[boost] Re: compressed_pair requirements?

From: Howard Hinnant (hinnant_at_[hidden])
Date: 2005-05-27 14:37:04

On May 27, 2005, at 1:47 PM, Peter Dimov wrote:

> Howard Hinnant wrote:
>
>> 2.  Uninitialized is better because it is much faster, especially if
>> you have containers of such elements.  You can always explicitly
>> initialize if you want.
>
> Technically... you can't put an uninitialized (non-character) scalar 
> in a container; copying an uninitialized scalar is undefined behavior.

<gets an evil look in his eye...>

#include <array>
#include <vector>
#include <tupleio>
#include <iterator>
#include <algorithm>
#include <iostream>

int main()
{
     std::vector<std::tr1::array<int, 3> > v;
     std::tr1::array<int, 3> a;
     for (int i = 0; i < 10; ++i)
         v.push_back(a);
     std::ostream_iterator<std::tr1::array<int, 3> > out(std::cout, 
"\n");
     copy(v.begin(), v.end(), out);
}

(1 0 3195936)
(1 0 3195936)
(1 0 3195936)
(1 0 3195936)
(1 0 3195936)
(1 0 3195936)
(1 0 3195936)
(1 0 3195936)
(1 0 3195936)
(1 0 3195936)

Ha, ha, did it!  And you can't stop me from doing it again! <evil laugh 
follows> :-)

Whatta ya gonna do, send the UBP squad (Uninitialized Behavior Police) 
over to my house?  I'm setting the barricades up now.  I've got 3 dogs 
I can hold hostage too! (well, 2.5 dogs really, one of them is a 
Maltese - http://www.malteseonly.com/ - I'll release the little yapper 
first)  :-)

Ok, kidding aside, yes, you've got technical on your side.  But I've 
got practicality and performance on mine.  On most machines and for 
most scalars, you can copy uninitialized to your heart's content.  And 
when you can't, initialization is both easy and readable.  I find 
forced initialization for everyone everywhere just a little too 
Java-ish.

Next thing you know someone is going to want to take away my precious 
goto. :-)

-Howard

PS:  Whoa, it must be Friday afternoon! :-)