questions about pointers in container

[tradevol]

as this example,

question:

If my purpose is initialize data from xml files and store them in the
vector, so they can be used in class B by other member functions, do
you think functionP is a viable function(will a could go away after
out of the function)?
If not, is there a better solution than using functionPt?

I read that it is not a good design to have container for pointers(C++
FAQs), but I cannot see how I can get around it in my situation.

Newbie questions, thanks for the help

Chris

class A{
....
}

class B{

void functionPt(){
...
A* aPt ;
vPt.push_back(aPt);

}

void functionP(){
...
A a;
v.push_back(a);
}

private:
vector<A*vPt;
vector<A v;

}

[acehreli]

On Aug 3, 4:38 pm, [e-mail] wrote:

If my purpose is initialize data from xml files and store them in the
vector, so they can be used in class B by other member functions, do
you think functionP is a viable function(will a could go away after
out of the function)?

void functionP(){
...
A a;
v.push_back(a);

}

That would work. Yes, 'a' would be terminated at the end of functionP.
Since 'v' would be containing a *copy* of 'a' you are good.

If not, is there a better solution than using functionPt?

void functionPt(){
...
A* aPt ;
vPt.push_back(aPt);

}

You would need a vector of pointers if the objects that you create a
polymorphic. Otherwise go with functionP.

I read that it is not a good design to have container for pointers(C++
FAQs), but I cannot see how I can get around it in my situation.

If you have polymorphic objects, store them either as smart pointers
in a vector, or use a "pointer vector" that knowns to delete the
objects when itself is going away.

Using the Boost library's reference counted pointer:

#include <boost/shared_ptr.hpp>

typedef boost::shared_ptr<AAPtr;
typedef vector<APtrAPointers;

APointers my_a_collection;
my_a_collection.push_back(APtr(new SomeDecendentOfA()));

See the Boost library for their pointer vector.

Ali

[Daniel T.]

tr******@yahoo.com wrote:

as this example,

question:

If my purpose is initialize data from xml files and store them in the
vector, so they can be used in class B by other member functions, do
you think functionP is a viable function(will a could go away after
out of the function)?
If not, is there a better solution than using functionPt?

I read that it is not a good design to have container for pointers(C++
FAQs), but I cannot see how I can get around it in my situation.

Newbie questions, thanks for the help

Chris

class A{
....
}

class B{

void functionPt(){
...
A* aPt ;

aPt has not been initialized. Copying it into a vector is technically
undefined behavior.

vPt.push_back(aPt);

}

void functionP(){
...
A a;
v.push_back(a);
}

private:
vector<A*vPt;
vector<A v;

}

[tradevol]

On Aug 3, 7:50 pm, "Daniel T." <[e-mail]:

[e-mail] wrote:

as this example,

question:

If my purpose is initialize data from xml files and store them in the
vector, so they can be used in class B by other member functions, do
you think functionP is a viable function(will a could go away after
out of the function)?
If not, is there a better solution than using functionPt?

I read that it is not a good design to have container for pointers(C++
FAQs), but I cannot see how I can get around it in my situation.

Newbie questions, thanks for the help

Chris

class A{
....
}

class B{

void functionPt(){
...
A* aPt ;

aPt has not been initialized. Copying it into a vector is technically
undefined behavior.

vPt.push_back(aPt);

}

void functionP(){
...
A a;
v.push_back(a);
}

private:
vector<A*vPt;
vector<A v;

}

Thanks. My fault. My intention is A* aPt = new A;

[tradevol]

On Aug 3, 7:07 pm, [e-mail] wrote:

On Aug 3, 4:38 pm, [e-mail] wrote:

If my purpose is initialize data from xml files and store them in the
vector, so they can be used in class B by other member functions, do
you think functionP is a viable function(will a could go away after
out of the function)?
void functionP(){
...
A a;
v.push_back(a);

}

That would work. Yes, 'a' would be terminated at the end of functionP.
Since 'v' would be containing a *copy* of 'a' you are good.

If not, is there a better solution than using functionPt?
void functionPt(){
...
A* aPt ;
vPt.push_back(aPt);

}

You would need a vector of pointers if the objects that you create a
polymorphic. Otherwise go with functionP.

I read that it is not a good design to have container for pointers(C++
FAQs), but I cannot see how I can get around it in my situation.

If you have polymorphic objects, store them either as smart pointers
in a vector, or use a "pointer vector" that knowns to delete the
objects when itself is going away.

Using the Boost library's reference counted pointer:

#include <boost/shared_ptr.hpp>

typedef boost::shared_ptr<AAPtr;
typedef vector<APtrAPointers;

APointers my_a_collection;
my_a_collection.push_back(APtr(new SomeDecendentOfA()));

See the Boost library for their pointer vector.

Ali

Thanks Ali. Basically, when push_back is called, A's copy constructor
is triggered and there is a copy of a is made and stored in v. And
since v's scope is the class, the copy will not be destroyed as long
as the class instance is alive. please correct me if i am wrong.

Heard about boost, but never used it before. Will auto_ptr do the same
thing in this case (in case A could be polymorphic )?
thanks

Chris

[acehreli]

On Aug 3, 6:30*pm, [e-mail] wrote:

Will auto_ptr do the same
thing in this case (in case A could be polymorphic )?

Unfortunately, std::auto_ptr can not be used with standard containers
because it doesn't satisfy the requirement that the copies are
equivalent. Source auto_ptr becomes "null" when auto_ptr objects are
copied.

Ali

[Daniel T.]

On Aug 3, 9:20*pm, [e-mail] wrote:

On Aug 3, 7:50 pm, "Daniel T." <[e-mail]:

[e-mail] wrote:

as this example,

question:

If my purpose is initialize data from xml files and store them in the
vector, so they can be used in class B by other member functions, do
you think functionP is a viable function(will a could go away after
out of the function)?
If not, is there a better solution than using functionPt?

I read that it is not a good design to have container for pointers(C++
FAQs), but I cannot see how I can get around it in my situation.

Newbie questions, thanks for the help

Chris

class A{
* *....
}

class B{

void functionPt(){
*...
* A* aPt ;

aPt has not been initialized. Copying it into a vector is technically
undefined behavior.

* vPt.push_back(aPt);

}

void functionP(){
* ...
* A a;
* *v.push_back(a);
}

private:
* *vector<A*vPt;
* *vector<A*v;

}

Thanks. My fault. My intention is A* aPt = new A;

Now you have a leak.

[tradevol]

On Aug 4, 1:48*pm, "Daniel T." <[e-mail]:

On Aug 3, 9:20*pm, [e-mail] wrote:

On Aug 3, 7:50 pm, "Daniel T." <[e-mail]:

[e-mail] wrote:
as this example,

question:

If my purpose is initialize data from xml files and store them in the
vector, so they can be used in class B by other member functions, do
you think functionP is a viable function(will a could go away after
out of the function)?
If not, is there a better solution than using functionPt?

I read that it is not a good design to have container for pointers(C++
FAQs), but I cannot see how I can get around it in my situation.

Newbie questions, thanks for the help

Chris

class A{
* *....
}

class B{

void functionPt(){
*...
* A* aPt ;

aPt has not been initialized. Copying it into a vector is technically
undefined behavior.

* vPt.push_back(aPt);

}

void functionP(){
* ...
* A a;
* *v.push_back(a);
}

private:
* *vector<A*vPt;
* *vector<A*v;

}

Thanks. My fault. My intention is A* aPt = new A;

Now you have a leak.

not really. my ~B will iterate the vector and call delete.