Got confused with const_cast

Sample_Program-1

#include<iostream>
using namespace std ;

int main(){
const int i = 9;
int *j = const_cast<int*>(&i); //Ok
int *j = const_cast<int*>(i); //Error
}

Sample_Program-2

#include<iostream>
using namespace std ;

int main(){
const int i = 9;
int j = const_cast<int&>(i);//Ok
int j = const_cast<int>(i);//Error
}


I was just learning some c++ concept and met with the above 2 concepts . Can anyone please explain the concept i marked as error in the above 2 sample program ?

Your compiler should have given you some error messages. What do they say and what do you think they mean?

My compiler gives only error message but dont explain it . I am new to c++ . Thats why i have mentioned in my post "Can anyone please explain the conpt i marked as error in the above 2 sample program ?"

Quote:

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Originally Posted by vikuseth

My compiler gives only error message but dont explain it .

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Well, for your first example (once I fix an irrelevant error whereby you declare j twice), my compiler complains that there was an:

Code:

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invalid const_cast from type 'const int' to type 'int*'

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Doesn't that explain to you what is the problem? The problem is that the const_cast is invalid. Why is it invalid? Because a const_cast is supposed to be used to remove (or add) const-ness, but here you are also changing from a non-pointer type to a pointer type.

For your next example, I get:

Code:

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invalid use of const_cast with type 'int', which is not a pointer, reference, nor a pointer-to-data-member type

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This tells you that the const_cast is invalid because what you are trying to cast "is not a pointer, reference, nor a pointer-to-data-member type".

So, did you actually read the error messages that your compiler gave you and tried to make sense to what they said?

So constant cast only works for pointer , reference or pointer to datamember ?

Quote:

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Originally Posted by vikuseth

So constant cast only works for pointer , reference or pointer to datamember ?

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Consider your second example. You could have done without a cast:

Code:

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int j = i;

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The fact that i is const does not matter since its value is copied to j.

On the other hand, consider:

Code:

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const int x = 123;
const int* i = &x;
int* j = i;

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Now this is a problem, because you could potentially write:

Code:

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*j = 456;

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which would result in undefined behaviour since you're changing a constant. However, maybe for some reason (e.g., working with a legacy API) you really do need j to be an int*, not a const int*, but you know for sure that the value will not be changed through the pointer. In that case, it is appropriate to write:

Code:

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int* j = const_cast<int*>(i);

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