Class reference member problem

[tech]

Hi, i have the following situation

class B
{

static B createBobj();
}

Class A
{
func()
{
m_obj = createobj();
}

B& m_obj;
};

Obviously doesn't compile so

In class A i want to hold a reference to an instance of class B
created by the
factory method createBObj. i can't initialise my reference member in
constructor as
class A needs to do some other initialisation before creating instance
of B.

So how to solve this problem?

 

[Martin Vuille]

ps.com:

In class A i want to hold a reference to an instance of class
B created by the
factory method createBObj. i can't initialise my reference
member in constructor as
class A needs to do some other initialisation before creating
instance of B.

So how to solve this problem?

Have class A hold a pointer to the instance of B instead?

MV

 

[anon]

Hi, i have the following situation

class B
{

static B createBobj();

I hope it is not private

}

Class A
{
func()
{
m_obj = createobj();

Here you point your reference to a temporary object, which is destroyed
when func() ends.
By the way, where is createobj() declared in class A?

}

B& m_obj;
};

Obviously doesn't compile so

Yes, for many reasons

In class A i want to hold a reference to an instance of class B
created by the
factory method createBObj. i can't initialise my reference member in
constructor as
class A needs to do some other initialisation before creating instance
of B.

So how to solve this problem?

Can you change m_obj to be of type B or B*?

 

[tech]

ok should be

func()
{
m_obj = B:createobj();

}

ok if i change m_obj to pointer, createobj returns object
so it will go out of scope anyway, so how do i solve this?
Where do i create the instance of B so it doesn't go out of scope
given that creatobj returns obj not on heap.

 

[anon]

> ok should be
>
> func()
> {
> m_obj = B:createobj();

m_obj = B::createobj();

>
> }
>
> ok if i change m_obj to pointer, createobj returns object
> so it will go out of scope anyway, so how do i solve this?

In the createobj() function create an object of type B, and return a pointer
for example
B* createobj()
{
B *p = new B;
return p;
}
then do not forget to delete created object

> Where do i create the instance of B so it doesn't go out of scope
> given that creatobj returns obj not on heap.

The way you presented, I think it is not possible because createobj()
returns a temporary object of type B.
If you make m_obj of type B, then you can do what you did, but you are
invoking the B's copy-constructor

 

[Daniel T.]

Hi, i have the following situation

class B
{

public: // I'm assuming either createBobj is public, or
// A is a friend of B.

static B createBobj();
}

// I'm assuming you meant to put a ';' after that closing brace.

Class A

// I'm assuming you meant "class A"

{
func()

// I'm assuming func() returns void

{
m_obj = createobj();

// I'm assuming that was supposed to be "m_obj = B::createBobj();"

}

B& m_obj;
};

Obviously doesn't compile so

Obviously.

In class A i want to hold a reference to an instance of class B
created by the
factory method createBObj. i can't initialise my reference member in
constructor as
class A needs to do some other initialisation before creating instance
of B.

So how to solve this problem?

Lastly, I'm assuming you can't change class B, and that B has proper
copy construction semantics...

class A {
B m_obj;
void func() {
m_obj = B::createBobj();
}
};

It will not hold the exact instance created by createBobj(), but it will
hold an instance that is in the same state.

Or did I make too many assumptions...

 

[James Kanze]

ok should be

func()
{
m_obj = B:createobj();
}

ok if i change m_obj to pointer, createobj returns object
so it will go out of scope anyway, so how do i solve this?

You need to make a copy.

Where do i create the instance of B so it doesn't go out of
scope given that creatobj returns obj not on heap.

Either in the class, or on the heap. If m_obj has type B, the
above will copy assign the temporary to it. If m_obj has type
B*, then:

m_obj = new B( B::creatobj() ) ;

will copy construct an object on the heap, and assign its
address to m_obj.