7 Standard conversions [conv]

7.5 Qualification conversions [conv.qual]

A cv-decomposition of a type T is a sequence of cvi and Pi such that T is cv0 P0 cv1 P1 cvn1 Pn1 cvn U” for n>0, where each cvi is a set of cv-qualifiers, and each Pi is “pointer to”, “pointer to member of class Ci of type”, “array of Ni”, or “array of unknown bound of”. If Pi designates an array, the cv-qualifiers cvi+1 on the element type are also taken as the cv-qualifiers cvi 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, cv1,cv2,,cvn, is called the cv-qualification signature of T.

Two types T1 and T2 are similar if they have cv-decompositions with the same n such that corresponding Pi components are the same and the types denoted by U are the same.

A prvalue expression of type T1 can be converted to type T2 if the following conditions are satisfied, where cvji denotes the cv-qualifiers in the cv-qualification signature of Tj:60

• T1 and T2 are similar.

• For every i>0, if const is in cv1i then const is in cv2i, and similarly for volatile.

• If the cv1i and cv2i are different, then const is in every cv2k for 0<k<i.

[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.