7 Standard conversions [conv]

7.5 Qualification conversions [conv.qual]

1
#
A cv-decomposition of a type T is a sequence of and such that T is “ ⋯ U” for , where each is a set of cv-qualifiers ([basic.type.qualifier]), and each is “pointer to” ([dcl.ptr]), “pointer to member of class of type” ([dcl.mptr]), “array of ”, or “array of unknown bound of” ([dcl.array]).
If designates an array, the cv-qualifiers on the element type are also taken as the cv-qualifiers of the array.
[ Example
:
The type denoted by the type-id const int ** has two cv-decompositions, taking U as “int” and as “pointer to const int”.
— end example
 ]
The n-tuple of cv-qualifiers after the first one in the longest cv-decomposition of T, that is, , is called the cv-qualification signature of T.
2
#
Two types and are similar if they have cv-decompositions with the same n such that corresponding components are the same and the types denoted by U are the same.
3
#
A prvalue expression of type can be converted to type if the following conditions are satisfied, where denotes the cv-qualifiers in the cv-qualification signature of :60
  • (3.1)
    and are similar.
  • (3.2)
    For every , if const is in then const is in , and similarly for volatile.
  • (3.3)
    If the and are different, then const is in every for .
[ Note
:
If a program could assign a pointer of type T** to a pointer of type const T** (that is, if line #1 below were allowed), a program could inadvertently modify a const object (as it is done on line #2).
For example,
int main() {
  const char c = 'c';
  char* pc;
  const char** pcc = &pc;       // #1: not allowed
  *pcc = &c;
  *pc = 'C';                    // #2: modifies a const object
}
— end note
 ]
4
#
[ Note
:
A prvalue of type “pointer to cv1 T” can be converted to a prvalue of type “pointer to cv2 T” if “cv2 T” is more cv-qualified than “cv1 T”.
A prvalue of type “pointer to member of X of type cv1 T” can be converted to a prvalue of type “pointer to member of X of type cv2 T” if “cv2 T” is more cv-qualified than “cv1 T”.
— end note
 ]
5
#
[ Note
:
Function types (including those used in pointer to member function types) are never cv-qualified ([dcl.fct]).
— end note
 ]
60)
These rules ensure that const-safety is preserved by the conversion.