Re: [Boost-users] [Interprocess] Problem with reserve for vector ofweak_ptr

Subject: Re: [Boost-users] [Interprocess] Problem with reserve for vector ofweak_ptr
From: Cliff Green (cliffg_at_[hidden])
Date: 2010-05-28 12:20:44

> Resize can trim the vector and I don't want it.
> I think "reserve" is the right function for my need.
>
> When I use push_back instead of operator[] there is no pb.

As Ion is probably just pointing out, there's a fundamental difference 
between resize and reserve. Resize modifies the "logical size" of the 
container, creating (by default construction) new entries, or removing (with 
destruction) entries. Reserve does not change the "logical size", and it's 
only use is as an implementation optimization (you can pre-assign internal 
buffer space, so that when adding new entries later on a memory allocation 
will already have occurred).

For a vector, you can create one and then reserve one all you want, but the 
size is still 0 (until "push_back" or "insert", etc is performed), and 
indexing into the vector will be out of bounds.

Read Josuttis C++ standard library book, or any good reference on the 
difference.

Cliff