Constant and pointer

shabbir's Avatar author of Constant and pointer
This is an article on Constant and pointer in C.

Introduction



In C you can effectively change the value of the constant variable. Just compile the following program and you will see the output.

Code: C
void main()
{
    int const i=123;
    int *ip;
   
    /* Changing constant integer i.e. const int/int const */
   
    printf(" %d %x \n",i,&i);
    ip=&i;
    *ip=456;
    printf(" %d %x %x %d \n",i,&i,ip,*ip);
   
}
Remember to name the file as <filename>.c and not <filename>.cpp You can also download the attachment.

But when you put the same code in the cpp file the compiler would give you an error message. I am talking with respect to MS compiler.

Getting into the constants and pointer



This was about the introduction and how .c and .cpp compiler treated the const. Now I would like to go deep into what does const means in cpp and what are the combination you can use with the object pointer and how the complicated the situation can become.

const int *p
int const *p
int * const p
const * int p // Warning
* const int p // Error
* int const p // Error

Lets take each one of them and see what it means how it behaves

const int *p & int const *p



Read it as "Pointer to constant int". For how to read it refer to the last section of the article, How to read. Now the expression means its a pointer pointing to the constant value. As an example
Code: CPP
cout<<"Pointer to constant int"<<endl;
{
    int i = 123;
    const int * p = &i;
    cout <<"p = i = "<<*p<<endl;
    p = &i;
//  *p = 321; Pointer to constant objects so cant change values
    cout <<"p = i = "<<*p<<endl;
}
cout<<"Pointer to constant int"<<endl;
{
    int i = 123;
    int const * p = &i;
    cout <<"p = i = "<<*p<<endl;
    p = &i;
//  *p = 321; Pointer to constant objects so cant change values
    cout <<"p = i = "<<*p<<endl;
}

int * const p



Read it as "Constant pointer". For how to read it refer to the last section of the article, How to read. Now the expression means that the pointer is constant
Code: CPP
cout<<"Constant pointer"<<endl;
{
    int i = 123;
    int * const p = &i;
    cout <<"p = i = "<<*p<<endl;
//  p = &i; Constant Pointer so unable to change
    *p = 321;
    cout <<"p = i = "<<*p<<endl;
}

const * int p



This is also not a valid expression and would generate a warning and should be avoided.

Code: CPP
cout<<"Pointer to constant int"<<endl;
{
    int i = 123;
    const * int p = &i;
    // warning: 'int ' storage-class or type specifier(s) unexpected here; ignored
    cout <<"p = i = "<<*p<<endl;
    p = &i;
//  *p = 321; Pointer to constant objects so cant change values
    cout <<"p = i = "<<*p<<endl;
}

* const int p & * int const p



Declaring a variable in this manner will generate and error.

How to Read



You have to read pointer declarations right-to-left.

const int * p means "p points to an int that is const"
int * const p means "p is a const pointer to an int"
Attached Files
File Type: zip ConstPointer.zip (712 Bytes, 44 views)
peterdrew, Scripting likes this
aisha.ansari84's Avatar, Join Date: Feb 2008
Contributor
i m really confused with such sorts ,but your article gave some concept
peterdrew's Avatar, Join Date: Jul 2010
Newbie Member
It's very good; thanks!
mbtshoes's Avatar, Join Date: Jul 2010
Newbie Member
i think so
Scripting's Avatar, Join Date: Jun 2010
John Hoder
Really cool, shabbir.

THX for this
shabbir's Avatar, Join Date: Jul 2004
Go4Expert Founder
The pleasure is mine.
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Banned
Thanks for the blog loaded with so many information. Stopping by your blog helped me to get what I was looking for.
gurumanoh's Avatar, Join Date: Aug 2010
Newbie Member
good work...
wdliming's Avatar, Join Date: Sep 2010
Go4Expert Member
good!! I like it!!
kien_vn's Avatar, Join Date: Aug 2010
Go4Expert Member
It's very clear to understand - "What is pointer"