C++ class member callback and external library - boost

Question

I would like to solve this issue about class member function callback.
Imagine you have a function from an external library (which cannot be modified!) like this:
void fortranFunction(int n, void udf(double*) );
I would like to pass as the udf function above a function member of an existing class. Please look at the following code:
// External function (tipically from a fortran library)
void fortranFunction(int n, void udf(double*) )
{
// do something
}
// User Defined Function (UDF)
void myUDF(double* a)
{
// do something
}
// Class containing the User Defined Function (UDF)
class myClass
{
public:
void classUDF(double* a)
{
// do something...
};
};
int main()
{
int n=1;
// The UDF to be supplied is myUDF
fortranFunction(n, myUDF);
// The UDF is the classUDF member function of a myClass object
myClass myClassObj;
fortranFunction(n, myClassObj.classUDF); // ERROR!!
}
The last line of the code above results in a compilation error, because you cannot declare the classUDF member function as a static function.
Do you know if it is possible to solve this issue?
Probably Boost libraries could help me, but I do not know how (please consider that fortranFunction cannot be modified because is from an external library).
Thanks a lot!
Alberto

Answer 1

I don't understand, why can't you declare classUDF as static like this
class myClass {
public:
static void classUDF(double *a) {
...
}
};
and then pass it like
fortranFunction(n, myClass::classUDF);

Answer 2

You might try that solution (a little bit hacky, but I think, it should work for you):
void fortranFunction(int n, void udf(double*))
{
double d = static_cast<double>(n);
udf(&d);
}
class myClass {
public:
void classUDF(double* a) {
}
};
#ifdef _MSC_VER
#define THREADLOCALSTATIC __declspec(thread) static
#define THREADLOCAL
#else
#define THREADLOCALSTATIC static ___thread
#define THREADLOCAL ___thread
#endif
struct _trampolinebase {
THREADLOCALSTATIC _trampolinebase* current_trampoline;
};
THREADLOCAL _trampolinebase* _trampolinebase::current_trampoline = 0;
#undef THREADLOCAL
#undef THREADLOCALSTATIC
template<class CBRET, class CBARG1, class T>
struct _trampoline1 : _trampolinebase
{
typedef CBRET (T::*CALLBACKFN)(CBARG1);
_trampoline1(T& target, CALLBACKFN& callback)
: callback_(callback)
, target_(target)
{
assert(current_trampoline == 0);
current_trampoline = this;
}
static CBRET callback(CBARG1 a1) {
_trampoline1* this_ = static_cast<_trampoline1*>(current_trampoline);
current_trampoline = 0;
return this_->trampoline(a1);
}
private:
CBRET trampoline(CBARG1 a1) {
return (target_.*callback_)(a1);
}
CALLBACKFN& callback_;
T& target_;
};
template<class FRET, class FARG1, class CBRET, class CBARG1, class T, class F>
FRET call1_1(T& target, CBRET (T::*callback)(CBARG1), F& fortranfunction, FARG1 a)
{
typedef typename _trampoline1<CBRET, CBARG1, T> trampoline;
trampoline t(target, callback);
return fortranFunction(a, trampoline::callback);
}
int main()
{
int n=1;
myClass myClassObj;
call1_1<void,int,void,double*>(myClassObj, &myClass::classUDF, fortranFunction, 1);
}
With the 'threadlocal' stuff, this will work in multithreaded calls, too. You may omit that, if you don't use a multithreaded environment. It also works with recursive calls (e.g. if the callback calls another fortran function).
This solution works only for one single argument plus callback for the fortran function and one single argument in the callback function itself, but you should be able to extend it easily. This is also, why I called it 'call1_1' (fortran function with 1 argument, callbackfunction with 1 argument). FRET is the return type of the fortran function, FARG1 the type of the first argument (int in this case). CBRET and CBARG are the same for the callback function.
Before the fortran function is actually called, the target object is stored within a global (thread-local) variable. The fortran function calls a static callback function, which finally calls your member function.
I invented the trampolinebase to instantiate the static member, I could also have used a global variable for that (but for some reason, I don't like global variables too much) ;-)