The alloc_n_copy Member

The alloc_n_copy member is called when we copy or assign a StrVec. Our StrVec class, like vector, will have valuelike behavior (§ 13.2.1, p. 511); when we copy or assign a StrVec, we have to allocate independent memory and copy the elements from the original to the new StrVec.

The alloc_n_copy member will allocate enough storage to hold its given range of elements, and will copy those elements into the newly allocated space. This function returns a pair (§ 11.2.3, p. 426) of pointers, pointing to the beginning of the new space and just past the last element it copied:

pair<string*, string*>
StrVec::alloc_n_copy(const string *b, const string *e)
{
    // allocate space to hold as many elements as are in the range
    auto data = alloc.allocate(e - b);
    // initialize and return a pair constructed from data and
    // the value returned by uninitialized_copy
    return {data, uninitialized_copy(b, e, data)};
}

alloc_n_copy calculates how much space to allocate by subtracting the pointer to the first element from the pointer one past the last. Having allocated memory, the function next has to construct copies of the given elements in that space.

It does the copy in the return statement, which list initializes the return value (§ 6.3.2, p. 226). The first member of the returned pair points to the start of the allocated memory; the second is the value returned from uninitialized_copy (§ 12.2.2, p. 483). That value will be pointer positioned one element past the last constructed element.