OOP39-CPP. Do not use pointer-to-member operators to access non-existent members

C++ 2003, Section 5.5 "Pointer-to-member operators", paragraph 4, says:

If the dynamic type of the object does not contain the member to which the pointer refers, the behavior is undefined.

So, trying to use a pointer-to-member operator to access a non-existent member leads to undefined behavior and must be avoided.

Non-Compliant Code Example

In this non-compliant code example there is an abstract base class Shape and a derived class Circle that contains a member function area. The last line of the code following the class definitions results in undefined behavior because there is no member function corresponding to area() in the class Shape.

class Shape {  // abstract
  // ...
public:
  virtual void draw() = 0;
  // ...
};

class Circle : public Shape {
  double radius;
public:
  Circle(double new_radius) : radius(new_radius) {}   
  void draw() {
    // ...
  }
  virtual double area() {
    return PI*radius*radius;
  }
};

// ...

Shape *circ = new Circle(2.0);
double(Shape::*circ_area)() = static_cast<double(Shape::*)()>(&Circle::area);
cout << "Area: " << (circ->*circ_area)() << endl;

Compliant Solution (Modifiable Base Class)

If the developer is able to change the base class when it is realized that the area() method is required in the derived class, then a pure virtual area() method should be added to the class Shape:

class Shape {  // abstract
  // ...
public:
  virtual void draw() = 0;
  virtual void area() = 0;
  // ...
}

Compliant Solution (Non-modifiable Base Class)

In many cases, the base class is not modifiable. In this case, one must call the derived method directly.

Circle *circ = new Circle(2.0);
cout << "Area: " << (circ->area)() << endl;

Risk Assessment

Bibliography

[ISO/IEC 14882-2003] Section 5.5 "Pointer-to-member operators"

OOP37-CPP. Write constructor member initializers in the canonical order      013. Object Oriented Programming (OOP)      014. Concurrency (CON)