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inheritance

Started by Crispy Dec 14, 2002 at 6:31 AM 3 replies 200+ views

Original Post

Dec 14, 2002 at 6:31 AM

Hi, why will this give me a "''TPDLight:osition'' is not an unambiguous base class of ''TProjectileBase''" error?

  
class TPDLight
{
   protected:
   	TVector3f Position;
   ...
};

class TProjectileBase : public TPDLight
{
   private:
      TVector3f Orientation;

      float Speed;

   public:
   	TProjectileBase(TVector3f pPosition, TVector3f pOrientation, float pSpeed)
      	: Orientation(pOrientation), Speed(pSpeed), TPDLight::Position(pPosition) {}; //<- here


   ...
};

I''m not accessing TPDLight:

osition as a class, but rather as a member, right? Crispy

Stary

Dec 14, 2002 at 6:49 AM

quote:
Original post by Crispy
Hi,
why will this give me a "'TPDLight:osition' is not an unambiguous base class of 'TProjectileBase' " error?

Because you're calling a member of a different class in the initialization list for your constructor, which makes the compiler think that it's a constructor call for a base class. Since the initialization list is run "before" the constructor itself, there's no guarantee that TPDLight:

osition even exists yet.

One possible solution to this is to put the initialization of Position inside the constructor itself, which would mean that Position would be constructed twice. Another solution is to create a constructor for the base class:

      
class TPDLight
{
   public:
        TPDLight(const TVector3f &pPosition) : Position(pPosition) {} // <- new constructor

   protected:
   	TVector3f Position;
   ...
};

class TProjectileBase : public TPDLight
{
   private:
      TVector3f Orientation;

      float Speed;

   public:
   	TProjectileBase(TVector3f pPosition, TVector3f pOrientation, float pSpeed)
      	: TPDLight(pPosition), Orientation(pOrientation), Speed(pSpeed) {}; // <- call the constructor instead of the protected inherited member


   ...
};

Edit: If you wonder, the const TVector3f & instead of TVector3f is to avoid copying the TVector3fs all the time. You should probably consider it for other non-copying calls as well.

[edited by - Stary on December 14, 2002 7:55:39 AM]

joanusdmentia

Dec 14, 2002 at 6:54 AM

you can''t initialize a base classes members from the initializer list of the constructor, it has to be done by the constructor of the base class itself. you can do this by

  
class TPDLight
{
   protected:
   	TVector3f Position;
   public:
        TPDLight(TVector3f &pPosition) : Position(pPosition) {}
};

class TProjectileBase : public TPDLight
{
   private:
      TVector3f Orientation;
      float Speed;
   public:
      TProjectileBase(TVector3f &pPosition, TVector3f &pOrientation, float pSpeed)
        : Orientation(pOrientation), Speed(pSpeed), TPDLight(pPosition) {}
};

joanusdmentia

Dec 14, 2002 at 6:57 AM

damn it!!! beat me!!

Crispy

Dec 14, 2002 at 7:12 AM

Heck. This is precisely what I was thinking of avoiding. I figured that the derivee (fancy for base class

) would be created right after the derived class, allowing the members to be access, totally ignoring the fact that this what constructors are for - to control the creation flow.

Thanks guys,
Crispy

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