[Dev-C++] Deliberate crashes

[Bob W. <bo...@ph...>]

I will appologize in advance for the length of this message. Anyone who =
has much experience in programming knows pointers are an extremely =
powerful and useful construct, but that they can be the quickest path to =
the hellfire and damnation of runtime erros and program crashes if =
careful attention is not paid to how they are used.=20

With that as a backdrop here is a quote from Sams Teach Yourself C++ in =
24 Hours, third Edition by Jesse Liberty.=20


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When you call a delete on a pointer, the memory it points to is freed. =
Calling delete on that pointer again will crash your program! When you =
delete a pointer, set it to 0 (null). Calling delete on a null pointer =
is guaranteed to be safe. For example

  Animal: *pDog =3D new Animal;
  delete pDog; // frees the memory
  pDog =3D 0; // sets the pointer to null
  // . . .
  delete pDog; // harmless

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But it does not work that way. As a matter of fact with both the =
Bloodshed/GNU compiler supplied with the book and VC++ 6.0 it works just =
the opposite. The book uses this program to demonstrate allocating and =
deleting a pointer.=20


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 // Listing 9.4
 // Allocating and deleting a pointer

 #include <iostream>

 int main()
 {
     int localVariable =3D 5;
     int * pLocal=3D &localVariable;
     int * pHeap =3D new int;
     if (pHeap =3D=3D NULL)
     {
         std::cout << "Error! No memory for pHeap!!";
         return 1;
     }
     *pHeap =3D 7;
     std::cout << "localVariable: " << localVariable << "\n";
     std::cout << "*pLocal: " << *pLocal << "\n";
     std::cout << "*pHeap: " << *pHeap << "\n";
     delete pHeap;
     pHeap =3D new int;
     if (pHeap =3D=3D NULL)
     {
         std::cout << "Error! No memory for pHeap!!";
         return 1;
     }
     *pHeap =3D 9;
     std::cout << "*pHeap: " << *pHeap << "\n";
     delete pHeap;
     return 0;
 }



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If you compile and run the program the output is as advertised in the =
book;

  localVariable: 5
  *pLocal: 5
  *pHeap: 7
  *pHeap: 9
However if you modify the code by setting pHeap to null and call delete =
on pHeap immedately after as shown below. The program still runs without =
error.=20


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 // Listing 9.4
 // Allocating and deleting a pointer

 #include <iostream>

 int main()
 {
     int localVariable =3D 5;
     int * pLocal=3D &localVariable;
     int * pHeap =3D new int;
     if (pHeap =3D=3D NULL)
     {
         std::cout << "Error! No memory for pHeap!!";
         return 1;
     }
     *pHeap =3D 7;
     std::cout << "localVariable: " << localVariable << "\n";
     std::cout << "*pLocal: " << *pLocal << "\n";
     std::cout << "*pHeap: " << *pHeap << "\n";
     delete pHeap;
     delete pHeap;                    // Try to crash program
     pHeap =3D new int;
     if (pHeap =3D=3D NULL)
     {
         std::cout << "Error! No memory for pHeap!!";
         return 1;
     }
     *pHeap =3D 9;
     std::cout << "*pHeap: " << *pHeap << "\n";
     delete pHeap;
     return 0;
 }



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If set pHeap to null, however, and then try to assign *pHeap a value, as =
shown below, the program will crash.=20


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 // Listing 9.4
 // Allocating and deleting a pointer

 #include <iostream>

 int main()
 {
     int localVariable =3D 5;
     int * pLocal=3D &localVariable;
     int * pHeap =3D new int;
     if (pHeap =3D=3D NULL)
     {
         std::cout << "Error! No memory for pHeap!!";
         return 1;
     }
     *pHeap =3D 7;
     std::cout << "localVariable: " << localVariable << "\n";
     std::cout << "*pLocal: " << *pLocal << "\n";
     std::cout << "*pHeap: " << *pHeap << "\n";
     pHeap =3D 0;
     *pHeap =3D33;
     pHeap =3D new int;
     if (pHeap =3D=3D NULL)
     {
         std::cout << "Error! No memory for pHeap!!";
         return 1;
     }
     *pHeap =3D 9;
     std::cout << "*pHeap: " << *pHeap << "\n";
     delete pHeap;
     return 0;
 }


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I have several questions.

    1.. How could someone who is expert enough to write a beginning book =
on C++ make such an error
    2.. Is it perhaps because the ANSI/ISO definition of the language =
and what a compiler should do has changed?=20
    3.. Have GNU and the Gates crowd made similar unilateral changes to =
solve a pesky problem?
    4.. Did I misread something which seems fairly straight forward?
    5.. Are there other pointer inconsistances hiding in the weeds =
waiting to pounce on the unsuspecting?