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Forward declared returned reference


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#1 Juliean   GDNet+   -  Reputation: 2720

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Posted 17 March 2014 - 04:37 PM

Hello,

 

I have a class like this:

class TabBarItem;

class TabBar :
    public Widget
{
public:
    TabBarItem& AddItem(const std::wstring& stText);
}

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?



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#2 Servant of the Lord   Crossbones+   -  Reputation: 20948

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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.


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#3 Juliean   GDNet+   -  Reputation: 2720

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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.



#4 Strewya   Members   -  Reputation: 1579

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Posted 18 March 2014 - 02:48 AM

http://msdn.microsoft.com/en-us/library/6c2dk0ah.aspx
 

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.


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