why does this happen with const pointer to int?

[rajhansk]

I wrote the following two codes. but one is giving erro

  #include <iostream>

using namespace std;

int main()
{
    int var1=10;
    int *const ptr;
    ptrt=&var1;


    cout<<*ptr;

    return 0;
}

why does the above code is giving error?
and the same code with minor difference is running fine?

#include <iostream>

using namespace std;

int main()
{
    int var1=10;
    int *const ptr=&var1;
    
    cout<<*ptr;

    return 0;
}

is it always necessary to initialize the const pointer to int at the declaration itself ??

[mbozzi]

is it always necessary to initialize the const pointer to int at the declaration itself ??

Basically, yes. However, note the differences between
int const*, int *const, and int const* const:
Which respectively denote
- an integer-constant pointer (i.e., a pointer to integer-constant)
- an integer pointer-constant (i.e., a constant pointer to integer)
- an integer-constant pointer-constant (i.e., a constant pointer to integer-constant).

given int* const, it is the pointer which is constant, not the pointed-to integer.

[rajhansk]

can you tell me where can anybody use this int * const??

I mean, I wanted to know the practical use of this method..

Thanks in advance

[AbstractionAnon]

can you tell me where can anybody use this int * const??

Any time you don;t want a pointer changed.

A common use of this is when specifying a pointer as an argument to a function. Since a pointer is passed by value, changing the local copy of the pointer is usually a logic error. Declaring the pointer argument as const prevents this.