I have experience with OCaml. You had to write a stub for every function you wanted to use to convert the types even C int <-> OCaml int. Linking was painful a well. I don't even want to thing about mapping C++ objects.
What about other popular languages? Is it always a pain?
EDIT:
Please avoid duplicates. And state C and C++ interfacing capabilities separately.
EDIT 2:
Please be specific. "X can call C" doesn't give too much information.
Python has a very good C API. It can be integrated to C++ also very easily and conveniently using the boost::python C++ binding for the Python C API.
Interfacing with C++ objects will always be painful, as there's no standard ABI (Binary interface).
Python has a great C interface using built-in ctypes module. In order to interface C++ modules, SWIG can be used.
There are varying degrees of enjoyment available among mainstream languages and their C interfaces. Gladly, you can use SWIG for most of them.
Python is a powerful and easy to learn high level language and has good documentation of extending it with C and C++:
http://docs.python.org/extending/extending.html
Using this approach is simple, but you'd write your extensions explicitly for Python. Using SWIG (see Extending Python with C++ for a nice little tutorial), you create the C/ C++ code as if it was to be run by itself plus an interface file that SWIG takes to create some wrapping code for you that you can use in Python (or other languages, for that matter).
Java has a nice native interface with JNI, C# has something very similar.
Nearly all of the scripting languages (Perl, Python, Lua, PHP, Ruby, Tcl) are designed to be embedded into C and C++.
A good survey paper of the relative merits of the APIs:
H. Muhammad and R. Ierusalimschy. C APIs in extension
and extensible languages. Journal of Universal Computer
Science, 13(6):839–853, 2007.
See also this very similar question (and my answer in particular ;)).
You might like this recent comp.compilers thread about various foreign-function interfaces. It was suggested that Haskell had one of the nicest.
D is designed to be easily intefaced to C.
D 2.0 does have a limited interface to C++ code.
R is extensible via C, C++ and Fortran, this is described in the R Extensions Manual and the contributed Rcpp package makes it easier to call C++ functions.
Lua's integration abilities with C (and by extension C++) are absolutely first class. Its a wonderful little language, too. I don' think it gets nearly the love it deserves.
link: http://www.lua.org/
Common Lisp implementations will often have a FFI that works really well with C. The nice thing is that you don't have to write any C code to use C libraries; all you need to do is write the declaration of the C function is Lisp.
Factor has copied this FFI system from Lisp.
Falcon is a good alternative to Python in this case : http://falconpl.org