CS 509, Advanced Programming II

Fall 2003 - Test 4

1. Explain why the code

   int * invalid_pointer() {
     int x;
     return &x;
     }
   

   causes instant disaster (according to Koenig and Moo), but the code

   int * valid_pointer() {
     static int x;
     return &x;
     }
   

   does not.

   ---

   invalid_pointer() returns a pointer to a variable that exists only while the function is executing. Once the function returns, x ceases to exist, and the pointer returned now points to hyperspace (or, more accurately and insidiously, points to where x used to be, which will look o.k. until something else occupies x's space). See Section 10.6 in Koening and Moo for more information.

   A number of people wrote that the problem occurs because x is unitinialized. This is true, but that problem occurs in both functions, which the question asks about something that's wrong in one function but write in the other.

   ---

2. Given the (partial) class

   class Str {
     public:
       template < class In >
       Str(In b, In e) {
         std::copy(b, e, std::back_inserter(data));
         }
     private:
       vector<char> data;
     };
   

   Explain why the range constructor is a template function.

   ---

   Iterators are part of a particular container; each container defines a different iterator. Without function templates, you'd have to define a range constructor for every possible iterator:

   Str(std::vector<char>::iterator b, std::vector<char>::iterator e) {
     std::copy(b, e, std::back_inserter(data));
     }
   
   Str(std::list<char>::iterator b, std::list<char>::iterator e) {
     std::copy(b, e, std::back_inserter(data));
     }
   
   Str(char * b, char * e) {
     std::copy(b, e, std::back_inserter(data));
     }
   

   There were a lot of generic answers to this question, as in "The constructor uses a template parameter because the type of the arguments doesn't matter." In addition to being generic, it's not quite correct because types do matter; for example, you can't pass in booleans.

   ---

3. Many of your colleagues attempted to solve the fourth assignment by searching the photograph for an identical configuration of angles taken from the trace. For example, if the trace formed a isosceles right triangle, the code would look for triplets in the photograph that formed one 90- and two 45-degree angles.

   Describe a test case that demonstrates how this approach can lead to incorrect answers.

   ---

   The test case is

   0 0 1  2 0 1  2 1 1  0 1 1
   box 0 0  1 0  1 1  0 1 
   

   The photograph shows a rectangle of height 1 and width 2; the trace is of a square. Both have four right angles, but a box doesn't match a rectangle.

   Almost everybody tried to answer this question using the example given in the question. Unfortunately, using matching angles works when the trace has three points; it's only when you move to traces with more than three points that the problems begin.

   ---

4. The following code compiles successfully

   void time::add() {
     increase();
     }
   

   The following code doesn't compile successfully:

   int main() {
     time t;
     t.increase();  // error here.
     }
   

   Show the declaration (not definition) of time.

   (Here's a hint: this question has a correct answer.)

   ---

   From main() we can conclude increase() is, or should be, a member function of the time class. From time::add() we can conclude that increase() is a member function of time, and that main() is calling it correctly.

   We can conclude that time::add() is allowed to call increase() but main() is not; that is, increase() has non-public access permissions.

   The answers to this question was all over the place.

   ---

This page last modified on 25 November 2003.