TKey

class TKey : public TNamed

    protected:

       virtual void Create(Int_t nbytes)
      virtual Int_t Read(const char* name)

    public:

                          TKey()
                          TKey(const char* name, const char* title, const TClass* cl, Int_t nbytes)
                          TKey(const TString& name, const TString& title, const TClass* cl, Int_t nbytes)
                          TKey(const TObject* obj, const char* name, Int_t bufsize)
                          TKey(const void* obj, const TClass* cl, const char* name, Int_t bufsize)
                          TKey(Long64_t pointer, Int_t nbytes)
                          TKey(const TKey&)
                  virtual ~TKey()
             virtual void Browse(TBrowser* b)
           static TClass* Class()
             virtual void Delete(Option_t* option = "")
             virtual void DeleteBuffer()
             virtual void FillBuffer(char*& buffer)
            virtual char* GetBuffer() const
                 TBuffer* GetBufferRef() const
      virtual const char* GetClassName() const
                  Short_t GetCycle() const
           const TDatime& GetDatime() const
      virtual const char* GetIconName() const
                  Short_t GetKeep() const
                    Int_t GetKeylen() const
                    Int_t GetNbytes() const
                    Int_t GetObjlen() const
         virtual Long64_t GetSeekKey() const
         virtual Long64_t GetSeekPdir() const
      virtual const char* GetTitle() const
                    Int_t GetVersion() const
          virtual ULong_t Hash() const
          virtual TClass* IsA() const
           virtual Bool_t IsFolder() const
             virtual void Keep()
             virtual void ls(Option_t* option = "") const
                    TKey& operator=(const TKey&)
             virtual void Print(Option_t* option = "") const
            virtual Int_t Read(TObject* obj)
             virtual void ReadBuffer(char*& buffer)
             virtual void ReadFile()
         virtual TObject* ReadObj()
            virtual void* ReadObjectAny(const TClass* expectedClass)
             virtual void SetBuffer()
             virtual void SetParent(const TObject* parent)
             virtual void ShowMembers(TMemberInspector& insp, char* parent)
            virtual Int_t Sizeof() const
             virtual void Streamer(TBuffer& b)
                     void StreamerNVirtual(TBuffer& b)
            virtual Int_t WriteFile(Int_t cycle = 1)

Data Members

    protected:

         Int_t fVersion    Key version identifier
         Int_t fNbytes     Number of bytes for the object on file
         Int_t fObjlen     Length of uncompressed object in bytes
       TDatime fDatime     Date/Time of insertion in file
       Short_t fKeylen     Number of bytes for the key itself
       Short_t fCycle      Cycle number
      Long64_t fSeekKey    Location of object on file
      Long64_t fSeekPdir   Location of parent directory on file
       TString fClassName  Object Class name
         Int_t fLeft       Number of bytes left in current segment
         char* fBuffer     Object buffer
      TBuffer* fBufferRef  Pointer to the TBuffer object

Class Description

                                                                      
  The TKey class includes functions to book space on a file,          
   to create I/O buffers, to fill these buffers,                      
   to compress/uncompress data buffers.                               
                                                                      
  Before saving (making persistent) an object on a file, a key must   
  be created. The key structure contains all the information to       
  uniquely identify a persistent object on a file.                    
     fNbytes    = number of bytes for the compressed object+key       
     fObjlen    = Length of uncompressed object                       
     fDatime    = Date/Time when the object was written               
     fKeylen    = number of bytes for the key structure               
     fCycle     = cycle number of the object                          
     fSeekKey   = Address of the object on file (points to fNbytes)   
                  This is a redundant information used to cross-check 
                  the data base integrity.                            
     fSeekPdir  = Pointer to the directory supporting this object     
     fClassName = Object class name                                   
     fName      = name of the object                                  
     fTitle     = title of the object                                 
                                                                      
  The TKey class is used by ROOT to:                                  
    - to write an object in the Current Directory                     
    - to write a new ntuple buffer                                    
                                                                      
  The structure of a file is shown in TFile::TFile.                   
  The structure of a directory is shown in TDirectory::TDirectory.    
  The TKey class is used by the TBasket class.                        
  See also TTree.                                                     
                                                                      

*-*-*-*-*-*-*-*-*-*-*TKey default constructor*-*-*-*-*-*-*-*-*-*-*-*-*
*-*                  ========================

*-*-*-*-*-*-*-*-*-*-*-*-*Create a TKey object to read keys*-*-*-*-*-*-*-*
*-*                      =================================
  Constructor called by TDirectory::ReadKeys and by TFile::TFile
  A TKey object is created to read the keys structure itself

 Create a TKey object for a TObject* and fill output buffer

 Create a TKey object for any object obj of class cl  and fill output buffer

 Read object from disk and call its Browse() method.
 If object with same name already exist in memory delete it (like
 TDirectory::Get() is doing), except when the key references a
 folder in which case we don't want to re-read the folder object
 since it might contain new objects not yet saved.

*-*-*-*-*-*-*-*-*-*-*-*-*Create a TKey object *-*-*-*-*-*-*-*-*-*-*-*-*-*
*-*                      ====================

*-*-*-*-*-*-*-*-*-*-*TKey default destructor*-*-*-*-*-*-*-*-*-*-*-*-*
*-*                  =======================

*-*-*-*-*-*-*-*-*-*-*-*Delete an object from the file*-*-*-*-*-*-*-*-*-*-*
*-*                    ==============================
 Note: the key is not deleted. You still have to call "delete key".
 This is different from the behaviour of TObject::Delete()!

*-*-*-*-*-*-*-*-*-*-*-*Delete key buffer(s)*-*-*-*-*-*-*-*-*-*-*-*-*-*-*
*-*                    ====================

*-*-*-*-*-*-*-*-*-*-*-*-*Return cycle number associated to this key*-*-*-*
*-*                      ==========================================

*-*-*-*-*-*-*-*-*-*-*-*-*Returns the "KEEP" status*-*-*-*-*-*-*-*-*-*-*-*
*-*                      =========================

*-*-*-*-*-*-*-*-*-*-*-*Encode key header into output buffer-*-*-*-*-*-*-*
*-*                    ====================================

 This Hash function should redefine the default from TNamed

 Check if object referenced by the key is a folder.

*-*-*-*-*-*-*-*-*-*-*-*-*Set the "KEEP" status*-*-*-*-*-*-*-*-*-*-*-*-*-*
*-*                      =====================
 When the KEEP flag is set to 1 the object cannot be purged

*-*-*-*-*-*-*-*-*-*-*-*-*List Key contents-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*
*-*                      =================

*-*-*-*-*-*-*-*-*-*-*-*-*Print key contents*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*
*-*                      ==================

 To read a TObject* from the file

  The object associated to this key is read from the file into memory
  Once the key structure is read (via Streamer) the class identifier
  of the object is known.
  Using the class identifier we find the TClass object for this class.
  A TClass object contains a full description (i.e. dictionary) of the
  associated class. In particular the TClass object can create a new
  object of the class type it describes. This new object now calls its
  Streamer function to rebuilt itself.

  see TKey::ReadObjectAny to read any object non-derived from TObject

  NOTE:
  In case the class of this object derives from TObject but not
  as a first inheritance, one must cast the return value twice.
  Example1: Normal case:
      class MyClass : public TObject, public AnotherClass
   then on return, one can do:
    MyClass *obj = (MyClass*)key->ReadObj();

  Example2: Special case:
      class MyClass : public AnotherClass, public TObject
   then on return, one must do:
    MyClass *obj = dynamic_cast<MyClass*>(key->ReadObj());

  Of course, dynamic_cast<> can also be used in the example 1.

  To read an object (non deriving from TObject)  from the file

  If expectedClass is not null, we checked that that actual class of
  the object stored is suitable to be stored in a pointer pointing
  to an object of class 'expectedClass'.  We also adjust the value
  of the returned address so that it is suitable to be cast (C-Style)
  a  a pointer pointing to an object of class 'expectedClass'.

  So for example if the class Bottom inherits from Top and the object
  stored is of type Bottom you can safely do:

     TClass *TopClass = TClass::GetClass("Top");
     Top *ptr = (Top*) key->ReadObjectAny( TopClass );
     if (ptr==0) printError("the object stored in the key is not of the expected type\n");

  The object associated to this key is read from the file into memory
  Once the key structure is read (via Streamer) the class identifier
  of the object is known.
  Using the class identifier we find the TClass object for this class.
  A TClass object contains a full description (i.e. dictionary) of the
  associated class. In particular the TClass object can create a new
  object of the class type it describes. This new object now calls its
  Streamer function to rebuilt itself.

*-*-*-*-*-*-*-*-*-*-*-*-*To read an object from the file*-*-*-*-*-*-*-*-*
*-*                      ===============================
  The object associated to this key is read from the file into memory
  Before invoking this function, obj has been created via the
  default constructor.

*-*-*-*-*-*-*-*-*-*-*-*Decode input buffer-*-*-*-*-*-*-*-*-*-*-*-*-*-*
*-*                    ===================

*-*-*-*-*-*-*-*-*-*-*-*-*Read the key structure from the file-*-*-*-*-*-*
*-*                      ====================================

  Set parent in key buffer

*-*-*-*-*-*-*-*Return the size in bytes of the key header structure*-*-*-*
*-*-*-*-*-*-*-*====================================================
Int_t nbytes = sizeof fNbytes;      4
            += sizeof(Version_t);   2
            += sizeof fObjlen;      4
            += sizeof fKeylen;      2
            += sizeof fCycle;       2
            += sizeof fSeekKey;     4 or 8
            += sizeof fSeekPdir;    4 or 8
             =                     22

*-*-*-*-*-*-*-*-*Stream a class object*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*
*-*              =========================================

*-*-*-*-*-*-*-*-*-*-*Write the encoded object supported by this key*-*-*-*
*-*                  ==============================================

 The function returns the number of bytes committed to the file.
 If a write error occurs, the number of bytes returned is -1.

 title can keep 32x32 xpm thumbnail/icon of the parent object.

 returns title (title can contain 32x32 xpm thumbnail/icon)

Inline Functions

           const char* GetClassName() const
                 char* GetBuffer() const
              TBuffer* GetBufferRef() const
        const TDatime& GetDatime() const
                 Int_t GetKeylen() const
                 Int_t GetNbytes() const
                 Int_t GetObjlen() const
                 Int_t GetVersion() const
              Long64_t GetSeekKey() const
              Long64_t GetSeekPdir() const
                 Int_t Read(TObject* obj)
                  void SetBuffer()
               TClass* Class()
               TClass* IsA() const
                  void ShowMembers(TMemberInspector& insp, char* parent)
                  void StreamerNVirtual(TBuffer& b)
                  TKey TKey(const TKey&)
                 TKey& operator=(const TKey&)