Question

I encountered an error as the topic says: assignment of data-member ‘RootBoxT<CORE::DoubleWrapper>::innerBox_’ in read-only structure. I “might” know why does this happen, but I need to learn more about the rules.

I made all instances for RootBoxT as const, and the member variable *innerBox_ of class RootBoxT is also const type. In one of the methods of class RootBoxT, I need to update the *innerBox_, so I called delete to free the memory of innerBox_ and assigned it with another const pointer.

My questions is that:

1. Is this the right method, that when I need to update the value of a const pointer, I just delete it and assign it with another const pointer?
2. Is this error cause by the fact that the instance of RootBoxT is const but one of the methods changed the place that a member variable points to? If not, why does it happen? If so, should I just change these instances to non-const?

I really messed up with const keyword for my current project, as I’m rather new to C++. Thanks.

Answer 1

If you have something like this:

struct S
{
   int * i;
};

and a const S object (s), the pointer s.i is const but not the int it points to (*s.i). This means you can write:

*s.i = 4;
delete s.i;

but not:

s.i = new int (5);   // illegal
int * j = 0;
s.i = j;             // illegal

because that would modify the pointer value, not what it points to.