CS 509, Advanced Programming II

Fall 2001 - Test 1

1. The for statement allows the index variable to be declared within the for statement itself, which limits the scope of the index to the for:

   for (int i = 0; i < 10; i++) {
     std::cout << i << "\n";     // Ok, i defined.
     }
   std::cout << i << "\n";      // Not ok, i is not defined.
   

   The while statement does not have this feature. Show how it can be simulated using standard C++ features; that is, show how the effect of

   while (int i = expr) {
     body
     }
   

   can be implemented using standard C++. Assume the value of the declaration statement

   int i = expr
   

   is the value of the expression expr.

   ---

   This code is almost the same as the code given in the Koenig and Moo to implement a for loop:

   { int i = expr;
     while (i) {
       body
       i = expr
       }
   }
   

   This code can be simplified a bit by using the fact that the expression v = e returns the value of e:

   { int i;
     while (i = expr) {
       body
       }
   }
   

   In addition to being shorter than the original, this code also handles the case where body has a continue statement.

   This form of the while statement was originally proposed for inclusion to the C++ standard, but it was never accepted.

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2. A colleague of yours believes that the stream eof condition is detected without having to read the eof position. Write a small piece of code that, when compiled and run, will convincingly demonstrate that your colleague's belief is wrong.

   ---

   One code example is

   #include <iostream>
   
   int main() {
     std::cout << "before read, cin.eof() = " << std::cin.eof() << "\n";
   
     char c;
     std.cin >> c;
   
     std::cout << "after read, cin.eof() = " << std::cin.eof() << "\n";
     }
   

   When compiled and run with cin connected to an empty file (such as /dev/null), it produces

   $ g++ -ansi -pedantic -Wall t.cc
   
   $ a.out < /dev/null
   before read, cin.eof() = 0
   after read, cin.eof() = 1
   
   $ 
   

   ---

3. What does "§39.4.3/857" mean?

   ---

   It's Koenig and Moo's notation for cross-references; "§s/p" refers to the part of Section s found on page p; in the example given, it refers to the part of Section 39.4.3 found on page 857.

   ---

4. A program contains the following two lines of code, which produce the indicated results when compiled

   std::string s1 = "ho" + h1 + h2;   // Compile time type error.
   std::string s2 = "ho" + h2 + h1;   // No compile time type error.
   

   What are the types of h1 and h2? Explain.

   ---

   Looking at the declaration for s2, "ho" + h2 compiles without error, so it must be that h2 has type std::string or int. If h2 has type std::string, then h1 must have type std::string or char *. If h2 has type int, then h1 must have type int. To summarize, the three possibilities arising from s2's declaration are

   1. string = char * + int + int

   2. string = char * + string + string

   3. string = char * + string + char *

   Looking at the declaration for s1, we can eliminate possibility 1 because if

   char * + int + int
   

   is legal for s2, it's also legal for s1, which is contrary to the claim made in the problem statement. By similar reasoning, we can eliminate possibility 2. That leaves only possibility 3. Checking against the problem statement, with h2 of type std::string and h1 of type char * we get

   char * + string + char *
   

   is legal as claimed, and

   char * + char * + string
   

   is illegal as claimed.

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This page last modified on 21 September 2001.