Must size() == end() - begin()? What about the cast?

Question

From what I understand, the purpose of size_type and difference_type is not merely the sign -- it was also meant to address e.g. segme

Answer 1

Here's what the C++11 standard has to say on various things here:

§ 23.2.1

Expression: difference_type
Return Type: signed integer type
Operational Semantics: -
Assertion/note, pre-/post-condition: is identical to the difference type of iterator and const_iterator
Complexity: compile-time

Expression: size_type
Return Type: unsigned integer type
Operational Semantics: -
Assertion/note, pre-/post-condition: size_type can represent any non-negative value of difference_type
Complexity: compile-time

Expression: size()
Return Type: size_type
Operational Semantics: distance(begin(),end()) 
Assertion/note, pre-/post-condition: -
Complexity: constant

Let's make sure size() is equivalent to end() - begin():

§ 24.4.4/4

distance():
Effects: If InputIterator meets the requirements of random access iterator, 
returns (last - first); otherwise, returns the number of increments needed 
to get from first to last

Since your container has random-access iterators, this holds true. That's that. As you can see in the first box,

size_type can represent any non-negative value of difference_type

From that, we have that the cast from difference_type to size_type should be valid for all non-negative values.