 <p>The ISO C++ Standard specifies that all virtual methods of a class that are not pure-virtual must be defined. </p> <p>Simply put the rule is:<br> If your derived class overiddes the Base class virtual method then it should provide a definition as well, If not then the Base class should provide the definition of that method. </p> <p>As per the above rule in your code example, <code>virtual void bar();</code> needs a definition in the Base class.</p> <p>Reference:</p> <p><strong>C++03 Standard: 10.3 Virtual functions [class.virtual]</strong> </p> <blockquote> <p>A virtual function declared in a class shall be defined, or declared pure (10.4) in that class, or both; but no diagnostic is required (3.2).</p> </blockquote> <p>So either you should make the function pure virtual or provide a definition for it. </p> <p>The <strong><a href="http://gcc.gnu.org/faq.html#vtables" rel="nofollow">gcc faq</a></strong> doccuments it as well: </p> <blockquote> <p>The ISO C++ Standard specifies that all virtual methods of a class that are not pure-virtual must be defined, but does not require any diagnostic for violations of this rule <code>[class.virtual]/8</code>. Based on this assumption, GCC will only emit the implicitly defined constructors, the assignment operator, the destructor and the virtual table of a class in the translation unit that defines its first such non-inline method.</p> <p>Therefore, if you fail to define this particular method, the linker may complain about the lack of definitions for apparently unrelated symbols. Unfortunately, in order to improve this error message, it might be necessary to change the linker, and this can't always be done.</p> <p>The solution is to ensure that all virtual methods that are not pure are defined. Note that a destructor must be defined even if it is declared pure-virtual <code>[class.dtor]/7</code>.</p> </blockquote>