Constant and pointer

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:

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:

cout<<"Pointer to constant int"<<endl;
{
	int i = 123;
	const int * p = &i;
	cout <<"p = i = "<<*p<<endl;
	p = &i;
//	*p = 321; [URL=http://www.go4expert.com/articles/difference-constant-pointers-pointer-t26816/]Pointer to constant[/URL] 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:

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:

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"

 

The pleasure is mine.