7 Standard conversions [conv]

7.5 Qualification conversions [conv.qual]

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, and each is “pointer to”, “pointer to member of class of type”, “array of ”, or “array of unknown bound of”.
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.
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.
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
  • and are similar.
  • For every , if const is in then const is in , and similarly for volatile.
  • 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
]
[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
]
[Note
:
Function types (including those used in pointer to member function types) are never cv-qualified ([dcl.fct]).
end note
]
These rules ensure that const-safety is preserved by the conversion.