The pointer-to-member operators ->* and .* group left-to-right.
pm-expression: cast-expression pm-expression .* cast-expression pm-expression ->* cast-expression
The binary operator .* binds its second operand, which shall be of type “pointer to member of T” (where T is a completely-defined class type) to its first operand, which shall be of class T or of a class of which T is an unambiguous and accessible base class. The result is an object or a function of the type specified by the second operand.
The binary operator ->* binds its second operand, which shall be of type “pointer to member of T” (where T is a completely-defined class type) to its first operand, which shall be of type “pointer to T” or “pointer to a class of which T is an unambiguous and accessible base class.” The expression E1->*E2 is converted into the equivalent form (*(E1)).*E2.
Abbreviating pm-expression.*cast-expression as E1.*E2, E1 is called the object expression. If the dynamic type of E1 does not contain the member to which E2 refers, the behavior is undefined.
The restrictions on cv-qualification, and the manner in which the cv-qualifiers of the operands are combined to produce the cv-qualifiers of the result, are the same as the rules for E1.E2 given in [expr.ref]. [ Note: it is not possible to use a pointer to member that refers to a mutable member to modify a const class object. For example,
struct S {
S() : i(0) { }
mutable int i;
};
void f(){
const S cs;
int S::* pm = &S::i; // pm refers to mutable member S::i
cs.*pm = 88; // ill-formed: cs is a const object
}
— end note ]
If the result of .* or ->* is a function, then that result can be used only as the operand for the function call operator (). [ Example:
(ptr_to_obj->*ptr_to_mfct)(10);
calls the member function denoted by ptr_to_mfct for the object pointed to by ptr_to_obj. — end example ] In a .* expression whose object expression is an rvalue, the program is ill-formed if the second operand is a pointer to member function with ref-qualifier &. In a .* expression whose object expression is an lvalue, the program is ill-formed if the second operand is a pointer to member function with ref-qualifier &&. The result of a .* expression whose second operand is a pointer to a data member is of the same value category ([basic.lval]) as its first operand. The result of a .* expression whose second operand is a pointer to a member function is a prvalue. If the second operand is the null pointer to member value ([conv.mem]), the behavior is undefined.