The C++ language comes with a standard library, and although implementations are welcome to implement the library types in whatever manner they choose, they are constraints imposed by the standard which often force one of a small number of possible implementations.

Inside STL: The vector

By Raymond Chen

The std::vector is one of those types which is constrained to the point that there’s really only one viable implementation.

Internally, a std::vector basically looks like this:

template<typename T>
struct vector
{
    T* first;
    T* last;
    T* end;
};

The first is a pointer to the beginning of a single memory allocation that holds the vector contents.

The last is a pointer one past the end of the last valid vector element.

The end is a pointer one past the end of the last allocated memory for the vector.

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