Strange result

"Alf P. Steinbach" <>
Sun, 09 Mar 2008 09:40:37 +0100
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).

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
     boost::shared_ptr<std::string> myString;
     RefString( size_t n, char c )
     : myString( new std::string( n, c ) )

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

     Op_(): s( 200, ' ' ), v( 100, s ) {}
     void operator()() { v.insert( v.begin(), s ); }

template< class String >
void doTest()
     using namespace std;
     typedef Op_<String> Op;
     vector<Op> op( 100000 );

     boost::progress_timer timer;
     for( size_t i = 0; i < op.size(); ++i )

template< class String >
void doNamedTest()
     std::cout << typeid(String).name() << ": ";

int main()
     for( int i = 1; i <= 5; ++i )
         std::cout << "TEST #" << i << ":" << std::endl;
         std::cout << std::endl;


Cheers, & TIA.,

- Alf

A: Because it messes up the order in which people normally read text.
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