b53e0a16   
   From: daniel.kruegler@googlemail.com   
      
   Am 20.09.2011 09:17, schrieb int21h:   
   > I have a protocol that describes packages as sets of bytes. Some bytes   
   > have several   
   > information included, like struct Format below. I need to do something   
   > like "Named Parameter Idiom". But I don't succeed in providing an   
   > "accessor" that returns something different as&BYTE (original type).   
   > How can I accomplish this (ctor with BYTE param, operator= didn't   
   > work)?   
   >   
   > Below is a small sample showing what I try to do. Appreciate your   
   > help.   
   >   
   > typedef unsigned char BYTE;   
   >   
   > template   
   > struct Base {   
   > 	BYTE data_[SIZE];   
   > };   
   >   
   > struct Format {   
   > 	// Does not help: Format(const BYTE&) {}   
   > 	BYTE layout : 3;   
   > 	BYTE length : 5;   
   > };   
   >   
   > template   
   > struct Package : public Base  {   
   > 	static const int OFFSET_SOURCE = 1;   
   > 	static const int OFFSET_TARGET = 2;   
   > 	static const int OFFSET_FORMAT = 0;   
   >   
   >          // kind of " Named Parameter Idiom"   
   > 	BYTE&  source()   { return data_[OFFSET_SOURCE]; }   
   > 	BYTE&  target()   { return data_[OFFSET_TARGET]; }   
      
   Both the source() and the target() implementation should be ill-formed,   
   because Base is a dependent base class and there is no data_ member in   
   Package. You can use this->data_[whatever] instead of data_[whatever],   
   though. This will defer the resolution of the name 'data_' until   
   instantiation time.   
      
   > 	// Next line won't compile: cannot convert from  'BYTE' to   
   >          // 'Format&'   
   > 	// Don't want to use a reinterpret_cast either.   
   > 	Format&  format() { return data_[OFFSET_FORMAT]; }   
   > };   
      
   This approach cannot work, because bit-fields are no addressable units.   
   If it is important to you, that there exists a single memory area (The   
   array member data_), I suggest that you use normal bit-masking   
   techniques to get and set the corresponding bit ranges within   
   data_[OFFSET_FORMAT] that refer to layout and length of the format   
   (Otherwise you could have a member Format and a member data_[SIZE-1]).   
      
   HTH & Greetings from Bremen,   
      
   Daniel Krügler   
      
      
      
      
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