Re: I am confused with these concepts.

"Igor Tandetnik" <>
Sat, 26 Jan 2008 23:04:28 -0500
"rockdale" <> wrote in message

Why sometime we need to new a class to call its functions or to get it
public properties. But sometime we just simply declare it and then we
can access its funnctions and properties?

Doesn't your favorite C++ textbook explain the difference between
automatic objects and dynamically allocated ones? You should throw it
away then, and get a better one.

Also, sometime, I need to pur Class myClassName at the top of another
class to compile, but sometime I do not need?

Well, if that other class doesn't use myClassName in any way, then of
course myClassName doesn't need to be mentioned.

I have an sample class to represent my members (id and name, to

typedef struct {
int ID;
std::string Name;
} MembData;

What do you think you need a typedef for? Why not just

struct MembData { ... };

typedef std::vector<MembData> MembVec;
typedef MembVec::iterator MembItr;

class CMembDataClass
void Load(); //populate memb's data list into m_vecMemb
bool Remove(MembData aData);
bool Append(MembData aData);
bool Update(MembData aData);

MembVec m_vecMemb;

I have another class to use this MembDataClass, let call it

I always thought I need to new the MembDataClass (instantialize it to
get the public variable m_vecMemb) :

You sound like instantiating and using 'new' is the same thing. This is
not the case. 'new' is not the only way to create an instance of a class
(note how you never used 'new' on m_vecMemb, which itself is an instance
of class vector<MembData>).

Are you coming from Java background, by any chance?

CMembDataClass* pMyMemb = new CMembDataClass();
m_membList = pMyMemb->m_vecMemb;

You also need to do

delete pMyMemb;

when you are done with it. Otherwise, you are leaking resources.

But I also tried:

CMembDataClass myMemb;
m_membList = myMemb.m_vecMemb;

I thought it would give me error like access violation something

And exactly why would you believe such a thing? After all, you don't
appear to be surprised that you can do

MembVec m_membList;

but feel "CMembDataClass myMemb;" is a problem. What, in your opinion,
is the major difference between the two statements?

and if I did not call new to instantialize the MembDataClass, where is
the data stored at?

Read about automatic variables (aka local variables, aka variables with
automatic storage duration) in your C++ textbook.

When you write, say,

int f() {
    int x = 1;
    return x;

do you also wonder where x is stored at? For that matter, when you write

CMembDataClass* pMyMemb = new CMembDataClass();

where is the memory for pMyMemb (the pointer itself, not the pointed-to
object) come from? Think about it.

is it because my MembData is struct, if it is another class, would it
be a different behavior?

In C++, struct and class are almost the same. The only difference is
that members of a struct are public by default, and members of a class
are private by default. In other words,

struct S {...};

is equivalent to

class S {

The keyword 'struct' remains in C++ mostly for compatiblity with C.

and what is the case if m_vecMemb is a

What do you feel would be a problem with this?
With best wishes,
    Igor Tandetnik

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