Re: What is the output of this program?

I made another post but it was full of bugs. This one compiles and runs
and is more what I had in mind:

#include <string>
#include <algorithm>
#include <ctime>
#include <cstdlib>
#include <stdexcept>
#include <iostream>

void pushbacking( std::string& s, const std::string & source )
{
   s.reserve( source.size() );
   std::copy( source.begin(), source.end(), std::back_inserter( s ) );
}

void resizing( std::string & s, const std::string & source )
{
  s.resize( source.size() );
  std::copy( source.begin(), source.end(), s.begin() );
}

void test( std::string const& fName, void f( std::string&, const
std::string & ), const std::string & source )
{
     std::clock_t const startTime = std::clock();
     std::string s = "knurre, voff!";
     f( s, source );

     std::clock_t const endTime = std::clock();
     double const duration =
     double(endTime - startTime)/CLOCKS_PER_SEC;

    std::cout << fName << ": " << duration << " seconds." << std::endl;
}

int main()
{
  try
  {
    const std::string::size_type stringSize = 10000000; // or whatever
number
    std::string source ( stringSize, '*' );
    test( "pushbacking", pushbacking, source );
    test( "resizing", resizing, source );
    return EXIT_SUCCESS;
  }
 catch ( const std::exception & x )
  {
    std::cerr << "Whoops " << x.what() << '\n';
    return EXIT_FAILURE;
  }
}

On VC8 in release mode:

pushbacking: 1.313 seconds.
resizing: 0.015 seconds.

The point is that push_back requires bounds-checking at every iteration
and resizing does not. The only thing likely to be slow in resizing is
initialisation (which shouldn't be a major problem with std::string).

(Of course, using a std::string member function might be a better idea,
but can you always be certain that your collection is std::string?)

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