Day 20 |
Today you will learn
The single biggest problem in the software industry is buggy, unstable code. The biggest expense in many major programming efforts is testing and fixing. The person who solves the problem of producing good, solid, bulletproof programs at low cost and on time will revolutionize the software industry.
There are a number of discrete kinds of bugs that can trouble a program. The first is poor logic: The program does just what you asked, but you haven't thought through the algorithms properly. The second is syntactic: You used the wrong idiom, function, or structure. These two are the most common, and they are the ones most programmers are on the lookout for.
Research and real-world experience have shown beyond a doubt that the later in the development process you find a problem, the more it costs to fix it. The least expensive problems or bugs to fix are the ones you manage to avoid creating. The next cheapest are those the compiler spots. The C++ standards force compilers to put a lot of energy into making more and more bugs show up at compile time.
Bugs that get compiled in but are caught at the first test--those that crash every time--are less expensive to find and fix than those that are flaky and only crash once in a while.
A bigger problem than logic or syntactic bugs is unnecessary fragility: Your program works just fine if the user enters a number when you ask for one, but it crashes if the user enters letters. Other programs crash if they run out of memory, or if the floppy disk is left out of the drive, or if the modem drops the line.
To combat this kind of fragility, programmers strive to make their programs bulletproof. A bulletproof program is one that can handle anything that comes up at runtime, from bizarre user input to running out of memory.
It is important to distinguish between bugs, which arise because the programmer made a mistake in syntax; logic errors, which arise because the programmer misunderstood the problem or how to solve it; and exceptions, which arise because of unusual but predictable problems such as running out of resources (memory or disk space).
Exceptions are different, however. You can't eliminate exceptional circumstances; you can only prepare for them. Your users will run out of memory from time to time, and the only question is what you will do. Your choices are limited to these:
C++ exception handling provides a type-safe, integrated method for coping with the predictable but unusual conditions that arise while running a program.
This means that your code must be commented even if you don't expect anyone else to ever look at it. Six months after you deliver your code, you will read it with the eyes of a total stranger, bewildered by how anyone could ever have written such convoluted and twisty code and expected anything but disaster.
NOTE: Code rot is somewhat of a programmer's joke used to explain how bug-free code suddenly becomes unreliable. It does, however, teach an important lesson. Programs are enormously complex, and bugs, errors, and mistakes can hide for a long time before turning up. Protect yourself by writing easy-to-maintain code.
The basic idea behind exceptions is fairly straightforward:
try
{
SomeDangerousFunction();
}
catch blocks handle the exceptions thrown in the try
block. For example:
try
{
SomeDangerousFunction();
}
catch(OutOfMemory)
{
// take some actions
}
catch(FileNotFound)
{
// take other action
}
The basic steps in using exceptions are
2. Create catch blocks to catch the exceptions if
they are thrown, to clean up allocated memory, and to inform the user as
appropriate. Listing 20.1 illustrates the use of both try blocks
and catch blocks.
When an exception is thrown (or raised), control transfers to the catch block immediately following the current try block.
Listing 20.1. Raising an exception.
NOTE: Some older compilers do not support exceptions. Exceptions are, however, part of the emerging C++ standard. All major compiler vendors have committed to supporting exceptions in their next releases, if they have not already done so. If you have an older compiler, you won't be able to compile and run the exercises in this chapter. It's still a good idea to read through the entire chapter, however, and return to this material when you upgrade your compiler.
0: #include <iostream.h>
1:
2: const int DefaultSize = 10;
3:
4: class Array
5: {
6: public:
7: // constructors
8: Array(int itsSize = DefaultSize);
9: Array(const Array &rhs);
10: ~Array() { delete [] pType;}
11:
12: // operators
13: Array& operator=(const Array&);
14: int& operator[](int offSet);
15: const int& operator[](int offSet) const;
16:
17: // accessors
18: int GetitsSize() const { return itsSize; }
19:
20: // friend function
21: friend ostream& operator<< (ostream&, const Array&);
22:
23: class xBoundary {}; // define the exception class
24: private:
25: int *pType;
26: int itsSize;
27: };
28:
29:
30: Array::Array(int size):
31: itsSize(size)
32: {
33: pType = new int[size];
34: for (int i = 0; i<size; i++)
35: pType[i] = 0;
36: }
37:
38:
39: Array& Array::operator=(const Array &rhs)
40: {
41: if (this == &rhs)
42: return *this;
43: delete [] pType;
44: itsSize = rhs.GetitsSize();
45: pType = new int[itsSize];
46: for (int i = 0; i<itsSize; i++)
47: pType[i] = rhs[i];
48: return *this;
49: }
50:
51: Array::Array(const Array &rhs)
52: {
53: itsSize = rhs.GetitsSize();
54: pType = new int[itsSize];
55: for (int i = 0; i<itsSize; i++)
56: pType[i] = rhs[i];
57: }
58:
59:
60: int& Array::operator[](int offSet)
61: {
62: int size = GetitsSize();
63: if (offSet >= 0 && offSet < GetitsSize())
64: return pType[offSet];
65: throw xBoundary();
66: return pType[0]; // appease MSC
67: }
68:
69:
70: const int& Array::operator[](int offSet) const
71: {
72: int mysize = GetitsSize();
73: if (offSet >= 0 && offSet < GetitsSize())
74: return pType[offSet];
75: throw xBoundary();
76: return pType[0]; // appease MSC
77: }
78:
79: ostream& operator<< (ostream& output, const Array& theArray)
80: {
81: for (int i = 0; i<theArray.GetitsSize(); i++)
82: output << "[" << i << "] " << theArray[i] << endl;
83: return output;
84: }
85:
86: int main()
87: {
88: Array intArray(20);
89: try
90: {
91: for (int j = 0; j< 100; j++)
92: {
93: intArray[j] = j;
94: cout << "intArray[" << j << "] okay..." << endl;
95: }
96: }
97: catch (Array::xBoundary)
98: {
99: cout << "Unable to process your input!\n";
100: }
101: cout << "Done.\n";
102: return 0;
103: }
Output: intArray[0] okay...
intArray[1] okay...
intArray[2] okay...
intArray[3] okay...
intArray[4] okay...
intArray[5] okay...
intArray[6] okay...
intArray[7] okay...
intArray[8] okay...
intArray[9] okay...
intArray[10] okay...
intArray[11] okay...
intArray[12] okay...
intArray[13] okay...
intArray[14] okay...
intArray[15] okay...
intArray[16] okay...
intArray[17] okay...
intArray[18] okay...
intArray[19] okay...
Unable to process your input!
Done.
Analysis: Listing 20.1 presents a somewhat
stripped-down Array class, based on the template developed on
Day 19, "Templates." On line 23, a new class is contained within the declaration
of the boundary.
This new class is not in any way distinguished as an exception class. It is just a class like any other. This particular class is incredibly simple: It has no data and no methods. Nonetheless, it is a valid class in every way.
In fact, it is incorrect to say it has no methods, because the compiler automatically assigns it a default constructor, destructor, copy constructor, and the copy operator (operator equals); so it actually has four class functions, but no data.
Note that declaring it from within Array serves only to couple the two classes together. As discussed on Day 15, "Advanced Inheritance," Array has no special access to xBoundary, nor does xBoundary have preferential access to the members of Array.
On lines 60-66 and 69-75, the offset operators are modified to examine the offset requested and, if it is out of range, to throw the xBoundary class as an exception. The parentheses are required to distinguish between this call to the xBoundary constructor and the use of an enumerated constant. Note that Microsoft requires that you provide a return statement to match the declaration (in this case, returning an integer reference), even though if an exception is thrown on line 65 the code will never reach line 66. This is a compiler bug, proving only that even Microsoft finds this stuff difficult and confusing!
On line 89, the keyword try begins a try block that ends on line 96. Within that try block, 100 integers are added to the array that was declared on line 88.
On line 97, the catch block to catch xBoundary exceptions is declared.
In the driver program on lines 86-103, a try block is created in which each member of the array is initialized. When j (line 91) is incremented to 20, the member at offset 20 is accessed. This causes the test on line 63 to fail, and operator[] raises an xBoundary exception on line 65.
Program control switches to the catch block on line 97, and the exception is caught or handled by the catch on the same line, which prints an error message. Program flow drops through to the end of the catch block on line 100.
try
{
Function();
};
try
{
Function();
};
catch (OutOfMemory)
{
// take action
}
When trying to determine try block locations, look to where you allocate memory or use resources. Other things to look for are out-of-bounds errors, illegal input, and so forth.
The call stack tracks the execution path. If main() calls the function Animal::GetFavoriteFood(), and GetFavoriteFood() calls Animal::LookupPreferences(), which in turn calls fstream::operator>>(), all these are on the call stack. A recursive function might be on the call stack many times.
The exception is passed up the call stack to each enclosing block. As the stack is unwound, the destructors for local objects on the stack are invoked, and the objects are destroyed.
After each try block there is one or more catch statements. If the exception matches one of the catch statements, it is considered to be handled by having that statement execute. If it doesn't match any, the unwinding of the stack continues.
If the exception reaches all the way to the beginning of the program (main()) and is still not caught, a built-in handler is called that terminates the program.
It is important to note that the exception unwinding of the stack is a one-way street. As it progresses, the stack is unwound and objects on the stack are destroyed. There is no going back: Once the exception is handled, the program continues after the try block of the catch statement that handled the exception.
Thus, in Listing 20.1, execution will continue on line 101, the first line after the try block of the catch statement that handled the xBoundary exception. Remember that when an exception is raised, program flow continues after the catch block, not after the point where the exception was thrown.
Listing 20.2. Multiple exceptions.
0: #include <iostream.h>
1:
2: const int DefaultSize = 10;
3:
4: class Array
5: {
6: public:
7: // constructors
8: Array(int itsSize = DefaultSize);
9: Array(const Array &rhs);
10: ~Array() { delete [] pType;}
11:
12: // operators
13: Array& operator=(const Array&);
14: int& operator[](int offSet);
15: const int& operator[](int offSet) const;
16:
17: // accessors
18: int GetitsSize() const { return itsSize; }
19:
20: // friend function
21: friend ostream& operator<< (ostream&, const Array&);
22:
23: // define the exception classes
24: class xBoundary {};
25: class xTooBig {};
26: class xTooSmall{};
27: class xZero {};
28: class xNegative {};
29: private:
30: int *pType;
31: int itsSize;
32: };
33:
34: int& Array::operator[](int offSet)
35: {
36: int size = GetitsSize();
37: if (offSet >= 0 && offSet < GetitsSize())
38: return pType[offSet];
39: throw xBoundary();
40: return pType[0]; // appease MFC
41: }
42:
43:
44: const int& Array::operator[](int offSet) const
45: {
46: int mysize = GetitsSize();
47: if (offSet >= 0 && offSet < GetitsSize())
48: return pType[offSet];
49: throw xBoundary();
50: return pType[0];
51: return pType[0]; // appease MFC
52: }
53:
54:
55: Array::Array(int size):
56: itsSize(size)
57: {
58: if (size == 0)
59: throw xZero();
60: if (size < 10)
61: throw xTooSmall();
62: if (size > 30000)
63: throw xTooBig();
64: if (size < 1)
65: throw xNegative();
66:
67: pType = new int[size];
68: for (int i = 0; i<size; i++)
69: pType[i] = 0;
70: }
71:
72:
73:
74: int main()
75: {
76:
77: try
78: {
79: Array intArray(0);
80: for (int j = 0; j< 100; j++)
81: {
82: intArray[j] = j;
83: cout << "intArray[" << j << "] okay...\n";
84: }
85: }
86: catch (Array::xBoundary)
87: {
88: cout << "Unable to process your input!\n";
89: }
90: catch (Array::xTooBig)
91: {
92: cout << "This array is too big...\n";
93: }
94: catch (Array::xTooSmall)
95: {
96: cout << "This array is too small...\n";
97: }
98: catch (Array::xZero)
99: {
100: cout << "You asked for an array";
101: cout << " of zero objects!\n";
102: }
103: catch (...)
104: {
105: cout << "Something went wrong!\n";
106: }
107: cout << "Done.\n";
108: return 0;
109: }
Output: You asked for an array of zero objects!
Done.
The try block is changed to include catch statements for each condition other than negative, which is caught by the "catch everything" statement catch(...), shown on line 103.
Try this with a number of values for the size of the array. Then try putting in -5. You might have expected xNegative to be called, but the order of the tests in the constructor prevented this: size < 10 was evaluated before size < 1. To fix this, swap lines 60 and 61 with lines 64 and 65 and recompile.
Listing 20.3. Class hierarchies and exceptions.
0: #include <iostream.h>
1:
2: const int DefaultSize = 10;
3:
4: class Array
5: {
6: public:
7: // constructors
8: Array(int itsSize = DefaultSize);
9: Array(const Array &rhs);
10: ~Array() { delete [] pType;}
11:
12: // operators
13: Array& operator=(const Array&);
14: int& operator[](int offSet);
15: const int& operator[](int offSet) const;
16:
17: // accessors
18: int GetitsSize() const { return itsSize; }
19:
20: // friend function
21: friend ostream& operator<< (ostream&, const Array&);
22:
23: // define the exception classes
24: class xBoundary {};
25: class xSize {};
26: class xTooBig : public xSize {};
27: class xTooSmall : public xSize {};
28: class xZero : public xTooSmall {};
29: class xNegative : public xSize {};
30: private:
31: int *pType;
32: int itsSize;
33: };
34:
35:
36: Array::Array(int size):
37: itsSize(size)
38: {
39: if (size == 0)
40: throw xZero();
41: if (size > 30000)
42: throw xTooBig();
43: if (size <1)
44: throw xNegative();
45: if (size < 10)
46: throw xTooSmall();
47:
48: pType = new int[size];
49: for (int i = 0; i<size; i++)
50: pType[i] = 0;
51: }
52:
53: int& Array::operator[](int offSet)
54: {
55: int size = GetitsSize();
56: if (offSet >= 0 && offSet < GetitsSize())
57: return pType[offSet];
58: throw xBoundary();
59: return pType[0]; // appease MFC
60: }
61:
62:
63: const int& Array::operator[](int offSet) const
64: {
65: int mysize = GetitsSize();
66: if (offSet >= 0 && offSet < GetitsSize())
67: return pType[offSet];
68: throw xBoundary();
69: return pType[0];
70: return pType[0]; // appease MFC
71: }
72:
73: int main()
74: {
75:
76: try
77: {
78: Array intArray(5);
79: for (int j = 0; j< 100; j++)
80: {
81: intArray[j] = j;
82: cout << "intArray[" << j << "] okay...\n";
83: }
84: }
85: catch (Array::xBoundary)
86: {
87: cout << "Unable to process your input!\n";
88: }
89: catch (Array::xTooBig)
90: {
91: cout << "This array is too big...\n";
92: }
93:
94: catch (Array::xZero)
95: {
96: cout << "You asked for an array";
97: cout << " of zero objects!\n";
98: }
99:
100: catch (Array::xTooSmall)
101: {
102: cout << "This array is too small...\n";
103: }
104:
105: catch (...)
106: {
107: cout << "Something went wrong!\n";
108: }
109: cout << "Done.\n";
110: return 0
111: }
Output: This array is too small...
Done.
Analysis: The significant change is
on lines 26-29, where the class hierarchy is established. Classes xTooBig,
xTooSmall, and xNegative are derived from xSize,
and xZero is derived from xTooSmall.
The Array is created with size zero, but what's this? The wrong exception appears to be caught! Examine the catch block carefully, however, and you will find that it looks for an exception of type xTooSmall before it looks for an exception of type xZero. Because an xZero object is thrown and an xZero object is an xTooSmall object, it is caught by the handler for xTooSmall. Once handled, the exception is not passed on to the other handlers, so the handler for xZero is never called.
The solution to this problem is to carefully order the handlers so that the most specific handlers come first and the less specific handlers come later. In this particular example, switching the placement of the two handlers xZero and xTooSmall will fix the problem.
Listing 20.4. Getting data out of an exception object.
0: #include <iostream.h>
1:
2: const int DefaultSize = 10;
3:
4: class Array
5: {
6: public:
7: // constructors
8: Array(int itsSize = DefaultSize);
9: Array(const Array &rhs);
10: ~Array() { delete [] pType;}
11:
12: // operators
13: Array& operator=(const Array&);
14: int& operator[](int offSet);
15: const int& operator[](int offSet) const;
16:
17: // accessors
18: int GetitsSize() const { return itsSize; }
19:
20: // friend function
21: friend ostream& operator<< (ostream&, const Array&);
22:
23: // define the exception classes
24: class xBoundary {};
25: class xSize
26: {
27: public:
28: xSize(int size):itsSize(size) {}
29: ~xSize(){}
30: int GetSize() { return itsSize; }
31: private:
32: int itsSize;
33: };
34:
35: class xTooBig : public xSize
36: {
37: public:
38: xTooBig(int size):xSize(size){}
39: };
40:
41: class xTooSmall : public xSize
42: {
43: public:
44: xTooSmall(int size):xSize(size){}
45: };
46:
47: class xZero : public xTooSmall
48: {
49: public:
50: xZero(int size):xTooSmall(size){}
51: };
52:
53: class xNegative : public xSize
54: {
55: public:
56: xNegative(int size):xSize(size){}
57: };
58:
59: private:
60: int *pType;
61: int itsSize;
62: };
63:
64:
65: Array::Array(int size):
66: itsSize(size)
67: {
68: if (size == 0)
69: throw xZero(size);
70: if (size > 30000)
71: throw xTooBig(size);
72: if (size <1)
73: throw xNegative(size);
74: if (size < 10)
75: throw xTooSmall(size);
76:
77: pType = new int[size];
78: for (int i = 0; i<size; i++)
79: pType[i] = 0;
80: }
81:
82:
83: int& Array::operator[] (int offSet)
84: {
85: int size = GetitsSize();
86: if (offSet >= 0 && offSet < GetitsSize())
87: return pType[offSet];
88: throw xBoundary();
89: return pType[0];
90: }
91:
92: const int& Array::operator[] (int offSet) const
93: {
94: int size = GetitsSize();
95: if (offSet >= 0 && offSet < GetitsSize())
96: return pType[offSet];
97: throw xBoundary();
98: return pType[0];
99: }
100:
101: int main()
102: {
103:
104: try
105: {
106: Array intArray(9);
107: for (int j = 0; j< 100; j++)
108: {
109: intArray[j] = j;
110: cout << "intArray[" << j << "] okay..." << endl;
111: }
112: }
113: catch (Array::xBoundary)
114: {
115: cout << "Unable to process your input!\n";
116: }
117: catch (Array::xZero theException)
118: {
119: cout << "You asked for an Array of zero objects!" << endl;
120: cout << "Received " << theException.GetSize() << endl;
121: }
122: catch (Array::xTooBig theException)
123: {
124: cout << "This Array is too big..." << endl;
125: cout << "Received " << theException.GetSize() << endl;
126: }
127: catch (Array::xTooSmall theException)
128: {
129: cout << "This Array is too small..." << endl;
130: cout << "Received " << theException.GetSize() << endl;
131: }
132: catch (...)
133: {
134: cout << "Something went wrong, but I've no idea what!\n";
135: }
136: cout << "Done.\n";
137: return 0;
138: }
Output: This array is too small...
Received 9
Done.
Analysis: The declaration of xSize
has been modified to include a member variable, itsSize, on line
32 and a member function, GetSize(), on line 30. Additionally,
a constructor has been added that takes an integer and initializes the
member variable, as shown on line 28.
The catch statements on lines 113 to 135 are modified to name the exception they catch, theException, and to use this object to access the data stored in itsSize.
It is tedious and error-prone to have each of these catch statements individually print the appropriate message. This job belongs to the object, which knows what type of object it is and what value it received. Listing 20.5 takes a more object-oriented approach to this problem, using virtual functions so that each exception "does the right thing."
NOTE: Keep in mind that if you are constructing an exception, it is because an exception has been raised: Something has gone wrong, and your exception should be careful not to kick off the same problem. Therefore, if you are creating an OutOfMemory exception, you probably don't want to allocate memory in its constructor.
Listing 20.5.Passing by reference and using virtual functions in exceptions.
0: #include <iostream.h>
1:
2: const int DefaultSize = 10;
3:
4: class Array
5: {
6: public:
7: // constructors
8: Array(int itsSize = DefaultSize);
9: Array(const Array &rhs);
10: ~Array() { delete [] pType;}
11:
12: // operators
13: Array& operator=(const Array&);
14: int& operator[](int offSet);
15: const int& operator[](int offSet) const;
16:
17: // accessors
18: int GetitsSize() const { return itsSize; }
19:
20: // friend function
21: friend ostream& operator<<
22: (ostream&, const Array&);
23:
24: // define the exception classes
25: class xBoundary {};
26: class xSize
27: {
28: public:
29: xSize(int size):itsSize(size) {}
30: ~xSize(){}
31: virtual int GetSize() { return itsSize; }
32: virtual void PrintError()
33: {
34: cout << "Size error. Received: ";
35: cout << itsSize << endl;
36: }
37: protected:
38: int itsSize;
39: };
40:
41: class xTooBig : public xSize
42: {
43: public:
44: xTooBig(int size):xSize(size){}
45: virtual void PrintError()
46: {
47: cout << "Too big! Received: ";
48: cout << xSize::itsSize << endl;
49: }
50: };
51:
52: class xTooSmall : public xSize
53: {
54: public:
55: xTooSmall(int size):xSize(size){}
56: virtual void PrintError()
57: {
58: cout << "Too small! Received: ";
59: cout << xSize::itsSize << endl;
60: }
61: };
62:
63: class xZero : public xTooSmall
64: {
65: public:
66: xZero(int size):xTooSmall(size){}
67: virtual void PrintError()
68: {
69: cout << "Zero!!. Received: " ;
70: cout << xSize::itsSize << endl;
71: }
72: };
73:
74: class xNegative : public xSize
75: {
76: public:
77: xNegative(int size):xSize(size){}
78: virtual void PrintError()
79: {
80: cout << "Negative! Received: ";
81: cout << xSize::itsSize << endl;
82: }
83: };
84:
85: private:
86: int *pType;
87: int itsSize;
88: };
89:
90: Array::Array(int size):
91: itsSize(size)
92: {
93: if (size == 0)
94: throw xZero(size);
95: if (size > 30000)
96: throw xTooBig(size);
97: if (size <1)
98: throw xNegative(size);
99: if (size < 10)
100: throw xTooSmall(size);
101:
102: pType = new int[size];
103: for (int i = 0; i<size; i++)
104: pType[i] = 0;
105: }
106:
107: int& Array::operator[] (int offSet)
108: {
109: int size = GetitsSize();
110: if (offSet >= 0 && offSet < GetitsSize())
111: return pType[offSet];
112: throw xBoundary();
113: return pType[0];
114: }
115:
116: const int& Array::operator[] (int offSet) const
117: {
118: int size = GetitsSize();
119: if (offSet >= 0 && offSet < GetitsSize())
120: return pType[offSet];
121: throw xBoundary();
122: return pType[0];
123: }
124:
125: int main()
126: {
127:
128: try
129: {
130: Array intArray(9);
131: for (int j = 0; j< 100; j++)
132: {
133: intArray[j] = j;
134: cout << "intArray[" << j << "] okay...\n";
135: }
136: }
137: catch (Array::xBoundary)
138: {
139: cout << "Unable to process your input!\n";
140: }
141: catch (Array::xSize& theException)
142: {
143: theException.PrintError();
144: }
145: catch (...)
146: {
147: cout << "Something went wrong!\n";
148: }
149: cout << "Done.\n";
150: return 0;
151: }
Output: Too small! Received: 9
Done.
Analysis: Listing 20.5 declares a virtual
method in the xSize class, PrintError(), that prints
an error message and the actual size of the class. This is overridden in
each of the derived classes.
On line 141, the exception object is declared to be a reference. When PrintError() is called with a reference to an object, polymorphism causes the correct version of PrintError() to be invoked. The code is cleaner, easier to understand, and easier to maintain.
Listing 20.6. Using exceptions with templates.
0: #include <iostream.h>
1:
2: const int DefaultSize = 10;
3: class xBoundary {};
4:
5: template <class T>
6: class Array
7: {
8: public:
9: // constructors
10: Array(int itsSize = DefaultSize);
11: Array(const Array &rhs);
12: ~Array() { delete [] pType;}
13:
14: // operators
15: Array& operator=(const Array<T>&);
16: T& operator[](int offSet);
17: const T& operator[](int offSet) const;
18:
19: // accessors
20: int GetitsSize() const { return itsSize; }
21:
22: // friend function
23: friend ostream& operator<< (ostream&, const Array<T>&);
24:
25: // define the exception classes
26:
27: class xSize {};
28:
29: private:
30: int *pType;
31: int itsSize;
32: };
33:
34: template <class T>
35: Array<T>::Array(int size):
36: itsSize(size)
37: {
38: if (size <10 || size > 30000)
39: throw xSize();
40: pType = new T[size];
41: for (int i = 0; i<size; i++)
42: pType[i] = 0;
43: }
44:
45: template <class T>
46: Array<T>& Array<T>::operator=(const Array<T> &rhs)
47: {
48: if (this == &rhs)
49: return *this;
50: delete [] pType;
51: itsSize = rhs.GetitsSize();
52: pType = new T[itsSize];
53: for (int i = 0; i<itsSize; i++)
54: pType[i] = rhs[i];
55: }
56: template <class T>
57: Array<T>::Array(const Array<T> &rhs)
58: {
59: itsSize = rhs.GetitsSize();
60: pType = new T[itsSize];
61: for (int i = 0; i<itsSize; i++)
62: pType[i] = rhs[i];
63: }
64:
65: template <class T>
66: T& Array<T>::operator[](int offSet)
67: {
68: int size = GetitsSize();
69: if (offSet >= 0 && offSet < GetitsSize())
70: return pType[offSet];
71: throw xBoundary();
72: return pType[0];
73: }
74:
75: template <class T>
76: const T& Array<T>::operator[](int offSet) const
77: {
78: int mysize = GetitsSize();
79: if (offSet >= 0 && offSet < GetitsSize())
80: return pType[offSet];
81: throw xBoundary();
82: }
83:
84: template <class T>
85: ostream& operator<< (ostream& output, const Array<T>& theArray)
86: {
87: for (int i = 0; i<theArray.GetitsSize(); i++)
88: output << "[" << i << "] " << theArray[i] << endl;
89: return output;
90: }
91:
92:
93: int main()
94: {
95:
96: try
97: {
98: Array<int> intArray(9);
99: for (int j = 0; j< 100; j++)
100: {
101: intArray[j] = j;
102: cout << "intArray[" << j << "] okay..." << endl;
103: }
104: }
105: catch (xBoundary)
106: {
107: cout << "Unable to process your input!\n";
108: }
109: catch (Array<int>::xSize)
110: {
111: cout << "Bad Size!\n";
112: }
113:
114: cout << "Done.\n";
115: return 0;
116: }
Output: Bad Size!
Done.
Analysis: The first exception, xBoundary,
is declared outside the template definition on line 3. The second exception,
xSize, is declared from within the definition of the template,
on line 27.
The exception xBoundary is not tied to the template class, but can be used like any other class. xSize is tied to the template, and must be called based on the instantiated Array. You can see the difference in the syntax for the two catch statements. Line 105 shows catch (xBoundary), but line 109 shows catch (Array<int>::xSize). The latter is tied to the instantiation of an integer Array.
Others point out that exceptions offer a powerful and clean way to return through many layers of function calls without danger of memory leaks. A frequent example is this: The user requests an action in a GUI environment. The part of the code that catches the request must call a member function on a dialog manager, which in turn calls code that processes the request, which calls code that decides which dialog box to use, which in turn calls code to put up the dialog box, which finally calls code that processes the user's input. If the user presses Cancel, the code must return to the very first calling method, where the original request was handled.
One approach to this problem is to put a try block around the original call and catch CancelDialog as an exception, which can be raised by the handler for the Cancel button. This is safe and effective, but pressing Cancel is a routine circumstance, not an exceptional one.
This frequently becomes something of a religious argument, but there is a reasonable way to decide the question: Does use of exceptions in this way make the code easier or harder to understand? Are there fewer risks of errors and memory leaks, or more? Will it be harder or easier to maintain this code? These decisions, like so many others, will require an analysis of the trade-offs; there is no single, obvious right answer.
Nearly all modern development environments include one or more high-powered debuggers. The essential idea of using a debugger is this: You run the debugger, which loads your source code, and then you run your program from within the debugger. This allows you to see each instruction in your program as it executes, and to examine your variables as they change during the life of your program.
All compilers will let you compile with or without symbols. Compiling with symbols tells the compiler to create the necessary mapping between your source code and the generated program; the debugger uses this to point to the line of source code that corresponds to the next action in the program.
Full-screen symbolic debuggers make this chore a delight. When you load your debugger, it will read through all your source code and show the code in a window. You can step over function calls or direct the debugger to step into the function, line by line.
With most debuggers, you can switch between the source code and the output to see the results of each executed statement. More powerfully, you can examine the current state of each variable, look at complex data structures, examine the value of member data within classes, and look at the actual values in memory of various pointers and other memory locations. You can execute several types of control within a debugger that include setting breakpoints, setting watch points, examining memory, and looking at the assembler code.
Learn to use your debugger. It can be the most powerful weapon in your holy war against bugs. Runtime bugs are the hardest to find and squash, and a powerful debugger can make it possible, if not easy, to find nearly all of them.
Exceptions are normal, user-created objects, and as such may be passed by value or by reference. They may contain data and methods, and the catch block may use that data to decide how to deal with the exception.
It is possible to create multiple catch blocks, but once an exception matches a catch block's signature, it is considered to be handled and is not given to the subsequent catch blocks. It is important to order the catch blocks appropriately, so that more specific catch blocks have first chance and more general catch blocks handle those not otherwise handled.
This chapter also examined some of the fundamentals of symbolic debuggers, including using watch points, breakpoints, and so forth. These tools can help you zero in on the part of your program that is causing the error, and let you see the value of variables as they change during the course of the execution of the program.
A. Often, the same error can be generated in a number of
different parts of the code. Exceptions let you centralize the handling
of errors. Additionally, the part of the code that generates the error
may not be the best place to determine how to handle the error.
Q. Why generate an object? Why not just pass an error code?
A. Objects are more flexible and powerful than error codes. They can convey more information, and the constructor/destructor mechanisms can be used for the creation and removal of resources that may be required to properly handle the exceptional condition.
Q. Why not use exceptions for non-error conditions? Isn't it convenient to be able to express-train back to previous areas of the code, even when non-exceptional conditions exist?
A. Yes, some C++ programmers use exceptions for just that purpose. The danger is that exceptions might create memory leaks as the stack is unwound and some objects are inadvertently left in the free store. With careful programming techniques and a good compiler, this can usually be avoided. Otherwise, it is a matter of personal aesthetic; some programmers feel that by their nature exceptions should not be used for routine conditions.
Q. Does an exception have to be caught in the same place where the try block created the exception?
A. No, it is possible to catch an exception anywhere in the call stack. As the stack is unwound, the exception is passed up the stack until it is handled.
Q. Why use a debugger when you can use cout with conditional (#ifdef debug) compiling?
A. The debugger provides a much more powerful mechanism for stepping through your code and watching values change without having to clutter your code with thousands of debugging statements.
2. What is a try block?
3. What is a catch statement?
4. What information can an exception contain?
5. When are exception objects created?
6. Should you pass exceptions by value or by reference?
7. Will a catch statement catch a derived exception if it is looking for the base class?
8. If there are two catch statements, one for base and one for derived, which should come first?
9. What does catch(...) mean?
10. What is a breakpoint?
2. Modify the answer from Exercise 1, put data into the exception,
along with an accessor function, and use it in the catch block.
3. Modify the class from Exercise 2 to be a hierarchy of exceptions. Modify the catch block to use the derived objects and the base objects.
4. Modify the program from Exercise 3 to have three levels of function calls.
5. BUG BUSTERS: What is wrong with the following code?
class xOutOfMemory
{
public:
xOutOfMemory( const String& message ) : itsMsg( message ){}
~xOutOfMemory(){}
virtual const String& Message(){ return itsMsg};
private:
String itsMsg;
}
main()
{
try {
char *var = new char;
if ( var == 0 )
throw xOutOfMemory();
}
catch( xOutOfMemory& theException )
{
cout << theException.Message() << "\n";
}
}