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From "Patrick Happel" <phap...@quovadx.com>
Subject RE: std::vector intialization on HP-UX
Date Wed, 23 May 2007 19:24:16 GMT
Thanks for these answers, Martin!

 -Patrick 

-----Original Message-----
From: Martin Sebor [mailto:sebor@roguewave.com] 
Sent: Wednesday, May 23, 2007 12:09 PM
To: stdcxx-dev@incubator.apache.org
Subject: Re: std::vector intialization on HP-UX

Patrick Happel wrote:
> Hello all,
>   I'm working with versions 4.1.2 and 4.1.0 of the STDCXX on HP-UX 11i

> with aCC 3.63, and I've noticed that when a std::vector is constructed

> with a size parameter, it's actually allocated as that size + 32 
> additional elements.  For example:
>  
> #include <iostream>
> #include <vector>
>  
> int main() {
>  std::vector<long> testvec1(4);
>  std::cout << "Capacity: " << testvec1.capacity() << std::endl;  
> std::cout << "Size: " << testvec1.size() << std::endl;  return 0; }
>  
>   When run will output:
>  
> [phappel@moe 15d]$ ./stdvectest
> Capacity: 36
> Size: 4
>  
>   The HP-UX Standard Library will allocate 32 element sizes to a 
> vector declared with a size less than 32, but will allocate as 
> expected beyond that.  STDCXX, however, seems like it will always add
32 to the size.
> I'm curious if there is an explanation for this - is it perhaps some 
> quirk of memory allocation or template instantiation on HP-UX?  Any 
> insight would be appreciated!

Vector in stdcxx computes the capacity, C, from the desired size N, the
Golden Ratio R (1.618), and the minimum capacity Cmin (32), using the
following formula:

   C := max (N * R, N + Cmin)

This is based on the assumption that more vectors will grow than not,
and that by how much they will grow is a function of their current size.
The Golden Ratio is known to strike an optimum balance between the
amount of utilized space and the amount unused space. See, for example:
http://en.wikipedia.org/wiki/Golden_ratio

The HP implementation of vector (Rogue Wave libstd 2.2.1) seems to use a
simpler formula:

   C := max (32, N)

The classic implementation of vector shipped by HP (Rogue Wave libstd
1.2.1) as well as other popular implementations such as GNU libstdc++ or
IBM XLC++ simply use N:

   C := N

Martin


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