Re: Need to create a C lib - using C++ classes - is it possible
On May 25, 6:28 am, "Alf P. Steinbach" <al...@start.no> wrote:
* Ian Collins:
Alf P. Steinbach wrote:
* Angus:
We have a lot of C++ code. And we need to now create a
library which can be used from C and C++. Given that we
have a lot of C++ code using classes how can we 'hide' the
fact that it is C++ from C compilers?
The C++ code will need the C++ runtime library.
Within the standards of C and C++ the only way to achieve
that is to insist that the C code using the library is
called from a C++ main program.
Is it? In practice, the only constraint on any platform
I've used is that the application must be linked with the
C++ compiler diver.
It's not clear exactly what you mean, but I'm guessing you
mean using a C++ compiler and linker for the main program.
In that case you have a C++ main program.
Or not. You can very easily compile main as a C program, but
use the C++ compiler driver (supposing such a thing exists) to
link.
In a very real sense, you're both wrong, of course: the correct
answer is that it depends on the implementation. Under Unix (or
at least Solaris and Linux), you do have a separate C++ compiler
driver (invoked by g++ or CC, rather than by gcc or cc), and
linking with that will ensure that the C++ standard libraries
are linked in. You can also invoke the linker directly, both
under Unix or under Windows, and ensure that whatever you want
is linked in however you want, but you generally have to know
very well what you are doing, what libraries are actually
required, what commands or whatever might be necessary in
addition to ensure support for e.g. exceptions or dynamic
initialization of static variables, etc., etc. And some of that
functionality may not even be accessible directly at the linker
interface; at least some implementations of C++ have generated
special code in main to ensure the initialization of statics,
for example (in which case, main does have to be compiled as
C++).
All in all, it's more complicated than just putting everything
in an `extern "C"', but most of all, it's very, very
implementation dependent.
The best is to forget that silly idea. Using C library
from C++, OK. But C++ has additional requirements from
runtime library, so other way, generally !OK, unless you're
working at a low level where you wouldn't have to ask...
Why is it generally not OK? Provided the restrictions
regarding exception leakage are met, there shouldn't be a
problem.
E.g. static variables of class type, internal use of
exceptions, use of standard library features that
(implementation-specific) requires C++ runtime library,
including internal use of exceptions in standard library; this
is a FAQ, IIRC.
As a general rule, it's probably easier (and surer) if you
ensure that main is compiled as C++. With at least some
development systems, however, it's not absolutely necessary.
What is necessary, however, even if you compile main as C++, is
that you find out the implementation specific requirements, and
respect them.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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