Overload resolution by const qualifier (of member function pointer) ?

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When I overload a class's member function solely on the const qualifier of the parameter (which is a different class's member function pointer), the compiler resolves the overload and performs as expected.
When I attempt the same overload for a non-member function, the compiler (VS2010 Express) complains of linker errors.

Does anyone know why ?

C++

//Header.h

#pragma once

#include "targetver.h"
#include <stdio.h>
#include <tchar.h>
#include <iostream>

struct Component
{
	int Execute(bool X) { return 0; }

	int ConstExecute(bool X) const { return 0; }
};

struct Tester
{
	void Con( int (Component::*MFP)(bool) )
	{ std::cout << "Tester::Con Passed Non-Const MFP\n"; };

	void Con( int (Component::*MFP)(bool) const )
	{ std::cout << "Tester::Con Passed Const MFP\n"; };
};


void Con( int (Component::*MFP)(bool) )
{ std::cout << "Con Passed Non-Const MFP\n"; };

void Con( int (Component::*MFP)(bool) const )
{ std::cout << "Con Passed Const MFP\n"; };


//Main.cpp

#include "Header.h"

int _tmain(int argc, _TCHAR* argv[])
{	
	// In Main
	Tester MyTester;
	MyTester.Con( &Component::Execute );
	MyTester.Con( &Component::ConstExecute );

	Con( &Component::Execute );
        // LNK2005: Con(int (__thiscall Component::*)(bool) already defined
	Con( &Component::ConstExecute );
        // LNK2005: Con(int (__thiscall Component::*)(bool)const ) already defined

	char X;
	std::cin >> X;
	return 0;
}

Commenting out the calls to the non-member Con Function produces:
"Tester::Con Passed Non-Const MFP"
"Tester::Con Passed Const MFP"
as expected.

Posted 6-Nov-11 21:06pm

WodgerDodger

Updated 6-Nov-11 23:17pm

Richard MacCutchan

v3

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Comments

Andrew Brock 7-Nov-11 5:33am

This may be a limitation of the Express compiler.
I just ran this code (minus a few #includes) on VS2010 Ultimate with Service Pack 1. It worked as you would expect.

2 solutions

Solution 2

There is no inherent meaning when you define a non-member function const. In a class, a member function that is declared const means that it won't change the instance of the class you call it on. For a non-member function that makes no sense however! There is no 'instance' of a class that a non-member functino relates to, therefore a const qualifier is not possible. IMHO the compiler (not the linker!) should even issue an error.

You can make an overload based on the constness of a function's parameters though.

In short, you can not overload non-member functions by adding a const qualifier to the function, only by adding them to the parameters.

P.S.: ignore this, it's just a result of my bad reading skills...

Posted 7-Nov-11 0:00am

Stefan_Lang

Updated 7-Nov-11 0:20am

v2

Comments

Andrew Brock 7-Nov-11 6:08am

Your answer is correct, it just isn't relevant to this question.
The 2 functions both non-const (also the 2 in the Tester struct), they just take a different argument, either of a const or a non-const function. It is the function which he is passing in as a parameter that has the const/non-const.
His code is perfectly valid and makes sense (although I don't imagine it is particularly useful for most scenarios).

What I said in my answer is still correct, and since it was marked as the solution I imagine it fixed the issue.

Stefan_Lang 7-Nov-11 6:18am

Ah, I got confused by the brackets, thinking the function was const-qualified. I somehow only focused on ') const' and completely ignored everything to the left and right of that part...

After rereading your answer in light of that insight, it makes a lot more sense.

Andrew Brock 7-Nov-11 6:20am

It took me a bit to get that too (see my comment to the original question). The code does look a bit messy

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Andrew Brock · Accepted Answer · 2011-11-06T23:39:00

Ignore my comment. I wasn't thinking straight.

The issue is that this code is defined in a header file.
This means that for every .cpp file that includes this, the function code is defined (under the same name).

The #pragma once only stops this file getting included more than once by any 1 cpp file, it doesn't stop it getting included my multiple files.

Either give the function prototype in the header, and define the code in just 1 cpp file, or add the inline keyword to the start of the functions

C++

inline void Con( int (Component::*MFP)(bool) )
{ std::cout << "Con Passed Non-Const MFP\n"; };
 
inline void Con( int (Component::*MFP)(bool) const )
{ std::cout << "Con Passed Const MFP\n"; };

The inline keyword makes the compiler essentially copy and paste the function code everywhere the function is called, without defining an actual function. This is a bit of a hack up, but works.