Solved

pointer to an object that cant be modified indirectly

Posted on 2004-03-23
6
200 Views
Last Modified: 2010-04-01
How would you define a pointer P that is initialized to point to the location of an int object i . The definition should make it illegal to modify the value of i indirectly through P .
0
Comment
Question by:plumcrazy
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
  • 3
6 Comments
 
LVL 10

Expert Comment

by:Sys_Prog
ID: 10664287
int main()
{
      int i = 10 ;
      const int *p = &i ;
      *p = 20 ;
    system ( "PAUSE" ) ;
    return 0;
}


Amit
0
 
LVL 10

Expert Comment

by:Sys_Prog
ID: 10664305
The line '*p = 20' in the above shown code gives an error saying a 'assignment to a const location'

The following code demostrates that u can't change i indirectyly thru P, however u can change i directly

int main()
{
      int i = 10 ;
      const int *p = &i ;
      cout << "i : " << i <<endl ;
      cout << "*p : " << *p <<endl;

      *p = 20 ;                                    // Syntax Error (Comment this for the program to run )
      i = 30 ;
      cout << "i : " << i <<endl;
      cout << "*p : " << *p <<endl;
    system ( "PAUSE" ) ;
    return 0;
}


Amit
0
 
LVL 10

Accepted Solution

by:
Sys_Prog earned 125 total points
ID: 10664351
There are two important thing regarding using const with pointers

Constant Pointer - A pointer which cannot point to some other location after being initialized to point to some location. However     the  value at which the pointer points can be changed

        int * const p = &i ;

Pointer to const - A pointer which can be made to point to some other location. However the value pointed to by the pointer cannot be changed

        int const *p = &i ;
        const int *p = &i ;

It's easy to distinguish/write the two
read from right to left
Thus, for first case above, "p is a const pointer to a integer"
and for second case above, "p is a pointer to a const integer" OR "p is a pointer to a integer const"


You can combine the above both to have a const pointer to a const integer

HTH

Amit





0
 
LVL 5

Expert Comment

by:dennis_george
ID: 10664685
Hi,

I think if you want that nobody can change the value of int directly then declare it as

const int i = 10 ;

By this way even if you assign the address of int to a non constant pointer you will get a warning or error. (because you are advised to assign address of a const data type to a non const pointer) :: by const pointer I mean to say pointer to a constant location.

But if you only want that the pointer you declare should not change its value then Sys_prog is quite precise in his answer.

Dennis
0

Featured Post

Free Tool: Postgres Monitoring System

A PHP and Perl based system to collect and display usage statistics from PostgreSQL databases.

One of a set of tools we are providing to everyone as a way of saying thank you for being a part of the community.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

This article will show you some of the more useful Standard Template Library (STL) algorithms through the use of working examples.  You will learn about how these algorithms fit into the STL architecture, how they work with STL containers, and why t…
Many modern programming languages support the concept of a property -- a class member that combines characteristics of both a data member and a method.  These are sometimes called "smart fields" because you can add logic that is applied automaticall…
The goal of the tutorial is to teach the user how to use functions in C++. The video will cover how to define functions, how to call functions and how to create functions prototypes. Microsoft Visual C++ 2010 Express will be used as a text editor an…
The viewer will learn how to pass data into a function in C++. This is one step further in using functions. Instead of only printing text onto the console, the function will be able to perform calculations with argumentents given by the user.

749 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question