About vectors

[ProgrammerC++]

How does a vector know when to destroy itself?

[Skizmo]

It's a class. It doesn't destroy itself, the destructor is called as soon as the class is deleted.

[ProgrammerC++]

I'm assuming for each element inside a vector it's allocating memory internally.

So, this is never deleted if you dont use the vector anymore? And thus vectors cause memory leaks?

[GCDEF]

I'm assuming for each element inside a vector it's allocating memory internally.

So, this is never deleted if you dont use the vector anymore? And thus vectors cause memory leaks?

Of course not. The vector's memory is deleted when the vector is deleted or goes out of scope. If it's a vector of pointers, you need to delete the memory the pointers point to yourself.

[ProgrammerC++]

so if you have

vector<T*> v;

T * ptr = new T();

v.push_back( ptr );

delete ptr;

//nothing with v done after this
//leak?

[S_M_A]

No since the only thing v contains is a pointer to the deleted T object.

[ProgrammerC++]

Memory does not have to be allocated to store the pointer to the T object?

[TheComputer]

It's not about the vector, but about the object it contains. Vector is made to be a dynamic object container, there will be of course use of allocators to dispatch and reclaim memory automatically
Right after you delete an allocated object the area it points to will be set free, all the later works can't inspire any more the provocation of the same pointer except overriding

[ProgrammerC++]

So a memory leak occurs if I'm not clearing the vector

[TheComputer]

When did you allocate your vector ?
It's not the vector that you are allocating, it's the object of type T you are to deal with.
By the way, today I bought a sandwich. I have ketchup and chili sauces already

[ProgrammerC++]

vector is allocated on the stack, it can't grow there, so it has to allocate memory to grow and hold elements like my pointer

[GCDEF]

vector is allocated on the stack, it can't grow there, so it has to allocate memory to grow and hold elements like my pointer

It managers its own memory. If the vector contains pointers, it won't delete the what the pointers point to.

[ProgrammerC++]

Yes but where does it store the pointer to the object? It must be in newly allocated memory as soon as push_back is called

[Lindley]

Yes but where does it store the pointer to the object? It must be in newly allocated memory as soon as push_back is called

Yes, that's correct. However, the vector cleans up after itself; that memory is automatically deallocated as soon as the vector goes out of scope or is otherwise itself destroyed.

In short: If you allocate something, then you have to clean it up. If an object allocates something internally, then that object is expected to clean it up in its destructor (if not before). This, incidentally, is a guideline you should follow in creating your own classes as well.

So a memory leak occurs if I'm not clearing the vector

Actually, clearing doesn't affect things at all----the vector is designed not to deallocate memory when it's cleared, only when it's destroyed. If you actually want a vector to resize itself smaller, then you need to use the "swap trick". (Usually, there's no pressing desire to do this however, so it's more efficient to let the vector keep its excess memory just in case you ever add more to it.)

[ProgrammerC++]

How does a vector know it goes out of scope?