Tag Archives: c++

Operator overloading in C++

Posted on by
33

I did my first operator overloading in C++ today. My code ended up being quite different to the example code that I found, but I think I’m happy with it:

class FileData {
public:
  FileData( const string& path );
  static bool LessThan( const FileData& a, const FileData& b );
  bool operator< ( const FileData& );
  unsigned long size() const { return m_size; }
private:
  unsigned long m_size;
  char m_md5[ 16 ];
};

static bool operator< ( const FileData& a, const FileData& b ) {
  return FileData::LessThan( a, b );
}

bool FileData::operator< ( const FileData& b ) {
  return FileData::LessThan( *this, b );
}

bool FileData::LessThan ( const FileData& a, const FileData& b ) {
  unsigned long a_size = a.size();
  unsigned long b_size = b.size();
  if ( a_size == b_size ) {
    return memcmp( a.m_md5, b.m_md5, 16 ) < 0;
  }
  return a_size < b_size;
}

I'm not sure when it becomes either useful or necessary to define the operator both as a static and as a member function. I did both because I wasn't sure what was required.

Differences between C++ pointers and C++ references

Posted on by
41

I was curious about the difference between C++ pointers and C++ references, so I searched and found this which says that basically:

  1. It’s not necessary to initialise pointers at declaration time, but it is necessary to initialise references at declaration time.
  2. You can create an array of pointers, but you can’t create an array of references.
  3. You can assign null to pointers, but you can’t assign null to references.

Reading binary files in C++

Posted on by
Reply

I learned how to read binary files in C++ today. My function (which creates an MD5 hash of a file) ended up looking a little different to the example that I learned from:

  const int BUFFER_SIZE = 1024;

  int length;
  char buffer[ BUFFER_SIZE ];

  MD5_CTX ctx;
  MD5Init( &ctx );

  ifstream is( path.c_str(), ios::binary );

  while ( is.good() ) {

    is.read( (char *)buffer, BUFFER_SIZE );

    streamsize count = is.gcount();

    MD5Update( &ctx, (unsigned char *)buffer, count );

  }

  if ( is.eof() ) {

    // it's ok, we're at the end of the file

  }
  else if ( is.bad() ) {

    // bad bit is set
    cout << "Bad bit is set while reading '" << path << "'." << endl;

    cout << strerror( errno ) << endl;

    exit ( 1 );

  }
  else if ( is.fail() ) {

    cout << "Fail bit is set while reading '" << path << "'." << endl;

    cout << strerror( errno ) << endl;

    exit( 1 );

  }

  is.close();

Fatal error LNK1120: unresolved externals, caused by C functions in C++

Posted on by
43

I was receiving link error LNK1120 in Visual Studio after adding some C code to my C++ project, and the problem was, as I discovered here (and found more info on here), that I hadn’t declared my C code as “extern C”.

So basically I added #ifdef __cplusplus macros to check if it was C or C++ code and if C++ then outputting “extern C {” with a suitable “}” at the end of the file.

Don’t use memcmp to compare structs or classes

Posted on by
42

Structs and classes can be laid out with padding between the data members for alignment purposes, and this padding doesn’t get initialised unless you specifically zero it out yourself. So if you’re using memcmp and comparing memory at pointers to structs you might have a problem if you’re expecting structs with equal data members to always be equal.