Question

I don’t understand why a vector‘s iterator should be invalidated when a reallocation happens.

Couldn’t this have been prevented simply by storing an offset — instead of a pointer — in the iterator?

Why was vector not designed this way?

Answer 1

Just to add a citation to the performance-related justification: when designing C++, Stroustrup thought it was vital that template classes like std::vector approach the performance characteristics of native arrays:

One reason for the emphasis on run-time efficiency…was that I wanted
templates to be efficient enough in time and space to be used for
low-level types such as arrays and lists.

…

Higher-level alternatives — say, a range-checked array with a size()
operation, a multidimensional array, a vector type with proper numeric
vector operations and copy semantics, etc. — would be accepted by
users only if their run-time, space, and notational convenience
approached those of built-in arrays.

In other words, the language mechanism supplying parameterized types
should be such that a concerned user should be able to afford to
eliminate the use of arrays in favor of a standard library class.

Bjarne Stroustrup, Design and Evolution of C++, p.342.