Casting between integers and pointers in C++

Question

#include<iostream>
using namespace std;

int main()
{
  int *p,*c;
  p=(int*)10;
  c=(int*)20;
  cout<<(int)p<<(int)c;
}

Somebody asked me "What is wrong with the above code?" and I couldn't figure it out. Someone please help me.

Answer 1

The fact that int and pointer data types are not required to have the same number of bits, according to the C++ standard, is one thing - that means you could lose precision.

In addition, casting an int to an int pointer then back again is silly. Why not just leave it as an int?

I actually did try to compile this under gcc and it worked fine but that's probably more by accident than good design.

Answer 2

Some wanted a quote from the C++ standard (I'd have put this in the comments of that answer if the format of comments wasn't so restricted), here are two from the 1999 one:

5.2.10/3

The mapping performed by reinterpret_cast is implementation defined.

5.2.10/5

A value of integral type or enumeration type can be explicitly converted to a pointer. A pointer converted to an integer of sufficient size (if ant such exists on the implementation) and back to the same pointer type will have its original value; mappings between pointers and integers are otherwise implementation-defined.

And I see nothing mandating that such implementation-defined mapping must give a valid representation for all input. Otherwise said, an implementation on an architecture with address registers can very well trap when executing

p = (int*)10;

if the mapping does not give a representation valid at that time (yes, what is a valid representation for a pointer may depend of time. For instance delete may make invalid the representation of the deleted pointer).

Answer 3

Assuming I'm right about what this is supposed to be, it should look like this:

int main()
{
  int *p, *c;
  // Something that creates whatever p and c point to goes here, a trivial example would be.
  int pValue, cValue;
  p = &pValue;
  c = &cValue;
  // The & operator retrieves the memory address of pValue and cValue. 

  *p = 10;
  *c = 20;
  cout << *p << *c;
}

In order to assign or retrieve a value to a variable referenced by a pointer, you need to dereference it.

What your code is doing is casting 10 into pointer to int (which is the memory address where the actual int resides).