Editing Virtual method table (section)

==Example==
Consider the following class declarations in [[C++ syntax]]:
<syntaxhighlight lang="cpp">
class B1 {
public:
  virtual ~B1() {}
  void fnonvirtual() {}
  virtual void f1() {}
  int int_in_b1;
};

class B2 {
public:
  virtual ~B2() {}
  virtual void f2() {}
  int int_in_b2;
};
</syntaxhighlight>

used to derive the following class:
<syntaxhighlight lang="cpp">
class D : public B1, public B2 {
public:
  void d() {}
  void f2() override {}
  int int_in_d;
};
</syntaxhighlight>

and the following piece of C++ code:
<syntaxhighlight lang="cpp">
B2 *b2 = new B2();
D  *d  = new D();
</syntaxhighlight>

g++ 3.4.6 from [[GNU Compiler Collection|GCC]] produces the following 32-bit memory layout for the object <code>b2</code>:<ref group="nb">G++'s <code>-fdump-class-hierarchy</code> (starting with version 8: <code>-fdump-lang-class</code>) argument can be used to dump virtual method tables for manual inspection. For AIX VisualAge XlC compiler, use <code>-qdump_class_hierarchy</code> to dump class hierarchy and virtual function table layout.</ref>
<pre>
b2:
  +0: pointer to virtual method table of B2
  +4: value of int_in_b2

virtual method table of B2:
  +0: B2::f2()   
</pre>

and the following memory layout for the object <code>d</code>:
<pre>
d:
  +0: pointer to virtual method table of D (for B1)
  +4: value of int_in_b1
  +8: pointer to virtual method table of D (for B2)
 +12: value of int_in_b2
 +16: value of int_in_d

Total size: 20 Bytes.

virtual method table of D (for B1):
  +0: B1::f1()  // B1::f1() is not overridden

virtual method table of D (for B2):
  +0: D::f2()   // B2::f2() is overridden by D::f2()
                // The location of B2::f2 is not in the virtual method table for D
</pre>

Note that those functions not carrying the keyword <code>virtual</code> in their declaration (such as <code>fnonvirtual()</code> and <code>d()</code>) do not generally appear in the virtual method table. There are exceptions for special cases as posed by the [[default constructor]].

Also note the [[Virtual function#Virtual destructors|virtual destructors]] in the base classes, <code>B1</code> and <code>B2</code>. They are necessary to ensure <code>delete d</code> can free up memory not just for <code>D</code>, but also for <code>B1</code> and <code>B2</code>, if <code>d</code> is a pointer or reference to the types <code>B1</code> or <code>B2</code>. They were excluded from the memory layouts to keep the example simple. <ref group="nb">{{Cite web|url=https://stackoverflow.com/questions/17960917/why-there-are-two-virtual-destructor-in-the-virtual-table-and-where-is-address-o|title = C++ - why there are two virtual destructor in the virtual table and where is address of the non-virtual function （gcc4.6.3）}}</ref>

[[Method overriding|Overriding of the method]] <code>f2()</code> in class <code>D</code> is implemented by duplicating the virtual method table of <code>B2</code> and replacing the pointer to <code>B2::f2()</code> with a pointer to <code>D::f2()</code>.