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. segmented architectures and such, where they might be of different sizes. With that context, if I have a container with random-access iterators, is it safe for me to perform a `static_cast` between its `difference_type` and `size_type` values at will, on the grounds that `end() - begin()` must always be equal to `size()`, when either is casted? (The use case, for example, is to create a container whose size is equal to the number of elements between two iterators, or the reverse: to copy a container of a certain size onto a range delimited by iterators.) Anything I should watch out for before casting (e.g. loss of data)?

Accepted Answer

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.

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

Solution

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