问题
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?
回答1:
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.
回答2:
A cv-qualifier is a specifier, actually a type specifier.
Quoting C++03 7.1:
The specifiers that can be used in a declaration are
decl-specifier:
storage-class-specifier
type-specifier
function-specifier
friend
typedef
decl-specifier-seq:
decl-specifier-seq_opt
decl-specifier
... while type specifiers are defined:
type-specifier:
simple-type-specifier
class-specifier
enum-specifier
elaborated-type-specifier
cv-qualifier
As for the distinction between the word specifier and qualifier:
Each type which is a cv-unqualified complete or incomplete object type or is void has three corresponding cv-qualified versions of its type: a const-qualified version, a volatile-qualified version, and a const-volatile-qualified version. The term object type includes the cv-qualifiers specified when the object is created. The presence of a const specifier in a decl-specifier-seq declares an object of const-qualified object type; such object is called a const object. The presence of a volatile specifier in a decl-specifier-seq declares an object of volatile- qualified object type; such object is called a volatile object. The presence of both cv-qualifiers in a decl-specifier-seq declares an object of const-volatile-qualified object type; such object is called a const volatile object. The cv-qualified or cv-unqualified versions of a type are distinct types; however, they shall have the same representation and alignment requirements.
While it isn't stated obviously, the above paragraph shows the difference. I admit that the definitions could be more strict however.
来源：https://stackoverflow.com/questions/3610943/correlation-between-specifier-and-qualifier