Choosing Initialization Instead of Assignment

413

//Construct the object. fileHandle fh(strName);

//This will check to see if the file already exists.

if ( !fh.Exists() ) return –1;

//Now, we can do the “expensive” operation of

//opening and reading the file. if ( fh.Open() == false )

return –1;

//Read the file...

}

In this example, we first check all input for validity. If it isn’t valid, there is no cost to the fileHandle object being created. If the input is valid, we then pass the validation onto that object. First, we simply set the filename into the object and see whether that file exists. Then we try to open the file — which will buffer the data for it and do the overhead (that is, getting the operating system to open the file). Only then, if all those things work, do we actually read the file, which costs the most time.

Choosing Initialization

Instead of Assignment

The final optimization technique to look at is initializing data (rather than assigning data in constructors for classes). Under normal circumstances, the member data for a class is first constructed (using default constructors), and then assigned values based on input to the constructor (or defaults provided by the programmer). The problem with this approach is that the initialization is really done twice — first in the constructor and then in the assignment. This is wasteful, and leads to program slowdowns.

To see how to make this improvement, follow these steps:

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

In this example, the file is named ch65b.cpp, although you can use whatever you choose.

2. Type the code from Listing 65-3 into your file.

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

LISTING 65-3: INITIALIZING VERSUS ASSIGNING

#include <iostream> #include <string>

using namespace std;

class Point

{

private: int _x; int _y;

public:

Point(void)

{

cout << “Point: void constructor called” << endl;

_x = 0; _y = 0;

}

Point( int x, int y )

{

cout << “Point: full constructor called” << endl;

_x = x; _y = y;

}

Point( const Point& p )

{

cout << “Point: copy constructor called” << endl;

(continued)

414 Technique 65: Optimizing Your Code

LISTING 65-3 (continued)

_x = p._x; _y = p._y;

}

Point& operator=( const Point& p )

{

cout << “Point: operator= called” << endl;

_x = p._x; _y = p._y;

return *this;

}

int& X()

{

return _x;

}

int& Y()

{

return _y;

}

};
class Line
{
Point	_p1;
Point	_p2;
public:
Line(void)
{
}
Line(	int x1, int x2, int y1,			9
int	y2 )
: _p1(x1,y1),
			10
	_p2(x2,y2)

{

}

Line( const Point& p1, const Point &p2 )

{

_p1 = p1; _p2 = p2;

}

Point& TopLeft()

{

return _p1;

return _p2;

}

};

int main()

{

//First, create some points. Point p1(0,0);

Point p2(10,10);

//Now create some lines. cout << “Line 1: “ << endl; Line l1( 0,0, 10, 10);

cout << “Line 2: “ << endl; Line l2( p1, p2 );

}

In this case, we are using a very simple set of classes that implement a point and a line. Notice that in the Line class, there are two separate constructors. One takes four data values, indicating the starting and ending x and y coordinates (see 9). The second takes two point objects to define the same coordinates, as shown in 10. The difference in the two constructors is how the data is assigned to the internal member variables in the two cases. In the first case, the two points in the Line class are initialized within the initialization line of the constructor code. In the second case, the two points are initialized by assignment within the constructor body. As we will see when the program runs, these two choices have very different results.

3. Save the source file in the code editor and then close the editor application.

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

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

}

Point& BottomRight()

{

Choosing Initialization Instead of Assignment

415

If you have done everything correctly, you should see the following output on the console window:

$ ./a.exe
Point: full constructor called
Point: full constructor called		11
Line 1:
Point: full constructor called
Point: full constructor called		12
Line 2:
Point: void constructor called

Point: void constructor called

Point: operator= called

Point: operator= called

Notice that in the first case (shown at			11), we con-
struct the two points as a part of	constructing the

line. This is as expected because the Line object contains two point objects. However, those two objects are constructed using the full constructor for the Point class, using the data values we passed in. This means there is no additional overhead for creating the points. In the second case, shown at

12, however, we not only have the two Pointobjects being created, but also the overhead of two assignment statements. This means that twice as much work is being done. If you initialize things using the initialization process in C++ constructors, you avoid the overhead of the assignments.

66 Documenting the

Data Flow

Technique

Save Time By

Learning how the code operates

Improving the readability of your code by documenting the data flow

Adding an undo system

Testing your code

Beginning programmers are often afraid to make adjustments to existing code for fear of destroying the code’s functionality. Existing code is simply part of life in the programming world, and

you can’t be afraid to just dig in and make changes to the code base. Other than to offer simple encouragement, I can’t really give you any advice on how to work with existing code; however, I can give you ideas on how to make your code easier to work with.

If you want to save a lot of time for yourself and all of the programmers who come after you, document the flow of data through the system. Most programmers document how the code works, or how you interface to the objects in the system. This is a nice thing, but the problems that crop up in coding are normally related to data, not code. In this technique, we are going to explore the most important part of the programming system, the data flow.

Learning How Code Operates

If you really want to know how the code in a system operates, just watch how it manipulates data. The surest way to do so is to keep track of all changes in a system. Although we normally think of an object-oriented system as having member variables and methods to access those variables, there is really no reason to do things this way. We can simply implement a system that stores the data in properties and then accesses those properties through standard methods of the property manipulation class rather than through the parent class. The following steps show you this exact process, implementing a property holding class and providing methods to track the changes to the data as it goes through the system.

Learning How Code Operates 417

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

In this example, the file is named ch66.cpp, although you can use whatever you choose.

2. Type the code from Listing 66-1 into your file.

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

LISTING 66-1: IMPLEMENTING PROPERTIES AS A CLASS

#include <map> #include <string> #include <iostream> #include <stack> #include <stdlib.h>

using namespace std;

class State

{

string _name; string _value;

public:

State(void)

{

_name = “”; _value = “”;

}

State( const char *name, const char *value )

{

setName( name ); setValue( value );

}

State( const State& aCopy )

{

setName( aCopy.getName() ); setValue( aCopy.getValue() );

}

void setName( const char *n )

{

_name = n;

}

void setName( const string& n )

{

_name = n;

}

string getName(void) const

{

return _name;

}

void setValue( const char *v )

{

_value = v;

}

void setValue( const string& v )

{

_value = v;

}

string getValue( void ) const

{

return _value;

}

};
class Properties			1
{
map<string, string> _props;		2
stack<State>	_previous;

public:

Properties(void)

{

}

Properties( const Properties& aCopy )

{

map<string, string>::iterator iter; for ( iter = _props.begin(); iter !=

_props.end(); ++iter ) _props[ (*iter).first ] =

(*iter).second;

}

virtual ~Properties()

{

}

void setProperty( const char *name, int value )

{

// First, see if its there if ( _props.find(name) !=

_props.end() )

{

State sold(name, _props[name]. c_str() ); (continued) 3