Save and Restore

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A general purpose manager for saving and restoring values and objects

Quite often when writing code, one needs to save a value or object, perform
some operation on it, and then restore the original value. This mean
creating a variable to hold the saved state, and initialising it, at the
beginning of a block, and then writing code to restore it at the end.
Unfortunately, the code needed to save and restore an object is often
dependant on the object itself. For example, a device context (CDC) uses
SaveDC and RestoreDC; the working directory uses ::GetCurrentDirectory and
::SetCurrentDirectory; a document might serialise to/from an in-memory
file.

The following two template classes encapsulate the save and restore
procedure. They are general enough for most situations.

The TSaveAndRestoreGeneral template class is, as the name implies, the more
general of the two. It requires that you write a global Save and Restore
function for the object you are managing. This template class has two
paramters. The first is the type of object to be managed, and the second
the type of value used to hold the state information. The state
information may often not be the same type as the object being managed.
For example, a CDC uses an int as its state value (using SaveDC and
RestoreDC).

The TSaveAndRestoreSimple class is for the simpler cases where the object
being managed and the state information are the same type. In other words,
one simply copies the original value to save, and copies it back to
restore. The class includes the simple Save and Restore functions so you
don’t need to write them yourself.

Here is the source code for these two template classes .

// SaveAndRestore.h
// (c) 1997 Roger Onslow
#ifndef _TSaveAndRestore_
#define _TSaveAndRestore_

template <class T, class TSAVE> class TSaveAndRestoreGeneral {

     // Write specific versions of these for the object type you want to manage
public:
     friend void Save(const T& t, TSAVE& savevalue);
     friend void Restore(T& t, const TSAVE& savevalue);

     // Management of save/restore - not usually directly called
protected:
     T& m_variable; // reference to variable we are managing here
     TSAVE m_savevalue;  // the saved value
public:
     void SaveInto(TSAVE& savevalue) { ::Save(m_variable,savevalue); }
     void SetFrom(const TSAVE& setvalue) { ::Restore(m_variable,setvalue); }
     void RestoreNow() { SetFrom(m_savevalue); }

     // Control auto-restore
protected:
     bool m_bKeep;  // is set, do NOT restore in constructor
public:
     void Keep() { m_bKeep = true; }
     void NoKeep() { m_bKeep = false; }

     // Constructor and destructor do the save and restore
public:
     TSaveAndRestoreGeneral(T& t) : m_variable(t), m_bKeep(false) { SaveInto(m_savevalue); }
     ~TSaveAndRestoreGeneral() { if (! m_bKeep) RestoreNow(); }
};

template <class T> class AFX_EXT_CLASS TSaveAndRestoreSimple : public TSaveAndRestoreGeneral {
     // Simple save and restore functions just take current value
     friend void Save(const T& t, T& savevalue) { savevalue = t; }
     friend void Restore(T& t, const T& savevalue) { t = savevalue; }

     // Constructor just calls base class
public:
     TSaveAndRestoreSimple(T& t) : TSaveAndRestoreGeneral(t) {}
};

#endif

The TSaveAndRestoreGeneral class includes some additional member functions
to change the way the save and restore works.

Keep() turns off the automatic restore. In other words, if you make a
change to the managed object and want to keep it, rather that restore the
previous value, then call the Keep() function. This can be used for an
undo/redo situation, where you may make a change and then optionally want
to undo it. NoKeep() is the opposite function, and turns the auto-restore
back on.

RestoreNow() lets you restore the original value at any time, not just when
the save/restore manager goes out of scope.

SetFrom() and SaveInto() let you save and restore the state to an external
state variable. These functions would not usually be called outside their
use internally in the TSaveAndRestoreGeneral class, but have been made
public “just in case”.

Below is an example of how to define a save and restore manager for a
simple class (like CString) using TSaveAndRestoreSimple.

typedef TSaveAndRestoreSimple CSaveAndRestoreString;

And here is an example of how it would be used.

class CMyClass {
     CString m_mystring;
     void MyFunc () {
          CSaveAndRestoreString saveit(m_mystring);
          // ...
          // do things to m_mystring
          // ...
          // m_mystring is restored at the end
     }
};

When member function ?MyFunc? starts, the current value of ?m_mystring? is
saved. At the end of the function, when ?saveit? goes out of scope, the
value of ?m_mystring? is restored.

A more complex example would is to save and restore a device context. In
this case, a simple copy of the object is not appropriate, so we use
TSaveAndRestoreGeneral. A CDC uses an int to save and restore its current
state, so we use as our template parameters.

// SaveAndRestoreDC.h
// (c) 1997 Roger Onslow
#ifndef _CSaveAndRestoreDC_
#define _CSaveAndRestoreDC_

#include "SaveAndRestore.h"

// Save and Restore a DC - encapulates SaveDC and RestoreDC
inline void Save(const CDC& dc, int& savevalue) {
     savevalue = const_cast(&dc)->SaveDC();
}
inline void Restore(CDC& dc, const int& savevalue) {
     dc.RestoreDC(savevalue);
}

typedef TSaveAndRestoreGeneralint> CSaveAndRestoreDC;

#endif

Here, we have to write our Save and Restore functions. Notice the
const_cast required in the Save routine because, unfortunately, SaveDC is
NOT a const function (even though it does not change the CDC object).

A more interesting example is to use these classes to save and restore the
current working directory.

Firstly, we need a class that corresponds to, and encapsulates, the current
directory functions.

// CurrentDirectory.h
// (c) 1997 Roger Onslow
#ifndef _CCurrentDirectory_
#define _CCurrentDirectory_

class AFX_EXT_CLASS CCurrentDirectory {
     // Construction
public:
     CCurrentDirectory() {}   // nothing to do
     CCurrentDirectory(LPCTSTR dir) { *this = dir; }    // do an assignment

     // Convert to/from string
public:
     operator CString() const;     // return current dir as a string
     CCurrentDirectory& operator= (LPCTSTR dir);   // set current dir
};

#endif

This class lets you treat the current directory as a string. You can
assign a string to a variable of type CCurrentDirectory, and this will
change the current directory. Similarly you can use a value of type
CCurrentDirectory whereever you need a CString, and it will return the
current directory name. Notice that this object does not require any
storage – it has no member variables and no virtual functions.

Using this class, we can easily make a new class that saves and restores
the current directory.

// SaveAndRestoreCurrentDirectory.h
// (c) 1997 Roger Onslow
#ifndef _CSaveAndRestoreCurrentDirectory_
#define _CSaveAndRestoreCurrentDirectory_

#include "SaveAndRestore.h"
#include "CurrentDirectory.h"

// Save and Restore current directory
inline void Save(const CCurrentDirectory& cd, CString& savevalue)
{savevalue = cd; }

inline void Restore(CCurrentDirectory& cd, const CString& savevalue)
{ cd = savevalue; }

class AFX_EXT_CLASS CSaveAndRestoreCurrentDirectory :
	public TSaveAndRestoreGeneral
{
     CCurrentDirectory m_cd;
public:
     CSaveAndRestoreCurrentDirectory(LPCTSTR str) : m_cd(),
		TSaveAndRestoreGeneral(m_cd) { m_cd = str; }
};

#endif

This class is slightly different to the previous classes in that the
constructor takes the name of the temporary current directory required.

You can use this function in you OnFileOpen, OnFileSave and OnFileSaveAs
functions to force these to look in a given directory. For example

void CMyDoc::OnFileSave( ) {
     CSaveAndRestoreCurrentDirectory saveandrestore(DATADIR);
     CDocument::OnFileSave();
}

void CMyDoc::OnFileSaveAs( ) {
     CSaveAndRestoreCurrentDirectory saveandrestore(DATADIR);
     CDocument::OnFileSaveAs();
}

void CMyApp::OnFileNew( ) {
     CSaveAndRestoreCurrentDirectory saveandrestore(DATADIR);
     CWinApp::OnFileNew();
}

Here, DATADIR would be defined as the directory for your data files. For
example, you might use a variable that is initialised from a registry
setting.

In summary, these classes take a lot of the burden out of saving and
restoring objects and values. The specific logic to so for a given object
type can be encapsulated in a class and reused whenever it is required.

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