From: jkubicky AT cco DOT caltech DOT edu (Joseph J. Kubicky)
Subject: Problem getting pointer to member function - HELP!
To: djgpp AT sun DOT soe DOT clarkson DOT edu
Date: Wed, 29 Apr 92 2:56:26 PDT
Status: O


Newsgroups: comp.lang.c++
Subject: Problem with pointer to member function - HELP!


Newsgroups: comp.lang.c++
Subject: Problem with pointer to member function - HELP!



I'm having a problem with the following piece of code:


class Block {

protected:
	char name[MAX_NAME_LEN+1];
	int num_inputs;
	DATA_ID (*input_func[MAX_BLOCK_INPUTS])();
	DATA_ID current_output, next_output;

public:
	// Constructor basically just gives the Block a name
	Block(char *n);

	// functions for maintaining input-function list (our input link to rest of world)
	void ClearInputList() { num_inputs = 0; }	// clear out input list
	void AddInput(DATA_ID (*inp)()) {			// append an input onto input list
				if(num_inputs < MAX_BLOCK_INPUTS - 1)
					input_func[num_inputs++] = inp;
			}

	// function that returns data (called by blocks with us as their output)
	// just returns current_output
	DATA_ID GetOutput();

	// default (virtual) initializer function (called before start of each
	// new simulation run)
	virtual int Initialize();

			.
			.
};
			.
			.
			.

class SinGen : public Block {
	int period;
	int nextiter;
	DATA_ID magnitude;
public:
	// Overloaded constructor - we define output type based on which is
	// called
	SinGen(char *, int, int);
	SinGen(char *, int, double);

	Simulate();
	Initialize();
};
			.
			.
			.
main()
{
	Printval PV("Print");
	SinGen SG("Function Generator", 20, 1.0);

	PV.AddInput(SG.GetOutput);
	^^^^^^^^^^^^^^^^^^^^^^^^^^^ Problem with this line.
			.
			.
}


The problem is with the underlined line.  I was hoping that I could use
SG.GetOutput just like any other function reference and pass it like a
normal pointer to a function, but gcc (djgpp's latest release - 1.06) gives me
the following error message:

ERR> ... bad argument 0 for function 'Block:AddInput(auto struct DATA_ID(*)())'
ERR> (type was struct DATA_ID(class Block::*)())


Clearly, SG.GetOutput isn't a DATA_ID(*)().  I've tried explicitly typing
it as such, and the compiler just tells me it can't convert the argument.

The line compiles fine if I do something like:

	DATA_ID test();
	PV.AddInput(test);



DATA_ID is:

typedef struct {
	char type;
	union { ... } u;
} DATA_ID;



Is it even possible to use a reference to a member function in this 
context? 

Any help will be appreciated.


Jay Kubicky
jkubicky AT cco DOT caltech DOT edu