418 Technique 66: Documenting the Data Flow

LISTING 66-1 (continued)

_previous.push( sold );

}

else

{

State sold(name, “”); _previous.push( sold );

}

char szBuffer[20]; sprintf(szBuffer, “%d”, value ); _props[name] = szBuffer;

}

string getProperty( const char *name )

{

return _props[ name ];

}

}

// Pop off the last change State s = _previous.top();

_previous.pop();

// Apply it _props[s.getName()] = s.

getValue();

}

Here, because we’re tracking all changes to the object anyway, undoing one of those changes is trivial. Adding this sort of code to the system at the outset — rather than trying to build it in later — makes for a very robust system that’s also easy to debug.

4. Save the source code in your code editor.

Okay, there’s nothing particularly special about this code — it simply allows you to add new properties to an object, modify them as you see fit, and retrieve them. It also keeps track of all of the changes to a given object, which would allow you to log those changes, or even implement an undo system. To give you an idea of how simple it would be to add functionality to a system based on this object, the next step adds an

undo method for the Properties object. The

Properties class, shown at 1, keeps track of a
list of properties for other objects.	Within the
class, the State class (shown at	2) is used to

maintain a list of the various values for each

property. When a value is changed, a new State object is created with the old value and stored (see 3). This will allow us to implement undo very simply.

3. Add the following code to the class listing:

void undo()

{

if ( _previous.empty() ) return;

Testing the Properties Class

After you create a class, you should create a test driver that not only ensures your code is correct, but also shows people how to use your code. The following steps tell you how:

1. In the code editor of your choice, reopen the source file to hold the code for your test program.

In this example, I named the test program ch66.cpp.

2. Type the code from Listing 66-2 into your file.

Better yet, copy the code from the source file on this book’s companion Web site.

LISTING 66-2: THE PROPERTY TEST PROGRAM

int main()

{

Properties	p;
p.setProperty( “x”, 12 );			4
cout << “x	= “ << p.getProperty

(“x”).c_str() << endl;

Testing the Properties Class 419

p.setProperty(		“x”, 13 );		5
cout <<	“x = “	<<
p.getProperty(“x”).c_str() << endl;				6
p.undo();
cout <<	“x = “	<<

p.getProperty(“x”).c_str() << endl;

}

If you have done everything properly, you should see the following output from the program on the console window:

$ ./a.exe x = 12

x = 13 x = 12

The test program simply creates a Properties object and adds a new property called x to it

(shown at 4). We print out the value of that
property, then	change it 5. The value is then

printed out again to verify the change. At this

point, we undo the last change for the Properties object by using the undo method at 6. We would then expect the value of x to be its previous value, 12, rather than the current value of 13.

3. Save the source code in the source-code editor and close the editor application.

4. Compile the source code with your favorite compiler on your favorite operating system.

5. Run the program on your favorite operating system console.

As we expected, the value of x changes from the last set value, 13, to its previous setting of 12 due to the undo function call. The undo functionality, besides being useful in a program by itself, also shows you how to track changes to data within the program. This ability will make it very simple to debug applications by seeing exactly how things changed over time.

As you can see, the system properly stores the information for the properties and easily implements the undo system. This type of object could easily be dropped into a “regular” object to replace the entire member variable list.

Tracking and documenting the data flow within an application is the single most important thing you can do for programmers, maintainers, debuggers, and customer support personnel. Data flow is what the user cares about and the QA department uses to validate your system. By making it easy to see what changes and when it happens, you save yourself immense amounts of time later on in the development process.

67 Creating a Simple

Locking Mechanism

Technique

Save Time By

“Locking” functionality in an application

Creating a locking mechanism

Testing the locking mechanism

From time to time, programmers must “lock out” the functionality of a given application. There are many possible reasons for this necessity: If you’re running a multithreaded application, for example, you

need to keep multiple threads from hitting the same function at the same time. Or perhaps you’re writing an application in which a resource (such as a hardware device) can be accessed only at certain times.

“Locking” a program means denying the program access into a given block of code until a certain condition occurs. Not unlike a finite-state machine, a lock mechanism can force a system into certain transitions (movements from one state to another) only when they are ready to happen — which ensures a predictable outcome and avoids unforeseen circumstances. Locking mechanisms save time for the developer by reducing hard to reproduce errors and problems that can only be tested in multi-user environments.

There are many ways to implement locking in an application. Most operating systems provide heavyweight critical-section handlers that can be used to lock only small pieces of code at the hardware level. These mechanisms, however, are intended only for serious multithreading; they impose too much overhead in terms of processing time, memory required, and code required, for the average application to utilize. What is really needed is a more lightweight system — with little or no overhead — that you can use to lock global resources within your application code at run-time. Filling that need is the purpose of this technique.

Portability is one big advantage of implementing and employing your own locking mechanism instead of a system-level lock. Using your own locking mechanism allows your code to port easily from system to system without requiring extensive rewrites (often necessary when you use a new compiler or operating system). Even if you choose to use the underlying system support to lock your application, you should wrap that functionality in your own class so it’s the only place you have to make changes when you move to a new operating system, compiler, or version of the library code.

Creating the Locking Mechanism 421

Creating the Locking

Mechanism

Giving your code the appropriate locking functionality is pretty straightforward. The following steps show how to create a class that does the job:

1. In the code editor of your choice, create a new file to hold the code for the implementation of source file.

In this example, the file is named ch67.cpp, although you can use whatever you choose.

2. Type the code in Listing 67-1 into your file.

Better yet, copy the code from the source file on this book’s companion Web site.

LISTING 67-1: THE SIMPLE LOCKING-MECHANISM CLASS

#include <string> #include <vector>

using namespace std;

class LockException

{

string _msg; public:

LockException(void)

{

_msg = “Lock Exception”;

}

LockException( const char *msg )

{

_msg = “Lock Exception: “; _msg += msg;

}

LockException( const LockException& aCopy )

{

_msg = aCopy._msg;

}

const char *Message(void)

{

return _msg.c_str();

}

void setMessage( const char *msg )

{

_msg = msg;

}

};

class Lock

{

private:

static bool _bLock; bool _isLocked;

public:

Lock(void)

{

_isLocked = false;

}

Lock( const Lock& aCopy )

{

_isLocked = aCopy._isLocked;

}

virtual ~Lock()

{

unLock();

}

bool setLock()

{

if ( !_isLocked )

{

if ( _bLock == false )

{

_bLock = true; return true;

}

}

return false;

}

bool isLocked(void)

{

return _bLock;

}

void unLock( void )

{

if ( _isLocked )

{

if ( _bLock == true )

{

_bLock = false;

}

}

}

};

bool Lock::_bLock=false;

1

2