class/struct question

**darksaidin** · 08-03-2003

Hi, I just found something to confuse me and nothing that would explain it to me. Seems like I got to ask another newbie question

...

In a tutorial I found the following code:

Code:

struct CVector3 {
	public:
		float x, y, z;

		CVector3() {}
		CVector3(float fX, float fY, float fZ);	

		CVector3 operator+(CVector3 vVector);
		CVector3 operator-(CVector3 vVector);
		CVector3 operator*(float fNum);
		CVector3 operator/(float fNum);
};

At first it looked like a normal class, but then I saw the struct at the beginning.

Is this just "bad code" or is there a real meaning to this? If I replaced the struct with a class, would it make any difference ?

**darksaidin** · 08-03-2003

Originally posted by Salem
> If I replaced the struct with a class, would it make any difference ?
The only difference - as far as I know - is that classes default to private, and structs default to public.

Since this struct begins with public:
Changing it to a class should have no effect.

mhm, so it's the same ? What if it started with "private" ?

**darksaidin** · 08-03-2003

I seem to have some problems understanding this

Does struct really declare a type like class does ? I always thought struct would declare an instance unless you use typedef.

**7stud** · 08-03-2003

"Does struct really declare a type like class does ? I always thought struct would declare an instance unless you use typedef."

Yes, in C++ there is absolutely no difference between a struct and a class except for the default access, i.e. the access given to members or functions when you don't specify it.

This declares a type:

Code:

struct Book
{
     int location;
     int number;

};

This declares an instance:

struct Book a_book(2000, 3);

Or, this declares a type and an instance:

Code:

struct Book
{
     int location;
     int number;
}a_book;

**darksaidin** · 08-03-2003

Thanks, Salem and 7stud!

There is just one more thing... what exactly does this thing do then?

Code:

typedef struct _AUTHENTICATE_MESSAGE {
    UCHAR Signature[sizeof(NTLMSSP_SIGNATURE)];
    NTLM_MESSAGE_TYPE MessageType;
    STRING LmChallengeResponse;
    STRING NtChallengeResponse;
    STRING DomainName;
    STRING UserName;
    STRING Workstation;
    STRING SessionKey;
    ULONG NegotiateFlags;
} AUTHENTICATE_MESSAGE, *PAUTHENTICATE_MESSAGE;

A variable _AUTHENTICATE_MESSAGE, a type AUTHENTICATE_MESSAGE and a type PAUTHENTICATE_MESSAGE that is a pointer to an AUTHENTICATE_MESSAGE type?

Or maybe a type _AUTHENTICATE_MESSAGE, a variable AUTHENTICATE_MESSAGE and a pointer *PAUTHENTICATE_MESSAGE to a _AUTHENTICATE_MESSAGE type?

If I'd follow 7studs example, the second paragraph would be the correct one, but somehow I got the feeling that the first one is correct. Or neither of them ?

edit: ok, I think I got it:

- _AUTHENTICATE_MESSAGE is the tag which I could use to create a variable using the struct keyword.... obviously something from the past so I don't need that anymore in C++...
- AUTHENTICATE_MESSAGE is the type...
- *PAUTHENTICATE_MESSAGE a pointer to a variable of that type...
- there are no variables declared in that piece of code.

It that correct now ?

**Cat** · 08-03-2003

Originally posted by darksaidin
Thanks, Salem and 7stud!

There is just one more thing... what exactly does this thing do then?

Code:

typedef struct _AUTHENTICATE_MESSAGE { UCHAR Signature[sizeof(NTLMSSP_SIGNATURE)]; NTLM_MESSAGE_TYPE MessageType; STRING LmChallengeResponse; STRING NtChallengeResponse; STRING DomainName; STRING UserName; STRING Workstation; STRING SessionKey; ULONG NegotiateFlags; } AUTHENTICATE_MESSAGE, *PAUTHENTICATE_MESSAGE;

A variable _AUTHENTICATE_MESSAGE, a type AUTHENTICATE_MESSAGE and a type PAUTHENTICATE_MESSAGE that is a pointer to an AUTHENTICATE_MESSAGE type?

Or maybe a type _AUTHENTICATE_MESSAGE, a variable AUTHENTICATE_MESSAGE and a pointer *PAUTHENTICATE_MESSAGE to a _AUTHENTICATE_MESSAGE type?

If I'd follow 7studs example, the second paragraph would be the correct one, but somehow I got the feeling that the first one is correct. Or neither of them ?

edit: ok, I think I got it:

- _AUTHENTICATE_MESSAGE is the tag which I could use to create a variable using the struct keyword.... obviously something from the past so I don't need that anymore in C++...
- AUTHENTICATE_MESSAGE is the type...
- *PAUTHENTICATE_MESSAGE a pointer to a variable of that type...
- there are no variables declared in that piece of code.

It that correct now ?

_AUTHENTICATE_MESSAGE is the original name of the structure.
AUTHENTICATE_MESSAGE is a typedef for _AUTHENTICATE_MESSAGE.
PAUTHENTICATE_MESSAGE is a typedef for _AUTHENTICATE_MESSAGE *.

**7stud** · 08-03-2003

Just so you know, these typdef things can get very complicated. The basic idea of a typedef is simple, but I've never seen a tutorial anywhere or an explanation of how to figure out the complicated ones that are allowed. The one you listed was a fairly simple one, and I couldn't figure it out. I've seen much more complicated ones too.

Notice that the basic typedef for a struct ooks like this(which is different than declaring a type and a variable at the same time):

Code:

typedef struct Book
{
     int location;
     int number;

}B;

int main()
{
    B my_book;

    return 0;
}

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