Thread: pointers

If I use a pointer to an element in an std::vector, during reallocation of memmory, will my pointer point to an invalid location?

Code:

std::vector<int> a(10, 0);
int *ptr = &a[4];

I have had no problems till now...
Can some one tell me a way to counteract this problem???

Whenever you change the contents of the vector your pointer could become invalid. If you never change the vectors contents after getting the pointer to an element you are safe.
If the contents change you could use a vector of pointers.
Kurt

If I use a pointer to an element in an std::vector, during reallocation of memmory, will my pointer point to an invalid location?

Yes. Presumably exceeding the capacity() of the vector causes the internal memory management to use new to create a new block of memory for the elements, then all the elements are copied to the new block, and then the old block is deleted. So, pointers that point to the old block of memory are no longer valid because the memory was deleted.

Can some one tell me a way to counteract this problem???

Make sure the capacity is large enough to start, so you don't have to reallocate? Check out reserve().

Thanks I'll try that...

Instead of keeping a pointer to the memory, keep an index. The index will always be valid unless you erase enough elements to make the size smaller than the index (which would make the pointer unusable as well).