Throw exceptions

Listing 2 is the second implementation of readnums. It includes the basic logic in listing 1. However, this newer version also includes exception throwing code.

Listing 2:

readnums.2

1class Exception
2{
3    char message[256];
4  public:
5    Exception(const char *m) { strncpy(message, m, sizeof(message)); }
6    virtual const char *getMessage(void) { return message; }
7};
8
9class BadPath : public Exception
10{
11  public:
12    BadPath(const char *m):Exception(m) {}
13};
14
15class BadInt : public Exception
16{
17  public:
18    BadInt(const char *m):Exception(m) {}
19};
20
21class TooBig : public Exception
22{
23  public:
24    TooBig(const char *m):Exception(m) {}
25};
26
27unsigned readnums(const char *path, int array[], unsigned n)
28  throw (BadPath, BadInt, TooBig)
29{
30  // read up to n integers from the file at path
31  ifstream file;
32  unsigned i;
33
34  file.open(path);
35  if (!file.is_open())
36  {
37    throw BadPath(”bad_path,_file_cannot_be_opened_for_reading”);
38  }
39  i = 0;
40  while (!file.eof() && (i < n))
41  {
42    file >> array[i];
43    if (file.fail())
44    {
45      file.close();
46      throw BadInt(”bad_integer_format”);
47    }
48  }
49  file.close();
50  if (!file.eof() && (i == n))
51  {
52    throw TooBig(”too_many_integers_in_the_file”);
53  }
54  return i;
55}

Line 28 is a part of the function prototype that indicates this function can throw exceptions.

The conditional statement on line 35 checks to see if the ﬁle is open. If not, it throws a BadPath exception. Note that BadPath is a subclass of Exception. The keyword throw will be explained later. For the time being, we will simply see throw as a very special return.

The conditional statement on line 43 checks to see if an integer is read. If there is a format error, the conditional statement closes the ﬁle ﬁrst, then throws a BadInt exception.

The conditional statement on line 50 checks to see if there is anything else in the ﬁle. If so, it throws a TooBig exception.

3.2 Throw exceptions