Pointers and constants

This is a discussion on Pointers and constants within the C++ Programming forums, part of the General Programming Boards category; Hello everyone! I've just started to learn C++ and have a question... I've written the following code: Code: #include <iostream.h> ...

  1. #1
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    Pointers and constants

    Hello everyone! I've just started to learn C++ and have a question... I've written the following code:

    Code:
    #include <iostream.h> 
    
    int main()
    {
      const int i=1;
      int* pi;
      //pi = &i;  Error, since pi isn't declared as a pointer\
                      which points to a constant
      pi = (int*) &i; //  Address of constant i allocated \
                                to pointer pi with a cast
      *pi = 2;
      cout << "Address of i:\t\t"  << &i << endl;
      cout << "pi points to:\t\t"  << pi << endl;
      cout << "Value of i:\t\t" << i << endl;
      cout << "Value pointed to by pi:\t" << *pi << endl;
      return 0;
    }
    which produces in my computer the following output:

    Address of i: 0x0012FF7C
    pi points to: 0x0012FF7C
    Value of i: 1
    Value pointed to by pi: 2

    Now, my question is, why, if the address of i and the address pointed to by pi are the same, can I (apparently) have two different values stored in this same address? I'm sure I'm missing something here, maybe I have some conceptual misconception on the subject pointers... Any help would be much appreciated.

  2. #2
    and the hat of wrongness Salem's Avatar
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    Simple example of type-abuse.

    If you go out of your way to cast away all the useful warnings, don't complain when it all goes wrong.

    > cout << "Value of i:\t\t" << i << endl;
    The compiler knows i is const (despite your efforts), and simply generates the code
    cout << "Value of i:\t\t" << 1 << endl;
    without ever referring to the storage location called i
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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    Aha! That makes a lot of sense, I should've thought of it. But when you said compiler you meant preprocessor, right? (just an honest question from a newbie trying to figure out how this thing works...)

    Thanks!
    Last edited by Jasel; 10-25-2003 at 02:34 AM.

  4. #4
    and the hat of wrongness Salem's Avatar
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    No, I meant compiler.

    The pre-processor deals with #include, #if and #define
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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    That means, if I define a constant with #define, it will be dealt with by the preprocessor, whereas if I define it with const, then it will be the compiler who deals with it? Do you have any advice on what's better?

  6. #6
    and the hat of wrongness Salem's Avatar
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    const is very much preferred in C++, primarily because it is type-safe. The pre-processor is just an 'on the fly' editor of your code, doing some fairly crude search/replace text substitutions. Occasionally, this produces some real surprises for the programmer.

    In C, you have to use the pre-processor for things like
    Code:
    #define SIZE 10
    #define max(a,b) a>b?a:b  /* this is a dangerous macro */
    int arr[SIZE];
    Because you can't do
    Code:
    const int SIZE = 10;
    int arr[SIZE];  /* doesn't work in C */
    In C++, you would do this
    Code:
    const int SIZE = 10;
    inline int max( int a, int b ) {
        return a>b?a:b;
    }
    int arr[SIZE];
    Inline functions are as fast as the macro versions, but again they are type-safe.
    For example, this would compile OK using the #define, but would produce some odd results at run-time.
    It would not compile at all with the C++ inline function
    Code:
    char *p = max( "hello", "world" );
    In addition, the inline function does not suffer from multiple evaluation problems, like
    Code:
    int c = max( a++, b++ );
    In C (using the macro), this expands to
    Code:
    int c = a++ > b++ ? a++ : b++;
    Which results in either a or b being incremented TWICE (almost certainly not what you wanted / expected).
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  7. #7
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    I see... thank you very much for the detailed explanation!

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