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Posted 30 Mar 2007

Function Overload by Return

, 5 Apr 2007
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Choosing the right function based on the return type


This code shows one way to use the C++ overload rule using the function return type instead of its signature. Of course, nothing is changed in the language, and the result just looks like a return type overload.


The function overload, in other words, reusing an existent function name but with different parameters, is something more or less known by the C++, Java and .NET community. As a sine qua non condition, we know that the overloaded function signature must be always different from its homonyms colleagues. The code below illustrates this:

GUID guid;
wstring guidS;

CreateNewGUID(guidS); // calls void CreateNewGUID(wstring&)
CreateNewGUID(guid);  // calls void CreateNewGUID(GUID&) (the compiler knows that)

It's a pretty common way to accept different input formats. But what about the output? How to extract different information or the same information in different format using the overload feature (e.g. the above example) without passing the result variable as a output argument? Giving ourselves some imagination and thinking how a valid construction would be, we can think of something like this:

GUID guid;
wstring guidS;

guidS = CreateNewGUID(); // calls wstring CreateNewGUID()
guid = CreateNewGUID();  // calls GUID CreateNewGUID() (the compiler knows that?)

Opening again our old and hurt C++ theories book, we can see the above code DOES NOT work. Or, at least, should not be. Just defining a couple of functions like that causes the following error:

error C2556: 'GUID CreateNewGUID(void)' : overloaded function differs 
	only by return type from 'std::wstring CreateNewGUID(void)'

That's right. Well, the error is right. The return type is not a function property that identifies it uniquely. Only the signature can do that (the parameters received by the function). AFAIK, this "limitation" is hack proof. Anyway, nothing disallows us to use another feature besides ordinary functions:

struct CreateNewGUID
   // what is supposed to be here?

We got it! Now we can "call" our "function" creating a new instance of the struct and attributing the "return" to a wstring or to our GUID struct:

guidS = CreateNewGUID(); 	// instantiates a CreateNewGUID
guid = CreateNewGUID(); 	// instantiates a CreateNewGUID. 
			// The difference is in the "return"

Using the Code

Since we create a new type, and knowing that this new type is different from the already known wstring and GUID types, we should simply convert our new type into every desirable return type:

struct CreateNewGUID
   operator wstring () { ... } 	// the conversion is the "function call"

   operator GUID () { ... } 	// as there's a couple of conversions... 
				// over... underload!

This concludes our somewhat odd solution to overload a "function" by the return type:

// instantiates a CreateNewGUID e calls CreateNewGUID::operator wstring()
guidS = CreateNewGUID();

// instantiates a CreateNewGUID e calls CreateNewGUID::operator GUID()
guid = CreateNewGUID();

There's the entire source.

/** @file underload.cpp
@brief A way to make overload using just the return type.
@author Wanderley Caloni
#include <windows.h>
#include <objbase.h>

#include <iostream />
#include <string>

using namespace std;

struct CreateNewGUID
   operator wstring () // the first function...
      GUID guid = operator GUID();
      OLECHAR buf[40] = { };
      ::StringFromGUID2(guid, buf, sizeof(buf));
      return wstring(buf);

   operator GUID () // ... and its "underload"
      GUID guid = { };
      return guid;

Points of Interest

This code was developed a long time ago as an answer to a friend's doubt. I learnt while coding that C++ is even more powerful than we think it is. In one form or another, we can always get what we want.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Wanderley Caloni
Software Developer (Senior) Intelitrader
Brazil Brazil
15 years experience in Windows operating systems developing in information security companies;
great team relationship; problem solving using systemic vision, knowledge bases maintenance,
chronograms and people coordination.

Technical historic
Software and hardware inventory
Clipboard and PrintScreen protection using windows hooks and global messages manipulation
Driver writing system event log
DeviceIoControl user/kernel communication
Desktop remote control using VNC technique
Remote execution tool PsExec (SysInternals) like
Print control using regex (Boost) and shell hook
Access policies management during user logon/logoff (register and hooks)
Datgabase migration CTree -> SQL (OLE classes)
Windows authentication using custom GINA and DCOM; Credential Provider (Vista)
CTree database synchronism using custom DCOM service
Bootable Linux CD with bash scripts and disk cryptography tools using C language
Hard disk encryption and PenDrive (USB) storage control
Blue Screen analysis using memory dumps and WinDbg live (Gflags)
System account execution using custom COM service
MBR (Master Boot Record) customization library
Blowfish/SHA-1 encryption library using C++ and 16 bits Assembly
Log access driver using shared memory between user and kernel mode
Kernel mode API hook for 9X and NT platforms
16 bits Assembly loader; debugging using tool
Executable protection using embedded domain authentication recorded inside files resources
Internet Explorer 6/7 and Firefox 1/2 browsing protection using Assembly 32 bits code injection
Code, strings and execution protection library (using Win32 interruptions)
Centralized log generation library using shared memory and global events
Internet Explorer 6/7 BHO (Broser Helper Object) and ActiveX; Mozilla/Firefox XPI plugin
Projects management using Source Safe, Bazaar and Batch (Win) scripts
Kernel mode debugging using SoftIce and WinDbg for NT

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Comments and Discussions

GeneralCool!!!! Pin
Merlinblack2-Apr-07 13:46
memberMerlinblack2-Apr-07 13:46 
GeneralCool Pin
Renato T Forti2-Apr-07 8:57
memberRenato T Forti2-Apr-07 8:57 
GeneralRe: Cool Pin
Wanderley Caloni2-Apr-07 11:07
memberWanderley Caloni2-Apr-07 11:07 
GeneralNice job Pin
Anant wakode2-Apr-07 0:21
memberAnant wakode2-Apr-07 0:21 
GeneralRe: Nice job Pin
Wanderley Caloni2-Apr-07 3:34
memberWanderley Caloni2-Apr-07 3:34 
GeneralGreat idea! Pin
kuhx198031-Mar-07 22:27
memberkuhx198031-Mar-07 22:27 
GeneralRe: Great idea! Pin
Wanderley Caloni2-Apr-07 2:27
memberWanderley Caloni2-Apr-07 2:27 
QuestionBest practice? Pin
Scott Dorman30-Mar-07 14:38
memberScott Dorman30-Mar-07 14:38 
I always thought it was not recommended to create overloads that differ by return value.

In just two days, tomorrow will be yesterday.

AnswerRe: Best practice? Pin
Wanderley Caloni30-Mar-07 17:37
memberWanderley Caloni30-Mar-07 17:37 

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