Re: Strange result

From:
"Alf P. Steinbach" <alfps@start.no>
Newsgroups:
comp.lang.c++
Date:
Sun, 09 Mar 2008 11:35:56 +0100
Message-ID:
<13t7fcffm96uif0@corp.supernews.com>
* Paul Brettschneider:

Hi,

Alf P. Steinbach wrote:

1. The program below should theoretically not run on my old & clunky
machine,
since theoretically it allocates 2 to 4 GB. In reality, according to
Windows
Task Manager, it allocates only some 20 MB tops. And runs fine, though
slow...

2. With MSVC, and/or with 10.000 or fewer iterations and Op vector
elements, the inefficient reference strings are faster than std::string
string, as expected. On my machine, with g++ and 100.000 iterations, the
opposite happens, and the
machine trashes on allocation and deallocation for the ref strings. I
guess on a modern machine that limit must be higher (yet another factor of
10?), but I'm interested whether (1) this can be reproduced, and (2)
whether anyone has any explanation (at a guess something causes a lot of
memory to be allocated, but it doesn't show up in Task Manager).


I think you're missing copy constructors, see below:

Disclaimer: this is late for me, so thinking box not entirely sharp...

#include <boost/progress.hpp>
#include <boost/shared_ptr.hpp>
#include <iostream>
#include <ostream>
#include <vector>
#include <string>

class RefString
{
private:
     boost::shared_ptr<std::string> myString;
public:
     RefString( size_t n, char c )
     : myString( new std::string( n, c ) )
     {}


// Missing copy constructor for non-PODs:
     RefString(const RefString &s)
     : myString(s.myString)
     { };
     RefString &operator=(const RefString &s)
     { myString = s.myString; return *this; };


Nope, these are effectively the same as those generated automatically.

};

template< class String >
struct Op_
{
     String s;
     std::vector<String> v;

     Op_(): s( 200, ' ' ), v( 100, s ) {}


// Missing copy constructor for non-PODs:
     Op_(const Op_ &o)
     : s(o.s), v(o.v) { };
     Op_ &operator=(const Op_ &o)
     { s = o.s; v = o.v; return *this; };


Ditto, no need to reproduce what the language provides automatically.

Except if there's some bug in g++...

[snip]

HTH,
Paul


Well, it could have. :-) So thanks.

Cheers,

- Alf

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