name as a class.

edited Sep 08 '11 at 13:50

answered Sep 08 '11 at 13:41

Steve Jessop

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Insidious - since when are you allowed to have a function of the same name as a type? – Kerrek SB Sep 08 '11 at 13:43
@Kerrek: same name as a struct, since K&R C. structs occupy a different namespace from typedefs and functions. IIRC the way C++ introduces `mystruct` for use without the `struct` prefix isn't actually by typedefing it, there's just a name resolution rule somewhere that says if you can't find a name in scope, check for a struct in that scope before moving on to the next scope. – Steve Jessop Sep 08 '11 at 13:43
Nice, didn't know you could do that. Thanks! – Luchian Grigore Sep 08 '11 at 13:45

score 3 · Answer 3 · answered Sep 08 '11 at 13:37

No difference. The latter is the correct C++ syntax; the former is permissible as a legacy variant for recovering C programmers.

Note that struct and class are essentially the same and both define a class, so there's no special treatment for C-style POD structs in C++.

_{[Edit: Apparently there is a small difference, see Mark B's excellent answer.]}

Specifying struct in function signature

3 Answers3