This might bewilder you some more because C++ is a tricky language for a
beginner. It''s a simple thin wrapper class around C stdio that provides
RAII and some return value checks that throw exception when errors
occur. If you can work this out you're well on your way to being
competent. It's mainly meant as a demonstrator for
constructors/destructors which are fundamental to C++ programming.
#include <iostream>
#include <string>
#include <stdexcept>
#include <cstdio>
class File
{
public:
// default constructor
File() : m_file( 0 ) {}
// constructor - opens the file
File( const std::string & filename, const std::string & mode )
: m_file( 0 )
{
std::cout << "constructor\n";
open( filename, mode );
}
// move constructor - takes ownership of the underlying file object
File( File && rhs ) : m_file(0)
{
std::cout << "move constructor\n";
m_file = rhs.m_file;
rhs.m_file = 0;
}
// destructor
~File()
{
close();
}
public:
// opens a file
void open( const std::string & filename, const std::string & mode )
{
std::cout << "opening file " << filename << "\n";
m_file = fopen( filename.c_str(), mode.c_str() );
if (!m_file) throw std::runtime_error( "Error opening file: " +
std::string( strerror( errno ) ) );
}
// closes the files
void close()
{
if (m_file)
{
std::cout << "closing file\n";
fclose( m_file );
m_file = 0;
}
}
// reads from the file
int read( void * buffer, size_t size )
{
return fread( buffer, 1, size, m_file );
}
// writes to the file
int write( const void * buffer, size_t size )
{
int bytesWritten = fwrite( buffer, 1, size, m_file );
if (bytesWritten == 0) // I/O error
{
throw std::runtime_error( std::string( "Error writing to
file: " + std::string( strerror( errno ) ) ) );
}
return bytesWritten;
}
private:
FILE * m_file; // file handle
};
// factory function to demonstrate change of ownership
File openFile( const std::string filename, const std::string & mode )
{
File file( filename, mode );
return file;
}
int main( int argc, char * argv[] )
{
try
{
// open the files
File input = openFile( "DD:INPUT", "rb, type=record, noseek" );
File output( "DD:OUTPUT", "wb, type=record" );
// copy the input file to the output file
size_t bytesRead;
char buffer[32768];
while ( ( bytesRead = input.read( buffer, sizeof buffer ) ) )
{
output.write( buffer, bytesRead );
}
// <<<<< destructors run here when the file objects go out of scope
}
catch (std::exception & e)
{
std::cout << e.what() << "\n";
}
return 0;
}
On 30/05/2017 4:32 AM, Steve Beaver wrote:
Does anyone have a complete piece of C++ code that runs under MVS or Linux that
I can study? 99% of the stuff I write is HLASM and to a point I find C++
bewildering.
TIA
Steve
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