7 Expressions [expr]

A cv-decomposition of a type T is a sequence of $cv{}_i$ and $P_i$ such that T is “$cv{}_0$ $P_0$ $cv{}_1$ $P_1$ ⋯ $cv{}_{n-1}$ $P_{n-1}$ $cv{}_n$ U” for $n\ge 0$, where each $cv{}_i$ 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]).

If $P_i$ designates an array, the cv-qualifiers $cv{}_{i+1}$ on the element type are also taken as the cv-qualifiers $cv{}_i$ of the array.

The n-tuple of cv-qualifiers after the first one in the longest cv-decomposition of T, that is, $cv{}_1,cv{}_2,\dots ,cv{}_n$, 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 $i>0$, $cv{}_i^3$ is the union of $cv{}_i^1$ and $cv{}_i^2$;
• (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 $cv{}_i^3$ is different from $cv{}_i^1$ or $cv{}_i^2$, or the resulting $P_i^3$ is different from $P_i^1$ or $P_i^2$, then const is added to every $cv{}_k^3$ for $0<k<i$.

where $cv{}_i^j$ 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.

int main() {
const char c = 'c';
char* pc;
const char** pcc = &pc;       // #1: not allowed
*pcc = &c;
*pc = 'C';                    // #2: modifies a const object
}