Allocator code in C++ Primer

[pauldepstein]

I saw this code in the 4th edition of C++ Primer (Lippman, Lajoie and
Moo):

allocator<string> alloc;
string* sp = alloc.allocate(2);

alloc.construct(sp+1, string(b, e)); // b and e are string iterators.

I don't understand the reason for the "+1" in sp+1.

Wouldn't alloc.construct(sp, string(b,e)); be simpler and more
natural?

Is there a reason for the "+1"?

Many thanks for your help.


Paul Epstein

 

[Bo Persson]

I saw this code in the 4th edition of C++ Primer (Lippman, Lajoie
and Moo):

allocator<string> alloc;
string* sp = alloc.allocate(2);

alloc.construct(sp+1, string(b, e)); // b and e are string
iterators.

I don't understand the reason for the "+1" in sp+1.

Wouldn't alloc.construct(sp, string(b,e)); be simpler and more
natural?

Is there a reason for the "+1"?

I believe it is just to show that it can be done that way. You
allocate space for two strings, and choose to construct the second
one.


Bo Persson