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using Ctrl::Add; required for templates / overloaded virtual functions [message #27054]

Wed, 23 June 2010 14:14

kohait00
Messages: 939
Registered: July 2009
Location: Germany

Experienced Contributor

hi folks,

this one is sort of freak situation in my application. nevertheless i provide it here for discussion. consider the following example.

class MyD
	: public ArrayCtrl 
	//works with Label (it has no own Add() overloads, 
	//does not work with ArrayCtrl, it has own Add() overloads
{
public:
	typedef MyD CLASSNAME;
	virtual ~MyD() {}
};

GUI_APP_MAIN
{
	Label l;
	MyD md;
	
	//option 1: << desired >>
	
	void (MyD::* mfp)(Ctrl &) = NULL; 
	mfp = (void (MyD::*)(Ctrl &)) &MyD::Add; 
	//only works if ArrayCtrl:: class has a using Ctrl::Add
	//otherwise
	//error C2440: 'type cast' : cannot convert 
	//from 'overloaded-function' 
	//to 'void (__thiscall Upp::Ctrl::* )(Upp::Ctrl &)' 
	//None of the functions with this name in scope match the target type

	(md.*mfp)(l);
	
	//option 2: doenst work anyway, because binding to Ctrl::Add is clear.
	void (Ctrl::* mfp2)(Ctrl &) = NULL;
	mfp2 = (void (Ctrl::*)(Ctrl &)) &Ctrl::Add; //(void (Ctrl::*)(Ctrl &)) 
	(md.*mfp2)(l); //does *not* call overriden Add(Ctrl&), but the one from Ctrl::
	
	ttest().Run();
}

problem is following:

i need to reference the virtual & overloaded ArrayCtrl::Add(Ctrl &) member function with a member function pointer (in template environment, but this one produces same errors and solution is applicable there as well). i access the Add(Ctrl&) function from topmost derived class MyD and it works fine with all Ctrl's that have no own Add() function *overloads* (not virtual overrides). but i.e. ArrayCtrl which has Add(Value&) and others, prevents this one from compiling.

key line is

	mfp = (void (MyD::*)(Ctrl &)) &MyD::Add;

for which MSC produces C2440 error.

i managed to solve it by adding a
ArrayCtrl.h:427

using Ctrl::Add;

which enables the compiler to deduce stuff explicitly, since ArrayCtrl now explicitly provides access to Ctrl::Add(Ctrl&) and the line compiles, both in MSC and GCC.

now i think of it as a general point to take position to.

why not providing using Ctrl::XXX ; for functions, which we know are used often, are even virtual, are public as well, and are beeing overloaded by a deriving class?

(so far i found this to be the case for ArrayCtrl and DropList, sure there are some more)

what is your point?

PS: i could possibly circumvent the problem by specifying an intermediate class like that, exposing Ctrl::Add explicitly, but is it the clean way?

class MyArrayCtrl
	: public ArrayCtrl
{
public:
	typedef MyArrayCtrl CLASSNAME;
	virtual ~MyArrayCtrl() {}
	using Ctrl::Add;
};

sorry for the long post, problem is not trivial though
attached a test project

Attachment: ttest.rar
(Size: 1.68KB, Downloaded 179 times)

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