How do you define a mutable pointer to a const object?

Question

I have a class that contains a pointer to a constant VARIANT value outside the class, but sometimes I want to change this pointer to refer to a VARIANT member object of the class itself.

Most instances of this class will be const, so I have to declare the pointer as mutable.

In Visual C++ this code seems to do what I want:

VARIANT mutable const* m_value;

However, since mutable is meant to be a property of the pointer and not the pointee, I would think this to be the correct syntax:

VARIANT const * mutable m_value;

Similar to how you define a constant pointer (and not a pointer to a const object). Visual C++ does not accept this variant though.

warning C4518: 'mutable ' : storage-class or type specifier(s) unexpected here; ignored

Is Visual C++ right, or am I missing something? Could another more standard-conformant compiler behave differently?

Martin Ba · Accepted Answer · 2010-11-05 10:23:01Z

Comeau online seems to agree with VC++ here.

And it also makes sense! A class member can only be mutable once and there is no such thing as a non-const pointer to a mutable const object. "Mutable const object" doesn't make sense.

You should put the mutable in front of your declaration, as it is in the same area as, for example, static:

class A { static int const* m_p1; // static modifiable pointer to a const object; mutable int const* m_p2; // mutable pointer to a const object ... mutable int *const m_p3; // DOES NOT MAKE sense

m_p3 does not make sense - you declare the member as "always mutabel" and as "always const" at the same time.

Thank you for the answer! (and the tip about Comeau online, might come in handy in the future). Yeah, I guess it makes more sense that mutable behaves (and "binds") similar to static and not as const.