CODEWITHSUNDEEP
c++bit-fields
Itachi Uchiwa
Itachi Uchiwa

Reputation: 3164

C++ primer 5th edition ch 19 Bitfieds

I have this example from C++ primer 5th edition. ch-19 Bit fields:

 typedef unsigned int Bit;
 class File {
     Bit mode: 2;       // mode has 2 bits
     Bit modified: 1;   // modified has 1 bit
     Bit prot_owner: 3; // prot_owner has 3 bits
     Bit prot_group: 3; // prot_group has 3 bits
     Bit prot_world: 3; // prot_world has 3 bits
     // operations and data members of File
 public:
     // file modes specified as octal literals; see § 2.1.3 (p. 38)
     enum modes { READ = 01, WRITE = 02, EXECUTE = 03 };
     File &open(modes);
     void close();
     void write();
     bool isRead() const;
     void setWrite();C++ Primer, Fifth Edition
 };

 File &File::open(File::modes m)
 {
     mode |= READ;    // set the READ bit by default
     // other processing
     if (m & WRITE) // if opening READ and WRITE
     // processing to open the file in read/write mode
     return *this;
 }

What makes me wonder is: the mode member is just 2 bits, so it can hold values: 0, 1, 2, 3 ( 00 01 10 11 in binary). The value 3 is defined as an enumeration that specifies the opening mode as execute, but if it is opened here for execute then it is opened too for writing: 3 & 2 = 2 3 & 1 = 1, which I think is a mistake. Normally each mode is independent from any other mode.

I mean, for example, in the member function open(), mode |= READ is OK to set the read bit, which is the first one. Now if(m & WRITE) looks meaningless if the user already has opened the file for Executing 3 (11 in binary).

What do you think?

Upvotes: 1

Views: 83

Answers (1)

Remy Lebeau
Remy Lebeau

Reputation: 596041

The mode field is not being treated as a bitmask, and the modes type is not defining values in powers of 2. So you should not be using bitwise operations to set/query the mode, for exactly this reason that EXECUTE shares bits with READ and WRITE, which goes against your desire to have each mode be independent. In a proper bitmask, EXECUTE would be defined as 4 (100 in binary), not 3.

Upvotes: 4

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