8 Expressions [expr]

8.5 Pointer-to-member operators [expr.mptr.oper]

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” to its first operand, which shall be a glvalue 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” to its first operand, which shall be of type “pointer to U” where U is either T or 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.
Otherwise, the expression E1 is sequenced before the expression E2.
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 whose ref-qualifier is &, unless its cv-qualifier-seq is const.
In a .* expression whose object expression is an lvalue, the program is ill-formed if the second operand is a pointer to member function whose ref-qualifier is &&.
The result of a .* expression whose second operand is a pointer to a data member is an lvalue if the first operand is an lvalue and an xvalue otherwise.
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 member pointer value, the behavior is undefined.