Is it possible to change the type of the 'this' keyword?

Question

Now this may seem like a strange question, but I've come accross a compiler error that has just blown my mind and even though there is a fix, I'd like to know why it breaks.

So the pretense is I'm using an API called Corba, it's used for RPC in short. In my program I am having to create an object which derives from a Corba object.

class MyClass : public CorbaClass;

Simple stuff, now in one of the methods of MyClass I am calling a function which requires a CorbaClass*, so I simply pass in the this pointer, note that the method is not a const method so surely this should work fine ?

However I get an error when compiling which states that the parameter I'm trying to pass is actually of type CorbaClass* const. Now you can imagine my confusion, surely that is not possible, that would require this to be an lvalue would it not ? So all of a sudden this has me questioning parts of my knowledge of C++ itself !

Now it turns out in the documentation of TAO (the specific implementation of Corba I'm using) that they actually have a snippet of code for this exact situation and I see they do something strange. Instead of directly passing this, they call a method _this() and pass the return from that through, so I did a bit of digging and found the method was declared in the Corba base class as:

CorbaClass* _this(void);

Now this seems completely insane to me, but maybe that's because I've never come accross it before, does anyone know why any of the weird stuff with this is happening ?

I believe the compiler I'm using is GCC 4.5.1

Edit: Some example code

MyClass.h

    class MyClass
    : public POA_NotifyExt::ReconnectionCallback
{
public:
    void Initialise();

private:
    NotifyExt::ReconnectionRegistry_var m_ReconnectionRegistry;
}

MyClass.cpp

void Initialise()
{
    m_ReconnectionRegistry->register_callback( this );
}

Generates error:

error: no matching function for call to ‘NotifyExt::ReconnectionRegistry::register_callback(MyClass* const)’
orbsvcs/NotifyExtC.h:491:63: note: candidate is: virtual NotifyExt::ReconnectionRegistry::ReconnectionID NotifyExt::ReconnectionRegistry::register_callback(NotifyExt::ReconnectionCallback*)

Related: http://stackoverflow.com/questions/6067244/type-of-this-pointer — NathanOliver, Mar 25 '15 at 12:13
Are you sure you mean `CorbaClass* const` and not `CorbaClass const*`? — Kerrek SB, Mar 25 '15 at 12:19
I'm pretty sure that `this` is behaving as intended. Maybe your code isn't, but that's not your question. — Kerrek SB, Mar 25 '15 at 12:20
@LightnessRacesinOrbit No, it isn't, even if it was this would be a const CorbaClass* not a CorbaClass* const. — abcthomas, Mar 25 '15 at 12:21
Please post an [MCVE](http://stackoverflow.com/help/mcve) - your code which contains the failing call (including types of anything relevant involved), and the declaration of the function whose call fails. — Angew is no longer proud of SO, Mar 25 '15 at 12:23
@abcthomas: True. But you cannot be 100% sure, because this makes no sense. Be less sure, and more evidence-showing. — Lightness Races in Orbit, Mar 25 '15 at 12:23
I know it doesn't hence my confusion, I will post an MCVE but I'm not sure how much more information it will provide. — abcthomas, Mar 25 '15 at 12:25
@abcthomas: It will provide infinitely more than we have at present! Or, the act of constructing it will reveal to you the bug in your code. That's what usually happens. — Lightness Races in Orbit, Mar 25 '15 at 12:28
If your compiler thinks that `this` is a `T* const` then it is noncompliant. More likely you are misinterpreting the error message. But, not considering this possibility, you did not even show it to us! — Lightness Races in Orbit, Mar 25 '15 at 12:33
Not knowing a thing about TAO, is `POA_NotifyExt::ReconnectionCallback` a derivative of `NotifyExt::ReconnectionCallback` ? If not, then your class isn't either, and therefore doesn't fulfill the requirements of the parameter. — WhozCraig, Mar 25 '15 at 12:37
Aha fml ! No it isn't it's a different class and from what I can tell they share no ancestry. Thanks, completely couldn't see that, doesn't help that the '_this()' method returns a different type. But one last question, why does my error state I'm giving a MyClass* const ? — abcthomas, Mar 25 '15 at 12:43
That seems odd to me as well, a strange way of telling you `this` is non-modifiable. I'd be curious if `clang` reported a different message, something to the effect of "Candidate function not viable, no known conversion from `MyClass*` to `NotifyExt::ReconnectionCallback*`", which would have been considerably more informative. — WhozCraig, Mar 25 '15 at 12:52
@abcthomas `this` is an rvalue and cannot be assigned to. So it's possible it's implemented as `TheClass * const` internally, so that's what the compiler shows. — Angew is no longer proud of SO, Mar 25 '15 at 12:53
@Angew I suspect similar. A more modern gcc may report differently as well. — WhozCraig, Mar 25 '15 at 12:54
Interesting, well thanks, yeah this compiler is quite old but unfortunately it's the build chain which has been forced upon me by my customer ! Thanks for the feedback I'll put this down to a quirk of the compiler message. — abcthomas, Mar 25 '15 at 13:14

score 1 · Answer 1 · answered Mar 26 '15 at 22:14

register_callback expects a NotifyExt::ReconnectionCallback, not a POA_NotifyExt::ReconnectionCallback (note the different namespaces).

MyClass is what is called a Servant: a server-side object implementing a CORBA interface.

Since CORBA aims at being location and language independent, the client of an object does not refer directly to the Servant; it uses an "object reference" which contains the information necessary to reach the object. You thus need to pass an object reference to the CORBA object implemented by the MyClass Servant.

One way to obtain such a reference is to use the _this() member function.

Is it possible to change the type of the 'this' keyword?

1 Answers1