Re: Abstract class linking issue

"kanze" <>
12 Oct 2006 09:55:14 -0400
Richard wrote:

First question - let's get this out of the way since it might
be the solution to all my woes: Does it make sense to have a
.cpp file for a class that is declared as having pure virtual
functions in its .h file?

That depends partially on local conventions, but my usual rule
is to make the virtual destructor inline, and to forego the
source file. In my own code, an in-class definition of a
virtual destructor is practially a signature of an "interface";
as soon as you see it, you pretty well know that all other
functions will be pure virtual, and that there will be no source
file (nor generated object file).

Here's my predicament. I'll put this in general terms since
it's happening across several classes.

I have a base class, let's say it's named A. In A.h I DECLARE
several pure virtual ( = 0) functions, a virtual destructor,
and a constructor.

Why the constructor?

  Note that each of these is terminated with a semicolon, hence they're
not DEFINED. There is also a file A.cpp which defines the constructor
and destructor with empty curly braces. A.cpp and A.h compile into a
library file (.a) with GCC without errors.

You don't really have a library with just a single file, do you?

There's a class B that derives class A. All of the pure virtual
functions are overridden by B. Class B also has a default constructor
(no parameters) and destructor. Class B compiles into a library
without error also.

When I go to link my executable I include the two resulting libraries.
The linker produces the error "undefined reference to A::A()" which is
said to occur in the constructor for class B. I've ensured that the
folders containing A.h and B.h are in the "include search path", and
I've ensured that the libraries containing A and B are included during

Sounds like you've got an ordering problem. Most linkers
consider the libraries in the order they're given, without going
back. Since the only reference to A's constructor and
destructor is in the library containing B, that library must
precede the library containing A on the command line of the

In this particular case, I'd just put an in class (and thus,
inline) definition of the destructor in the base class, and skip
the constructor completely. More generally, however, you should
define a hierarchy of library dependancies, without cycles, and
ensure that the libraries are specified in the correct order
(determined by a topological sort governed by the dependancies)
on the command line which does the link.

James Kanze GABI Software
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