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);
}
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 *pint const *pint * const pconst * int p // Warning*
const int p // Error*
int const p // ErrorLets 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"
