What does "[ptr, ptr+len) must be a valid range" mean?

Question

In various places in the c++ Standard, the phrase:

Requires: [p, p + len) is a valid range.

(where p is a pointer, and len is a number) is used.

An alternate version for iterators is:

Requires: [first, last) is a valid range.

What does this mean?

Answer 1

I think the relevant part of the C++17 Standard is here

27.2.1 In general [ iterator.requirements.general ]

...

8 An iterator j is called reachable from an iterator i if and only if there is a finite sequence of applications of the expression ++i that makes i == j. If j is reachable from i, they refer to elements of the same sequence.

9 Most of the library’s algorithmic templates that operate on data structures have interfaces that use ranges. A range is a pair of iterators that designate the beginning and end of the computation. A range [i, i) is an empty range; in general, a range [i, j) refers to the elements in the data structure starting with the element pointed to by i and up to but not including the element pointed to by j. Range [i, j) is valid if and only if j is reachable from i. The result of the application of functions in the library to invalid ranges is undefined.

So extracting the pertinent parts:

8 An iterator j is called reachable from an iterator i if and only if there is a finite sequence of applications of the expression ++i that makes i == j...

9 ... Range [i, j) is valid if and only if j is reachable from i....

So it seems that a "valid range" is a pair of iterators that belong to the same container (or array), with j being farther along than i.

But also not forgetting:

in general, a range [i, j) refers to the elements in the data structure starting with the element pointed to by i and up to but not including the element pointed to by j.

So all of the iterators between i and j (but not necessarily including j) must be "in the data structure" (ie. not out-of-bounds).