C++Builder Console application: linking to a __closure based method - closures

Question

First off, forgive me on the title. Not really sure how to ask this question:
I have an application that I need to convert to a console application (note the application runs fine as a VCL style windows app). The app uses a few 3rd party widgets that have callback functions. However, when I attempt to compile it, I get 'cannot convert ...' errors, like this:
Cannot convert 'void(Tobject *, TErrorEventParams *)' to 'TErrorEvent'
TErrorEvent is defined as:
typedef void __fastcall (__closure* TErrorEvent)(System::TObject* Sender, TErrorEventParams *e);
The line causing the error is:
handler->OnError = errorHandler;
The code for errorHandler is:
void __fastcall errorHandler(System::TObject* Sender, TErrorEventParams *e)
{
memoLine = e->Description;
updateLog();
}

Answer 1

A __closure type is a pointer to a non-static class method. The compiler does not allow you to assign a standalone non-class function where a __closure is expected. It requires a pointer to a method of a class object. Karem's answer shows you one way to accomplish that.
However, there IS a way to use a non-class function, using the helper TMethod struct (which is how a __closure is implemented behind the scenes).
First, add an explicit 'this' parameter to your event handler:
void __fastcall errorHandler(void *This, TObject* Sender, TErrorEventParams *e)
{
memoLine = e->Description;
updateLog();
}
And then assign the event handler like this:
TMethod m;
m.Code = &errorHandler
m.Data = NULL; // any value you want to pass to the 'This' parameter...
handler->OnError = reinterpret_cast<TErrorEvent&>(m);

Answer 2

Have a look at this documentation:
http://docwiki.embarcadero.com/RADStudio/Tokyo/en/Closure
In short:
TErrorEvent is defined as a pointer to a class member function. So errorHandler has to be declared as a class member function.
The implementation could look somewhat like this:
class TMyClass
{
private:
TMyHandler* handler;
void __fastcall errorHandler(System::TObject* Sender, TErrorEventParams *e);
public:
__fastcall TMyClass();
} my_dummy_class;
__fastcall TMyClass::MyClass()
{
//handler has to be created
handler->OnError = errorHandler;
}
void __fastcall TMyClass::errorHandler(System::TObject* Sender, TErrorEventParams *e)
{
memoLine = e->Description;
updateLog();
}