7 Standard conversions [conv]

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 $n>0$, where each $c{v}_i$ is a set of cv-qualifiers, and each $P_i$ is “pointer to”, “pointer to member of class $C_i$ of type”, “array of $N_i$”, or “array of unknown bound of”. If $P_i$ 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 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, $c{v}_1,c{v}_2,\dots ,c{v}_n$, is called the cv-qualification signature of T.

Two types ${\text{T}}_1$ and ${\text{T}}_2$ are similar if they have cv-decompositions with the same n such that corresponding $P_i$ components are the same and the types denoted by U are the same.

A prvalue expression of type ${\text{T}}_1$ can be converted to type ${\text{T}}_2$ if the following conditions are satisfied, where $c{v}_i^j$ denotes the cv-qualifiers in the cv-qualification signature of ${\text{T}}_j$:60

• (3.1)

  ${\text{T}}_1$ and ${\text{T}}_2$ are similar.

• (3.2)

  For every $i>0$, if const is in ${cv}_i^1$ then const is in ${cv}_i^2$, and similarly for volatile.

• (3.3)

  If the ${cv}_i^1$ and ${cv}_i^2$ are different, then const is in every ${cv}_k^2$ 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 ]