q: stl vector of pointers / pointer consistancy

[laniik]

Hi. I have a question. If I have a vector of objects and on of object
pointers:

---
vector <Obj> objects;
vector <Obj*> objectPointers;
---

and i setup the vectors with somhthing like:

---
Obj o;
objects.push_back(o);
objectPointers.push_back(&objects[objects.size()-1])
---

now. when i grow the objects array:

---
for(int i=0;i<10000;i++)
{
Obj o;
objects.push_back(o);
}
---

is it possible that the pointer in the objectPointer array become
invalid when STL shifts the objects vector around in memory?

thanks!

Oliver

[Peter Koch Larsen]

"laniik" <la****@yahoo.com> skrev i en meddelelse
news:11*********************@z14g2000cwz.googlegro ups.com...

Hi. I have a question. If I have a vector of objects and on of object
pointers:

---
vector <Obj> objects;
vector <Obj*> objectPointers;
---

and i setup the vectors with somhthing like:

---
Obj o;
objects.push_back(o);
objectPointers.push_back(&objects[objects.size()-1])
---

now. when i grow the objects array:

---
for(int i=0;i<10000;i++)
{
Obj o;
objects.push_back(o);
}
---

is it possible that the pointer in the objectPointer array become
invalid when STL shifts the objects vector around in memory? It is not only possible but almost guaranteed the way your code is written.

/Peter

thanks!

Oliver

[Rolf Magnus]

laniik wrote:

Hi. I have a question. If I have a vector of objects and on of object
pointers:

---
vector <Obj> objects;
vector <Obj*> objectPointers;
---

and i setup the vectors with somhthing like:

---
Obj o;
objects.push_back(o);
objectPointers.push_back(&objects[objects.size()-1])
objectPointers.push_back(&objects.back())
now. when i grow the objects array:

---
for(int i=0;i<10000;i++)
{
Obj o;
objects.push_back(o);
}
---

is it possible that the pointer in the objectPointer array become
invalid when STL shifts the objects vector around in memory?

Yes. If objects reallocates its memory, all the pointers of objectPointers
will be invalidated.
One way to avoid that is by reserving space. In your example, put
objects.reserve(10000); before the loop. Then, enough memory for 10000
objects gets reserved, so as long as not more objects are added, the vector
will not reallocate.
Of course that is only an option if you know the size in advance.
Another solution is to not store pointers, but indexes. Those won't get
invalidated by push_back().