Question

Now and then when using GCC I get cryptic errors like this:

undefined reference to ‘vtable for classname’

When it’s not caused by a missing library, this not-very-descriptive error message always causes me to dig through code files line by line to find the missing implementation for a virtual function. Is there a way to make the linker tell me which virtual function it is missing, perhaps a flag or something? Or is it maybe telling me but I don’t understand what it’s saying?

Answer 1

From gcc faq:

When building C++, the linker says my
constructors, destructors or virtual
tables are undefined, but I defined
them

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
[class.virtual]/8. 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.

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.

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 [class.dtor]/7.

The solution that I adopt is search the classname and seek virtual methods declaration and check if there is any definition. I didn’t found any other solution for this.