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Message-ID: <4654832B.20806@roguewave.com>
Date: Wed, 23 May 2007 12:08:43 -0600
From: Martin Sebor <sebor@roguewave.com>
Organization: Rogue Wave Software
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To: stdcxx-dev@incubator.apache.org
Subject: Re: std::vector intialization on HP-UX
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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