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Reading a Binary File to a C++ Class

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Join Date: Jun 2005

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WolfPack

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Reading a Binary File to a C++ Class

Jun 9th, 2006

Is it possible to read data from a binary file to a C++ class using istream::read? I know it can be done for structures, but I couldnt do it for a class.

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#include <iostream>
#include <fstream>
 
class BITMAPFILEHEADER
{
private:
    char                type[ 2 ];              // Magic identifier
    unsigned int        size;                   // File size in bytes
    unsigned short int  reserved1;              // Reserved; always zero
    unsigned short int  reserved2;              // Reserved; always zero
    unsigned int        offset;                 // Offset to image data, bytes
 
public:
    friend std::istream& operator>>(std::istream& s, BITMAPFILEHEADER& r);
};
std::istream& operator>>(std::istream& s, BITMAPFILEHEADER& r);
#include <iostream>
#include <fstream>

class BITMAPFILEHEADER
{
private:
    char                type[ 2 ];              // Magic identifier
    unsigned int        size;                   // File size in bytes
    unsigned short int  reserved1;              // Reserved; always zero
    unsigned short int  reserved2;              // Reserved; always zero
    unsigned int        offset;                 // Offset to image data, bytes

public:
    friend std::istream& operator>>(std::istream& s, BITMAPFILEHEADER& r);
};
std::istream& operator>>(std::istream& s, BITMAPFILEHEADER& r);

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std::istream& operator>>(std::istream& s, BITMAPFILEHEADER& r)
{
#if 0
    // This does not work
    s.read( reinterpret_cast <char*>(&r), sizeof(BITMAPFILEHEADER) );
    if( s.bad() )
    {
        std::cerr << "Error reading data" << std::endl;
        exit( 0 );
    }
#endif
#if 1
     // This works
    s.read( r.type, 2 * sizeof(char) );
    if( s.bad() )
    {
        std::cerr << "Error reading data" << std::endl;
        exit( 0 );
    }

    s.read( (char*)&(r.size), sizeof(unsigned int) );
    if( s.bad() )
    {
        std::cerr << "Error reading data" << std::endl;
        exit( 0 );
    }

    s.read( (char*)&(r.reserved1), sizeof(unsigned short int) );
    if( s.bad() )
    {
        std::cerr << "Error reading data" << std::endl;
        exit( 0 );
    }

    s.read( (char*)&(r.reserved2), sizeof(unsigned short int) );
    if( s.bad() )
    {
        std::cerr << "Error reading data" << std::endl;
        exit( 0 );
    }

    s.read( (char*)&(r.offset), sizeof(unsigned int) );
    if( s.bad() )
    {
        std::cerr << "Error reading data" << std::endl;
        exit( 0 );
    }
#endif

    return s;
}

Last edited by WolfPack; Jun 9th, 2006 at 12:49 pm.

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Dave Sinkula Dave Sinkula is offline

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long time no c

Re: Reading a Binary File to a C++ Class

Jun 9th, 2006

I think it is better to take the serialized approach you presented. Even with a POD structure the binary read/write is inherently non-portable.

Nit - consider this:

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s.read( r.type, sizeof r.type );
//s.read( r.type, 2 * sizeof(char) );

// ...

s.read( (char*)&(r.size), sizeof r.size );
//s.read( (char*)&(r.size), sizeof(unsigned int) );

And FWIW:
http://www.parashift.com/c++-faq-lit....html#faq-36.6

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