If you have an STL vector which has been resized, is it safe to take the address of element 0 and assume the rest of the vector will follow in memory?
e.g.
vector<char> vc(100);
// do some stuff with vc
vc.resize(200);
char* p = &vc[0];
// do stuff with *p
Yes, that is a valid assumption (*).
From the C++03 standard (23.2.4.1):
The elements of a vector are stored contiguously, meaning that if v is a vector where T is some type other than bool, then it obeys the identity &v[n] == &v[0] + n for all 0 <= n < v.size().
(*) ... but watch out for the array being reallocated (invalidating any pointers and iterators) after adding elements to it.
The C++03 standard added wording to make it clear that vector elements must be contiguous.
C++03 23.2.4 Paragraph 1 contains the following language which is not in the C++98 standard document:
The elements of a
vectorare stored contiguously, meaning that ifvis avector<T, Allocator>whereTis some type other thanbool, then it obeys the identity&v[n] == &v[0] + nfor all0 <= n < v.size().
Herb Sutter talks about this change in one of his blog entries, Cringe not: Vectors are guaranteed to be contiguous:
... contiguity is in fact part of the vector abstraction. It’s so important, in fact, that when it was discovered that the C++98 standard didn’t completely guarantee contiguity, the C++03 standard was amended to explicitly add the guarantee.
Storage is always contiguous, but it may move as the vector's capacity is changed.
If you had a pointer, reference, or iterator on element zero (or any element) before a capacity-changing operation, it is invalidated and must be reassigned.
std::vector guarantees that the items are stored in a contiguous array, and is therefore the preferred replacement of arrays and can also be used to interface with platform-dependent low-level code (like Win32 API calls). To get a pointer to the array use:
&myVector.front();