Callback function in two classes - c++

Question

Hello I have a problem with a callback function. I'm relative new to c++.
I have two classes helper and exporter:
Helper:
class Helper {
typedef Bool (*IterationCallback)(BaseObject *op);
public: Int32 RecurseHierarchy (BaseObject* op, IterationCallback callback) {
Int32 count = 0;
while (op) {
if (callback(op)) {
count++;
count += RecurseHierarchy(op->GetDown(), callback);
op = op->GetNext();
}
}
return count;
}
};
Exporter:
class Exporter {
private: Helper helper;
private: Bool writeT3D (Filename exportT3D) {
string filepath = C4DStringToStdString(exportT3D.GetString());
t3DFile.open(filepath.c_str());
writeBegin();
// Iterate all objects in the document and writes an actor
BaseDocument *doc = GetActiveDocument();
Int32 count = helper.RecurseHierarchy(doc->GetFirstObject(), this->WriteActor);
writeEnd();
t3DFile.close();
return true;
}
};
And I'm getting the errorC3867 function call missing argument list and I should use &Exporter::WriteActor. But I can't solve the problem. Can someone help me?

Answer 1

The problem
Assuming WriteActor to be a member function of Exporter, then its type is probably:
Bool (Exporter::*)(BaseObject*)
which is not compatible with:
Bool (*)(BaseObject*)
The reason is that in the former an implicit parameter is passed of type Exporter* (normally accessible inside the member function as this), while in the latter this does not happen.
One solution
For the above reason, you need to explicitly pass this implicit argument of type Exporter*, before passing the resulting function to RecurseHierarchy.
In your case you can use std::bind as follows:
using namespace std::placeholders;
auto fn = std::bind(&Exporter::WriterActor, this, _1);
Int32 count = helper.RecurseHierarchy(doc->GetFirstObject(), fn);
and then modify Helper to take any callable object:
struct Helper {
template<typename Fn>
Int32 RecurseHierarchy(BaseObject* op, Fn callback) {
Int32 count = 0;
while (op) {
if (callback(op)) {
count++;
count += RecurseHierarchy(op->GetDown(), callback);
op = op->GetNext();
}
}
return count;
}
};
Live demo
The above change is especially important if you also want to allow std::function objects and lambdas to be passed.