A glvalue ([basic.lval]) of a non-function, non-array type T can be converted to a prvalue.53 If T is an incomplete type, a program that necessitates this conversion is ill-formed. If the object to which the glvalue refers is not an object of type T and is not an object of a type derived from T, or if the object is uninitialized, a program that necessitates this conversion has undefined behavior. If T is a non-class type, the type of the prvalue is the cv-unqualified version of T. Otherwise, the type of the prvalue is T.54
When an lvalue-to-rvalue conversion occurs in an unevaluated operand or a subexpression thereof (Clause [expr]) the value contained in the referenced object is not accessed. Otherwise, if the glvalue has a class type, the conversion copy-initializes a temporary of type T from the glvalue and the result of the conversion is a prvalue for the temporary. Otherwise, if the glvalue has (possibly cv-qualified) type std::nullptr_t, the prvalue result is a null pointer constant ([conv.ptr]). Otherwise, the value contained in the object indicated by the glvalue is the prvalue result.
[ Note: See also [basic.lval]. — end note ]
For historical reasons, this conversion is called the “lvalue-to-rvalue” conversion, even though that name does not accurately reflect the taxonomy of expressions described in [basic.lval].
In C++ class prvalues can have cv-qualified types (because they are objects). This differs from ISO C, in which non-lvalues never have cv-qualified types.