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QFile f("in.txt");
int n = -1;
if (f.open(QIODevice::ReadOnly | QIODevice::Text) {
   QByteArray buf;
   do {
       buf = f.readLine();
       n++;
   } while (!buf.isEmpty());
}

int n = 0;
QFile file("in.txt");
if (file.open(QFile::ReadOnly)) {
    char buf[1024]; // либо QVarLengthArray - для произвольной длины строки
    qint64 lineLength;
    forever {
        lineLength = file.readLine(buf, sizeof(buf));
        if (lineLength > 0)
            ++n;
        else if (lineLength == -1)
            //###
        else
            break;
   }
}

#include <iostream>
#include <fstream>
#include <iterator>
#include <algorithm>
#include <stdexcept>

#include <QtGlobal>
#include <QTime>
#include <QFile>
#include <QTextStream>
#include <QStringList>

using namespace std;

/**
** sendevent's variant.
** i think it's possible to get a correct "endl" from the Qt, but idk how =(
*/
#if defined(Q_OS_MAC)
    #define CURRENT_LINES_DELEMITER "\r"
#elif defined(Q_OS_WIN)
    #define CURRENT_LINES_DELEMITER "\r\n"
#elif defined(Q_OS_LINUX)
    #define CURRENT_LINES_DELEMITER "\n"
#else
    message("define your lines ending!")
#endif //-- Q_OS_MAC|Q_OS_WIN

unsigned long sendevent( const char* pFileName )
{
    QFile file( pFileName );
    unsigned long res = 0;
    if( file.open( QIODevice::ReadOnly ) )
    {
        res = file.readAll().count( CURRENT_LINES_DELEMITER );
        file.close();
    }

    return res;
}

/**
** nixMan's variant.
** google it yourselth how to change it for compille with MS VS
** and ignore whitespaces ;-)
*/
unsigned long nixMan( const char* pFileName ) {
   ifstream file( pFileName );
   if ( !file ) {
      throw runtime_error( string( "can`t open file: " ).append( pFileName ) );
   }
   return (unsigned long)distance(
      (istream_iterator< string >(file)),
      (istream_iterator< string >())
   );
}

/**
** garryHotDog's lines counter
*/
unsigned long garryHotDog( const char *pFileName )
{
    QFile file( pFileName );
    unsigned long linecount=0x00;
    
    if( file.open( QIODevice::ReadOnly ) )
    {
        QTextStream stream( &file );
        // количество строк.
        
        QString line;
        do 
        {
            line = stream.readLine();
            // увеличим счетчик строк
            if(!line.isNull()) { linecount++;}
        } while (!line.isNull());
        
        file.close();
    }
    
    return linecount;
}

unsigned long kambala( const char *pFileName )
{
    unsigned long n = 0;
    QFile f( pFileName );
    if ( f.open(QIODevice::ReadOnly | QIODevice::Text ) )
    {
        n = QString( f.readAll() ).split(CURRENT_LINES_DELEMITER).count();
        f.close();
    }

    return n;
}

unsigned long konstantin( const char *pFileName )
{
    unsigned long n = 0;
    QFile file( pFileName );
    if ( file.open( QFile::ReadOnly ) )
    {
        char buf[1024]; // либо QVarLengthArray - для произвольной длины строки
        qint64 lineLength;
        forever
        {
            lineLength = file.readLine(buf, sizeof(buf));
            if (lineLength > 0)
            {
                ++n;
            }
            else if (lineLength == -1)
            {
                break;
            }
        }
        file.close();
    }

    return n;
}


/**
** test stuff
*/

typedef unsigned long ( *pointer2function )( const char* );
const char* callTestFunction( pointer2function pFunc, const char *pArg )
{
    QTime time;
    unsigned long iLinesCount, iMsecs;
    
    time.start();
    iLinesCount = pFunc( pArg );
    iMsecs = time.elapsed();
    
    QString strRes( "%1 lines were found at %2 ms" );
    return strRes.arg( iLinesCount ).arg( iMsecs ).toAscii();
}

void createTestFile( const char *pFileName, const char *line,
                     unsigned long ulLinesAmount )
{
    QFile file( pFileName );
    if( file.open( QIODevice::WriteOnly ) )
    {
        QTextStream txtStream( &file );
        for( unsigned long i = 0; i < ulLinesAmount; ++i )
        {
            txtStream << line << i+1 << CURRENT_LINES_DELEMITER;
        }
        file.close();
        cout  << endl << "The " << pFileName << " contains " << ulLinesAmount
                << " lines \""<< line << "\"" << endl;
    }
}

void test( const char* pFile, const char *pLine, unsigned long ulLinesCount, int iTests )
{
    string strFileName( pFile );
    strFileName.append( ".txt" );

    const char *pFileName = strFileName.c_str();
    createTestFile( pFileName, pLine, ulLinesCount );
    pointer2function ptr2Func = NULL;

    for( int i = 0; i < iTests; ++i )
    {
        ptr2Func = &::nixMan;
        cout << callTestFunction( ptr2Func, pFileName ) << " by nixMan" << endl;

        ptr2Func = &::garryHotDog;
        cout << callTestFunction( ptr2Func, pFileName ) << " by harryHotDog" << endl;

        ptr2Func = &::sendevent;
        cout << callTestFunction( ptr2Func, pFileName ) << " by sendevent" << endl;

        ptr2Func = &::kambala;
        cout << callTestFunction( ptr2Func, pFileName ) << " by kambala" << endl;

        ptr2Func = &::konstantin;
        cout << callTestFunction( ptr2Func, pFileName ) << " by konstantin" << endl;
    }
}

int main(int /*argc*/, const char** /*argv*/) 
{
    unsigned long ulShortTest = 1024;
    unsigned long ulLongTest = ulShortTest*ulShortTest;

    test( "testShortWithoutSpaces", "line_line", ulShortTest, 1 );
    test( "testShortWithSpaces", "line line",  ulShortTest, 1 );
    test( "testLongtWithoutSpaces", "line_line", ulLongTest, 1 );
    test( "testLongtWithSpaces", "line line", ulLongTest, 1 );
    
    return 0;
}

istream operator>>(istream& is, MyCustomStringImp& s)

void createLongFile( const QString& strFileName )
{
    QFile srcFile( strFileName );
    if( srcFile.open( QIODevice::ReadWrite ) )
    {
        QByteArray baContent = srcFile.readAll();
        for( long i = 65536*64; i >= 0; --i )
        {
            srcFile.write( baContent );
        }
    }
}