RE: [boost] Re: std::complex design (Was: N-Dimensional array lib rary submission...)

From: Stewart, Robert (stewart_at_[hidden])
Date: 2002-03-05 14:44:24

From: rwgk [mailto:rwgk_at_[hidden]]
> 
> --- In boost_at_y..., Damien Fisher <damien_at_m...> wrote:
> > >    Why not?  If you write vector<complex<double> >   vec(10);  
> you have made
> > > a conscious decision to create a vector that started with 10 
> complex
> > > numbers. Its not unreasonable for a user to use one of those 
> complex
> > > numbers.  If you just wanted to reserve space for 10 complex 
> numbers you
> > > could have just used the default constructor then called reserve
> ().
> > > Besides, the default constructor is inline, so how slow could it 
> be?
> > > 
> > 100 x 100 matrix of complex numbers => 100 * 100 * 2 unnecessary
> > initializations.  and that's not even a particularly large example.
> 
> Unfortunately the reserve() solution eventually requires
> use of push_back(), which is also slow.

If you use push_back() following reserve(), there is no performance penalty.
The push_back()'ed object is copied into the next element.  If you exceed
the size allocated with reserve(), then push_back() will cause reallocation
and copying, so be sure to choose the right size when calling reserve().

Rob
Susquehanna International Group, LLP
http://www.sig.com