7 Expressions [expr]

7.3 Standard conversions [conv]

7.3.5 Qualification conversions [conv.qual]

A cv-decomposition of a type T is a sequence of

c v_{i}

and

P_{i}

such that T is “

c v_{0}

P_{0}

c v_{1}

P_{1}

⋯

c v_{n - 1}

P_{n - 1}

c v_{n}

U” for

n \geq 0

, where each

c v_{i}

is a set of cv-qualifiers ([basic.type.qualifier]), and each

P_{i}

is “pointer to” ([dcl.ptr]), “pointer to member of class

C_{i}

of type” ([dcl.mptr]), “array of

N_{i}

”, or “array of unknown bound of” ([dcl.array]).

P_{i}

designates an array, the cv-qualifiers

c v_{i + 1}

on the element type are also taken as the cv-qualifiers

c v_{i}

of the array.

[ Example

The type denoted by the type-id const int ** has three cv-decompositions, taking U as “int”, as “pointer to const int”, and as “pointer to 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,

c v_{1}, c v_{2}, \dots, c v_{n}

, 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

P_{i}

components are either the same or one is “array of

N_{i}

” and the other is “array of unknown bound of”, and the types denoted by U are the same.

The cv-combined type of two types T1 and T2 is the type T3 similar to T1 whose cv-decomposition is such that:

(3.1)
for every $i > 0$ , $c v_{i}^{3}$ is the union of $c v_{i}^{1}$ and $c v_{i}^{2}$ ;
(3.2)
if either $P_{i}^{1}$ or $P_{i}^{2}$ is “array of unknown bound of”, $P_{i}^{3}$ is “array of unknown bound of”, otherwise it is $P_{i}^{1}$ ;
(3.3)
if the resulting $c v_{i}^{3}$ is different from $c v_{i}^{1}$ or $c v_{i}^{2}$ , or the resulting $P_{i}^{3}$ is different from $P_{i}^{1}$ or $P_{i}^{2}$ , then const is added to every $c v_{k}^{3}$ for $0 < k < i$ .

where

c v_{i}^{j}

and

P_{i}^{j}

are the components of the cv-decomposition of Tj.

A prvalue of type T1 can be converted to type T2 if the cv-combined type of T1 and T2 is T2.

[ 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

Given similar types T1 and T2, this construction ensures that both can be converted to the cv-combined type of T1 and T2.

— 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

]