C++ for Scientists - Technische Universität Dresden

216 CHAPTER 7. EFFECTIVE PROGRAMMING: THE POLYMORPHIC WAY
pointers or references to the objects have to be used, which eliminates the possibility for a
compiler to optimize some parts of the code, i.e. inlining. Second, a dynamic cast has to be
used which can cause an exception at run time. This kind of problem is called binary-methodproblem,
which is explained in Section 7.5.3
Nevertheless, dynamic polymorphism in C++ is best at:
• Uniform manipulation based on base/derived class relationships: Different classes that
hold a base/derived relationship can be treated uniformly.
• Static type checking: All types are checked statically in C++.
• Dynamic binding and separate compilation: Code that uses classes in a hierarchy can
be compiled apart from the code of the entire hierarchy. This is possible because of the
indirection that pointers provide (both to objects and to functions).
• Binary interfacing: Modules can be linked either statically or dynamically, as long as the
linked modules lay out the virtual tables the same way.
Behind the Dynamic Polymorphism in C++
How virtual functions work:
• Normally when the compiler sees a member function call it simply inserts
instructions calling the appropriate subroutine (as determined by the
type of the pointer or reference)
• However, if the function is virtual a member function call such as
vc→ foo() is replaced with following: (∗((vc→ vtab)[0]))()
• The expression vc→ vtab locates a special ”secret” data member of the
object pointed to by vc. This data member is automatically present in
all objects with at least one virtual function. It points to a class-specific
table of function pointers (known as the class’s vtable)
• The expression (vc→ vtab)[0] locates the first element of the class’s
vtable of the object (the one corresponding to the first virtual function
foo() ). That element is a function pointer to the appropriate foo()
member function.
• Finally, the expression (∗((vc→ vtab)[0]))() dereferences the function
pointer and calls the function
• Special care must be taken with destructors in virtual class hierarchies.
The base class does not know anything about the derived classes and
so the derived class destructor has to be marked with virtual, too.