Re: C++ vs C when it comes to speed...

From:
"Mike Wahler" <mkwahler@mkwahler.net>
Newsgroups:
comp.lang.c++
Date:
Thu, 01 Mar 2007 22:34:26 GMT
Message-ID:
<SxIFh.7381$_73.1779@newsread2.news.pas.earthlink.net>
<mast2as@yahoo.com> wrote in message
news:1172787087.012856.47820@z35g2000cwz.googlegroups.com...

I am sure this topic has been discussed a thousand times


Yes, and the answer is the same: Neither language has
a 'speed', only specific implementations can be measured
for speed.

More below.

and I read a
few things about it today on the net. I also want to say I am trying
to start a polemic here, I am just curious and willint to learn and
improve the way I am approaching some coding issues that I have at the
moment. I use C++ programming for my work, but I am not a developper
so please be patient & tolerant in your answers ;-)

Okay for the last few days I have been struggling with mixing some C
and C++ code. I usually try to make my code C++ all the way through.
Recently because I had to implement some old C specs I decided for
some weird reasons to mix C and C++ code. Not a good thing to do I
know. Anyway I really struggled with the idea for the last few days
and spend quite some time going back & fort different versions of the
code, some had more C than C++, some where only C++. I must say that
in what I am doing, execution SPEED is important.

So today I decided to do this very simple test. I wrote the same
functionalities but one version is C++ the other C and run these
functions in a loop (10 000 times).

The C++ versions takes 5 seconds
The C version takes 1 second to execute


Since you don't show both versions, we can't know how
closely the algorithms for each match one another.

This a big difference. I realised that the difference is mostly coming
in the push_back function of the std::vector class. If I comment that
line out, the C++ code runs in 1 second. I used to find the STL lib
very very convenient but I never realised they had such an impact of
the application performances. Here is the program that I used for the
test... Maybe I am doing something wrong so I apologize in advance.

As I said in the pre-ambule of the post, I am trying to be
constructive. The feedbacks I would like to have are more:

1/ i am doing something wrong in the C++ implementation that would
slow it down.


I'll guess yes, probably, if you've done what I think in
the C version: preallocate storage (which you did not do
in the C++ code below).

2/ is it a good thing to do to use C coding in a C++ app


No, never. They are two separate, distinct languages, each
with its own 'best practices'. If you want a C program, write it
in C. If you want a C++ program, write it in C++.

if speed is
an issue


Almost without exception, the largest factor concerning time
performance is algorithms, not language or 'clever tricks'.
Note that with the C++ standard library, all the algorithm
details are not readily apparent.

and I want the app to run as fast as it could.

Thanks everyone.

// 1. comparing speed C vs C++


I suspect you're not comparing the same procedures between
the languages.

More below.

// Running on Max OS X, Power PC G4, 1.5 Ghz
// c++ -o ribparser ribparser.cpp

#include <stdlib.h>
#include <stdio.h>

#include <fstream>
#include <string>
#include <vector>

#include <ctime>

class RibParser
{
 std::string m_ribFile;
public:
 std::ifstream ifs;
 RibParser( std::string ribFile ) : m_ribFile( ribFile )
 {
   char rixm[512];
   try
   {
     ifs.open( ribFile.c_str() );
     if ( ifs.fail() )
     {
       sprintf( rixm, "%s: Can't open\n", ribFile.c_str() );
       throw( rixm );
     }
   }
   catch( char *rixm )
   {
     printf( rixm );
     ifs.close();
     exit( 0 );
   }
   int ch;


      In your C version, I suspect you've defined or allocated an array
whose
      size can accomodate the input. If so, to more closely approximate
that
      with this program, change the following line:

   std::vector<char> token;


to:

     std::vector<char> token(number_of_array_elements_in_C_program);

   while( ! ifs.eof() )


Change the above line to:

     while(ifs)

   {
     ch = ifs.get();
     // comment this line out and the app runs in 1 second
     token.push_back( ch );
   }
 }
 ~RibParser()
 {
   ifs.close();
 }
};

static const size_t ARRAY_INCR = 8;

void RibParserC( const char *ribFile )
{
 char *token;
 size_t tokenByteSize = 0;
 size_t tokenArraySize = ARRAY_INCR;
 FILE *source;
 if ( ( source = fopen( ribFile, "r" ) ) == NULL )
 {
   printf( "%s: Can't open\n", ribFile );
   fclose( source );
   exit( 0 );
 }
 token = (char*)malloc( ARRAY_INCR );
 int ch;
 do
 {
   ch = fgetc( source );
   token[tokenByteSize] = ch;
   tokenByteSize++;
   if ( ( tokenByteSize % tokenArraySize ) == 0 )
   {
     token = (char*)realloc( token, tokenArraySize + ARRAY_INCR );
     tokenArraySize += ARRAY_INCR;
   }
 } while ( ch != EOF );
 fclose( source );
 free( token );
}

int main( int argc, char ** argv )
{
 time_t start, end;
 time( &start );
 for ( size_t i = 0; i < 10000; ++i )
 {
   RibParser ribParser( "/Users/jean-colas/Desktop/comment.rib" );
 }
 time( &end );
 double diff = difftime( end, start );
 printf( "seconds %f %d\n", diff, CLOCKS_PER_SEC );

 time( &start );
 for ( size_t i = 0; i < 10000; ++i )
 {
   RibParserC( "/Users/jean-colas/Desktop/comment.rib" );
 }
 time( &end );
 diff = difftime( end, start );
 printf( "seconds %f %d\n", diff, CLOCKS_PER_SEC );

 return 0;
}

/////


With the small change I indicated above, I suspect you'll see
a dramatic improvement in time performance.

-Mike

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