what are vectors in c++?

[dineesha]

what are vector pointers ?and what does a vector addition or subtraction mean?

[Banfa]

A vector pointer would be a pointer to a vector.

vectors them selves are basically just dynamically size arrays of any given type and can be treated as such. They are 1 of the standard container classes provided by the C++ STL.

They are implemented as a template class. A template is used because this allows them to handle any type and a class is used because this then provides the mechanisms by which the vector can be dynamically resized.

One of the features of a vector is that it's memory is held internally as a contiguous block. This means that if a vector is resized in a way that forces a memory reallocation (specifically if it is made bigger) then it's entire memory has to be reallocated and the contents copied from the old memory block which is then discarded, this also invalidates any iterators currently on the vector.

Because of this when you are using vectors it you know you are going to add a series of new values where possible it is better to resize the vector to the new required size before adding each item rather than doing the resize as you add each item as it will involve copying less data.

Additionally when a vectors size is reduced it does not deallocate the unused memory, i.e. if a vector holding 1000000 entries is reduce so that it only holds 100 entries then it will still have memory allocated to hold 1000000 entries. The program has to specifically take action to release the unused memory in this case.