What does "cv-unqualified" mean in C++?

Question 1

There are fundamental types and compound types. Fundamental types are the arithmetic types, void, and std::nullptr_t. Compound types are arrays, functions, pointers, references, classes, unions, enumerations, and pointers to non-static members.

A cv-unqualified type is any of those types.

For any cv-unqualified type, there are three corresponding cv-qualified types:

const-qualified - with the const cv-qualifier
volatile-qualified - with the volatile cv-qualifier
const-volatile-qualified - with both the const and volatile cv-qualifiers

Note, however, that cv-qualifiers applied to an array type actually apply to its elements.

The cv-qualified and cv-unqualified types are distinct. That is int is a distinct type from const int.

Question 2

A type is "cv-unqualified" if it doesn't have any cv-qualifiers. A cv-qualifer is either const or volatile.

Question 3

cv stands for const and volatile (and more rarely mutable), two attributes qualifying a type. You can manipulate them with std::remove_const and the like in C++11.

The excellent cppreference site gives you more info.

To answer your question, a cv-unqualified type either doesn't have or is stripped from its cv-qualifiers. For instance int is the cv-unqualified part of const volatile int.

std::remove_cv<T>::type is the cv-unqualified partof T.

Question 4

cv-unqualified type is a type that hasn't been specified by any of cv-qualifiers. These define two basic properties of a type: constness and volatility. See C++03 3.9.3 CV-qualifiers §1:

A type mentioned in 3.9.1 and 3.9.2 is a cv-unqualified type. Each type which is a cv-unqualified complete or incomplete object type or is void (3.9) 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 (1.8) 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 volatilequalified object type; such object is called a volatile object.

The presence of both cv-qualifiers in a declspecifier-seq declares an object of const-volatile-qualified object type; such object is called a const volatile object.

Question 5

Generally it means "the same type, but with any cv-qualifier removed", so (for example) the cv-unqualified version of void volatile * const x would be void *x.

Note that here, however, I'm removing the cv-qualifiers from both the pointer itself and what it points at. In most cases, cv-unqualified will refer only to one object at a time, so a cv-unqualified version of the pointer itself would still be void volatile *x, whereas a cv-unqualfied version of what it points at would be void *const x.

Question 6

I think First step is to understand possible types & what CV means:

const and volatile appear in any type specifier, including decl-specifier-seq of declaration grammar, to specify constness or volatility of the object being declared or of the type being named.

const - defines that the type is constant.

volatile - defines that the type is volatile.

Explanation

For any type T (including incomplete types), other than function type or reference type, there are three more distinct types in the C++ type system: const-qualified T, volatile-qualified T, and const-volatile-qualified T.

Note: array types are considered to have the same cv-qualification as their element types. When an object is first created, the cv-qualifiers used (which could be part of decl-specifier-seq or part of a declarator in a declaration or part of type-id in a new-expression) determine the constness or volatility of the object, as follows:

const object - an object whose type is const-qualified or a non-mutable subobject of a const object. Such object cannot be modified: attempt to do so directly is a compile-time error, and attempt to do so indirectly (e.g., by modifying the const object through a reference or pointer to non-const type) results in undefined behavior.

volatile object - an object whose type is volatile-qualified, or a subobject of a volatile object, or a mutable subobject of a const-volatile object. Every access (read or write operation, member function call, etc.) made through a glvalue expression of volatile-qualified type is treated as a visible side-effect for the purposes of optimization (that is, within a single thread of execution, volatile accesses cannot be optimized out or reordered with another visible side effect that is sequenced-before or sequenced-after the volatile access. This makes volatile objects suitable for communication with a signal handler, but not with another thread of execution, see std::memory_order). Any attempt to refer to a volatile object through a non-volatile lvalue (e.g. through a reference or pointer to non-volatile type) results in undefined behavior.

const volatile object - an object whose type is const-volatile-qualified, a non-mutable subobject of a const volatile object, a const subobject of a volatile object, or a non-mutable volatile subobject of a const object. Behaves as both a const object and as a volatile object.

Ref: Cpp reference