Daniel Berlin <[EMAIL PROTECTED]> writes: > > In other words, we're asked to agree that the type of an object > > changes depending on how it is accessed. > > For the benefit of readers, only the first sentence of this para is > > the language of the standard; the rest isn't. > > > > That an object referred to through a volatile pointer must > > "temporarily" be treated as though it were declared volatile is the > > crux of this argument. > > Again, you could say the same about const, restrict, or any other > qualifier then, making them more or less useless as qualifiers.
I think there may be a difference. const is inherently a characteristic of the object. It applies at definition time. Casting away const in a reference does not change the definition. Whether making an assignment through a pointer after casting away const is legal depends upon how the definition of the object pointed to is handled (C99 6.7.3: "If an attempt is made to modify an object defined with a const-qualified type through use of an lvalue with non-const-qualified type, the behavior is undefined"). restrict is less interesting, since it simply provides a guarantee about a pointer. Casting away restrict does nothing useful--it just abandons the guarantee. Casting to restrict is by definition only valid if the guarantee holds--if the object is indeed restricted (C99 6.7.3.1: ".... If these requirements are not met, then the behavior is undefined"). volatile, on the other hand, is inherently a characteristic of the access, not of the object. Defining a volatile object does nothing in itself; it merely affects all accesses to the object. Thus casting to volatile should mean that all uses of the resulting pointer should be done in a volatile fashion--i.e., all reads and writes done precisely as specified by the standard's "abstract machine." Casting away from volatile means the reverse--there are no restrictions on the use of the object to which the pointer points. I am not aware of any other type qualifiers in C or C++. Ian
