Question

I was learning stl and never saw this kind of class like class classname :: reference {}

I searched on the net but could get good information ..

class bitset::reference {
  friend class bitset;
  reference();                                 // no public constructor
public:
  ~reference();
  operator bool () const;                      // convert to bool
  reference& operator= ( bool x );             // assign from bool
  reference& operator= ( const reference& x ); // assign from bit
  reference& flip();                           // flip bit value
  bool operator~() const;                      // return inverse value
};

• what is this :: reference here for ?

i saw this code here[enter link description here][1]
http://www.cplusplus.com/reference/stl/bitset/
i have worked on c++ before.

Answer 1

Did you look at the bitset class definition? There is something like this somewhere:

template<size_t _Bits>
class bitset
{
    ...
    class reference;
    ...
}

It’s much like putting the body of a funciton outside the class body. Now we are putting the body of a nested class outside of the parent class:

class bitset::reference
{
    /* class body */
}

By the way, in MSVC (C:\Program Files\Microsoft Visual Studio 9.0\VC\include\bitset) the’re actually defined inside each other:

// TEMPLATE CLASS bitset
template<size_t _Bits>
class bitset
{   // store fixed-length sequence of Boolean elements
typedef unsigned long _Ty;  // base type for a storage word
enum {digits = _Bits};  // extension: compile-time size()

public:
typedef bool element_type;  // retained

    // CLASS reference
class reference
    {   // proxy for an element
    friend class bitset<_Bits>;
    .
    .
    .

It’s the same for g++’s bitset.h, although a bit more complex.