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Sep 21st, 2006

interesting code......Help

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please help me understand the folowing code....

C++ Syntax (Toggle Plain Text)
 class A {
      int a;
  public:
     A() : a ( 0 ) {} 
     int get()  {
          cout << "Hello WOrld " << endl;
          return 0;
     }
 };
 
  int main( int argc, char** argv )
 {
      cout<< ((A*)3)->get() << endl;
       return 0;
 }
 class A {
      int a;
  public:
     A() : a ( 0 ) {} 
     int get()  {
          cout << "Hello WOrld " << endl;
          return 0;
     }
 };
 
  int main( int argc, char** argv )
 {
      cout<< ((A*)3)->get() << endl;
       return 0;
 }

this code runs! and prints the correct output if i return zero in get() function,, but when i return a it throws an exception ( i think becuase a is not initialized, infact not constructed ), but this code runs ......no idea:eek:

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Laiq Ahmed

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Sep 21st, 2006

Re: interesting code......Help

Click to Expand / Collapse Quote originally posted by Laiq Ahmed ...

please help me understand the folowing code....
C++ Syntax (Toggle Plain Text)
 class A {
      int a;
  public:
     A() : a ( 0 ) {} 
     int get()  {
          cout << "Hello WOrld " << endl;
          return 0;
     }
 };
 
  int main( int argc, char** argv )
 {
      cout<< ((A*)3)->get() << endl;
       return 0;
 }
class A { int a; public: A() : a ( 0 ) {} int get() { cout << "Hello WOrld " << endl; return 0; } }; int main( int argc, char** argv ) { cout<< ((A*)3)->get() << endl; return 0; }

The thing is that when an object is created, its constructor is called but not when the object pointer is created which is the thing in your case. (please get this verified, havent done C++ in quite a while)

So as a pointer to object is created, you dont specifically allocate any memory, the class variable doesnt exist. So as soon as the function si encountered, the "cout" stmt is executed and prints out the output but when encounters the reutrn a stmt, there is as such no "a" to which the object points to since no memory has been allocated.

In the end, yes since the memory is not allocated for the pointer and neither does it point to any object hence an execption is generated. Returning something which doesnt exist is not a very good thing

And btw your code doesnt technically run as an run time exception is generated, there is a difference you know

Hope it helped, bye.

Last edited by ~s.o.s~; Sep 21st, 2006 at 1:21 pm. Reason: Some additions...

~s.o.s~

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Sep 21st, 2006

Re: interesting code......Help

>>cout<< ((A*)3)->get() << endl;

This is using undefined behavor techniques. 3 isprobably not a valid address for an object of type class A.

C++ Syntax (Toggle Plain Text)
A* pA = new A; // allocate an instance of the class
pA->get();
// 
// and don't forget to deallocate it
delete pA;
A* pA = new A; // allocate an instance of the class
pA->get();
// 
// and don't forget to deallocate it
delete pA;

Another way to do it is to make the pointer point to an already existing object of the class

C++ Syntax (Toggle Plain Text)
A a; // an instance of class A
A* pA = &a; // pointer to the object
//
//
// you do NOT delete the pointer
A a; // an instance of class A
A* pA = &a; // pointer to the object
//
//
// you do NOT delete the pointer

Last edited by Ancient Dragon; Sep 21st, 2006 at 1:26 pm.

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Sep 21st, 2006

Re: interesting code......Help

Click to Expand / Collapse Quote originally posted by Ancient Dragon ...

>>cout<< ((A*)3)->get() << endl;

This is using undefined behavor techniques. 3 isprobably not a valid address for an object of type class A.

Oh yes and i overlooked this particular thing. Thanks for pointing it out Mr. Dragon. Maybe this is one of those quizzies questions where you are asked to predict the output of the code snippet.

Rule:
1. Before using any pointer dont forget to allocate memory to it or to point it to an existing object. Hence when using a pointer better ground it (make it NULL) so that you dont end up using it by mistake.

2. Delete only those pointers for those you have explicitly allocated memory using the "new" stmt. Forgettting to free the memory results in what is known as a memory leak.

Hope it helped, bye.

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Sep 22nd, 2006

Re: interesting code......Help

heyz it its undefined what about the following code

C++ Syntax (Toggle Plain Text)
 
class  A { 
  int a;
public:
  A() : a ( 0 )  {}
 
 int get() const {
       cout <<" in A()" << endl;
       return a; 
 }
};
 
int main()
{
       *((int*)0x00100ff)=14;
       cout<< reinterpret_cast<A*>(0x00100ff)->get() << endl;
       return 0;
}
 
class  A { 
  int a;
public:
  A() : a ( 0 )  {}
 
 int get() const {
       cout <<" in A()" << endl;
       return a; 
 }
};
 
int main()
{
       *((int*)0x00100ff)=14;
       cout<< reinterpret_cast<A*>(0x00100ff)->get() << endl;
       return 0;
}

it works ..........

Laiq Ahmed

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Sep 22nd, 2006

Re: interesting code......Help

yes its using undefined behavior too. You can't just hardcode an address into the program like that and expect it to work because only the computer knows where an object's address, unless you are using some obscure operating system such as some embedded systems.

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