C++ Hacks

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Bit twiddling

Various bit twiddling hacks can be found at S. Anderson's Bit Twiddling Hacks site. Additional hacks I use in my C++ projects can be found here:

Parallel swapping of odd and even bits of a word

Swap the odd and even bits of a word. Example with 16-bit words:

      b15b14b13b12 b11b10b9b8 b7b6b5b4 b3b2b1b0
      →
      b14b15b12b13 b10b11b8b9 b6b7b4b5 b2b3b0b1
    

The following function works for 32-bit words

      unsigned int swapOddEven( unsigned int v )
      {
          return ( ( v & 0xAAAAAAAA ) >> 1 ) | 
                 ( ( v & 0x55555555 ) << 1 );
      }
    

The first part

( v & 0xAAAAAAAA ) >> 1

selects the odd bits of v and shifts them to the even positions, the second part

( v & 0x55555555 ) << 1

selects the even bits of v and shifts them to the odd positions.

STL

Selective erasing of items from a std::set

Erase all elements of the set s meeting the condition:

      std::set<...> s;
     
      for( std::set<...>::iterator i = s.begin(); i != s.end(); )
      {
          if( condition( *i ) )
          {
              s.erase( i++ );
          }
          else
          {
              ++i;
          }
      }
    

You may alternatively use STLs remove_if algorithm, but this requires the specification of a condition predicate functor, which is not needed in the above solution.

T to string conversion

A template method to convert any type T to a std::string using an output string stream. The only condition on T is the existence of an output operator for T (std::ostream& operator<<( std::ostream&, const T& )

      #include <sstream>
      #include <string>
      
      /* creates a std::string from a value of type T */
      template<typename T> std::string toString( const T& value )
      {
          /* create an output string stream */
  	  std::ostringstream oss;
          /* output the value to the stream */
	  oss << value;
          /* return the streams string representation */
	  return oss.str();
      }
    

You can specialize this function for std::string parameters to avoid unnecessary conversions:

      template<> std::string toString( const std::string& value )
      {
          /* just return the value. no conversion needed */
	  return value;
      }