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The pointer-to-member operators .* and ->* are used to obtain an object or a function as though it were a member of an underlying object. For instance, the following are functionally equivalent ways to call the member function f() on the object o:.

struct S {
  void f() {}
};

void func() {
  S o;
  void (S::*pm)() = &S::f;
  
  o.f();
  (o.*pm)();
}

...

The C++ Standard, [expr.mptr.oper], paragraph 4 [ISO/IEC 14882-2014], states  the following:

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.

(A pointer-to-member expression of the form E1->*E2 is converted to its equivalent form, (*(E1)).*E2, so use of pointer-to-member expressions of either form behave equivalently in terms of undefined behavior.)

Further, the C++ Standard, [expr.mptr.oper], paragraph 6, in part, states , in partthe following:

If the second operand is the null pointer to member value, the behavior is undefined.

...

In this noncompliant code example, a pointer-to-member object is obtained from D::g but upcast but is then upcast to be a B::*. When called on an object whose dynamic type is D, the pointer-to-member call is well defined. However, in this noncompliant code example, the dynamic type of the underlying object is object is B, resulting which results in undefined behavior:.

struct B {
  virtual ~B() = default;
};

struct D : B {
  virtual ~D() = default;
  virtual void g() { /* ... */ }
};

void f() {
  B *b = new B;
 
  // ...
 
  void (B::*gptr)() = static_cast<void(B::*)()>(&D::g);
  (b->*gptr)();
  delete b;
}

...

In this compliant solution, the upcast is removed, rendering the initial code ill-formed and emphasizing the underlying problem : that B::g() does not exist. This compliant solution assumes that the programmer's intention was to use the correct dynamic type for the underlying object:.

struct B {
  virtual ~B() = default;
};

struct D : B {
  virtual ~D() = default;
  virtual void g() { /* ... */ }
};

void f() {
  B *b = new D; // Corrected the dynamic object type.
 
  // ...
  void (D::*gptr)() = &D::g; // Moved static_cast to the next line.
  (static_cast<D *>(b)->*gptr)();
  delete b;
}

...

In this noncompliant code example, a null pointer-to-member value is passed as the second operand to a pointer-to-member expression, resulting in undefined behavior:.

struct B {
  virtual ~B() = default;
};

struct D : B {
  virtual ~D() = default;
  virtual void g() { /* ... */ }
};
 
static void (D::*gptr)(); // Not explicitly initialized, defaults to nullptr.
void call_memptr(D *ptr) {
  (ptr->*gptr)();
}
 
void f() {
  D *d = new D;
  call_memptr(d);
  delete d;
}

...

In this compliant solution, gptr is properly initialized to a valid pointer-to-member value instead of to the default value of nullptr:.

struct B {
  virtual ~B() = default;
};
 
struct D : B {
  virtual ~D() = default;
  virtual void g() { /* ... */ }
};
 
static void (D::*gptr)() = &D::g; // Explicitly initialized.
void call_memptr(D *ptr) {
  (ptr->*gptr)();
}
 
void f() {
  D *d = new D;
  call_memptr(d);
  delete d;
}

...