• Create Account

Forward declared returned reference

Old topic!

Guest, the last post of this topic is over 60 days old and at this point you may not reply in this topic. If you wish to continue this conversation start a new topic.

3 replies to this topic

#1Juliean  GDNet+

Posted 17 March 2014 - 04:37 PM

Hello,

I have a class like this:

class TabBarItem;

class TabBar :
public Widget
{
public:
}

Now when I only include this header file and use that member method, I get this error under visual studio 2013:

error C2027: use of undefined type "acl::gui::TabBarItem"


when I use that method like this:

m_tabArea.AddItem(dock.GetName());


Storing the returned value in a reference fixes it:

auto& item = m_tabArea.AddItem(dock.GetName());


And so does including the headerfile of "TabBarItem". Why is this happening? I've had that issue for quite a while, and have always wondered. There is no copy being made even if I include the header file and leave the method without the assignement, and with pointers it works also. Is this some part of the specification or some obscure compiler error?

#2Servant of the Lord  Members

Posted 17 March 2014 - 05:07 PM

You are probably failing to #include "TabBarItem.h" in the file that calls "m_tabArea.AddItem(dock.GetName());"

Maybe the compiler creates a temporary (non-reference) variable to receive the return result (?) since you aren't explicitly receiving it yourself, but then encounters a compiler error since "TabBarItem.h" was never included, and so it only sees the declaration and not the definition. (I'm guessing - I don't know if it actually creates a temp variable)

References and Pointers can be used without being fully defined (because the memory a reference or pointer takes is constant (sizeof(int&) == sizeof(MyLargeCustomType&)), regardless of the type), but an actual variable's size depends on the full definition that the compiler needs to know.

It's perfectly fine to abbreviate my username to 'Servant' or 'SotL' rather than copy+pasting it all the time.
All glory be to the Man at the right hand... On David's throne the King will reign, and the Government will rest upon His shoulders. All the earth will see the salvation of God.
Of Stranger Flames -

#3Juliean  GDNet+

Posted 18 March 2014 - 02:41 AM

You are probably failing to #include "TabBarItem.h" in the file that calls "m_tabArea.AddItem(dock.GetName());"

Sure, I did not explicitely write this, but I didn't fail to include it, I simply didn't want, since I didn't use the TabBarItem itself here. Thats the point of forward declaration, isn't it, if I wanted to have TabBarItem everywhere I only call methods of TabBar, I would have included it into TabBar.h.

Maybe the compiler creates a temporary (non-reference) variable to receive the return result (?) since you aren't explicitly receiving it yourself, but then encounters a compiler error since "TabBarItem.h" was never included, and so it only sees the declaration and not the definition. (I'm guessing - I don't know if it actually creates a temp variable)

Thats my only possible explanation, but why is this really happening? There is no sane reason for the compiler to do this, is there? I double checked, there is no copy constructor and/or destructor invoked here, so at least there is no "real" variable generated in this line. Why the compile error then? Its not that much of an annoyance, but more a WTF.

#4Strewya  Members

Posted 18 March 2014 - 02:48 AM

It is possible to declare a pointer to a declared but undefined type. But Visual C++ does not allow a reference to an undefined type.

The following sample generates C2027.

// C2027_b.cpp
class A;
A& CreateA();

class B;
B* CreateB();

int main() {
CreateA(); // C2027
CreateB(); // OK
}

You're basically hitting a VC++ limitation with references to declared but undefined types.

devstropo.blogspot.com - Random stuff about my gamedev hobby

Old topic!

Guest, the last post of this topic is over 60 days old and at this point you may not reply in this topic. If you wish to continue this conversation start a new topic.