Correlation between specifier and qualifier?
c++, lvalue
Solution
Most of it doesn't make sense.
Specifier and qualifier are defined in the C++ standard. Qualifier is just an integral part of a specifier. For example, type specifier in a declaration can include cv-qualifiers. I don't see the reason to quote everything from the standard on this topic.
Cv-qualifiers are not restricted to lvalues. Rvalues of class types can also be cv-qualified. It is possible to cv-qualify an rvalue of non-class type, but it will have no effect and will be ignored.
The use of `const` qualifier that you show in your example with `foo` is just a syntactic form, which actually means that the const-qualifier is applied to the implied `this` parameter of the `foo` method: `const A* this`. I.e. in this case it does indeed qualify an lvalue, but it is `*this`, not `foo`.
The term qualifier also appears in the context of qualified names. Name like `some_class::some_member` (or `some_namespace::some_name`) are called qualified names and the `some_class::` part is a qualifier.
The idea that if something is an lvalue then you can modify it is totally incorrect. There are modifiable lvalues and non-modifiable lvalues. An object declared as `const int i = 5` is an lvalue, yet you can't modify it. Ordinary functions are also lvalues in C++, yet you can't modify a function.
Problem
`const` and `volatile` are called `cv-qualifier` by the C spec. What is exactly defference between `specifier` and `qualifier` (`cv-qualifier`)? Is a qualifier is a specifier as well? Is it necessarry that `qualifier` is with an lvalue only? What are qualifiers other than `cv-qualifier`? Does my above understanding make any sense?