Appending std::vector to itself, undefined behavior?

Question

This question made me uncertain about appending a vector to itself. So the question is: Following lines of code do what I expect, but is it standard conform?

vec.reserve(vec.size() * 2): vec.insert(vec.end(), vec.begin(), vec.end());

Following (without reserve()) still works, is it even standard conform?

vec.insert(vec.end(), vec.begin(), vec.end());

Or implementation depending?

changing the content of a container(such as std::vector) while iterating through it is UB — Aniket Inge
– Aniket Inge, Commented Feb 9, 2013 at 22:09
For the conformant way, see stackoverflow.com/q/17636690/103167 — Ben Voigt
– Ben Voigt, Commented Oct 18, 2014 at 19:52

templatetypedef · Accepted Answer · 2013-02-09 22:11:19Z

According to the C++03 ISO spec (§23.1.1, Table 67) (and as @AndyProwl has mentioned, in §23.2.3, table 11 of the C++11 ISO spec), as part of sequence requirements, the operation a.insert(p, i, j) in a sequence container has this precondition:

i, j are not iterators into a.

In other words, sequence containers are allowed to safely assume that if you do a range insertion operation, that range will not be defined from iterators over that original container.

As a result, if you try to insert a container's elements into itself, you are calling a standard library function and breaking a precondition. This results in undefined behavior, meaning that it might work on some platforms if the library implementers are nice people, but it could terribly and catastrophically fail with no justification.

Hope this helps!

@AndyProwl- Thanks! Out of curiosity, is there an easy way to get a legal copy of that ISO spec?
@templatedtypdef: Yes, but you have to pay for it. Or you can get a draft for free from here.
@AndyProwl: That's a very recent draft, with substantial post-C++11 changes. N3337 is much more like the actual C++11 standard, with only minor editorial changes.
Yes, this helps a lot, thanks for the standard reference and the explaining words! I was so sure that the resize() example is correct, ouch.
@NicolBolas: Thank you for the information. Where can I find a link to that draft?

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