Safely override C virtual functions

Question

I have a base class with a virtual function and I want to override that function in a derived class  Is there some way to make the compiler check if the function I declared in the derived class actually overrides a function in the base class  I would like to add some macro or something that ensures that I didn t accidentally declare a new function  instead of overriding the old one   Take this example   class parent   public    virtual void handle event int something  const          boring default code         class child   public parent   public    virtual void handle event int something           new exciting code         int main       parent  p   new child      p- gt handle event 1       Here parent  handle event   is called instead of child  handle event    because the child s method misses the const declaration and therefore declares a new method  This could also be a typo in the function name or some minor difference in the parameters types  It can also easily happen if the interface of the base class changes and somewhere some derived class wasn t updated to reflect the change   Is there some way to avoid this problem  can I somehow tell the compiler or some other tool to check this for me  Any helpful compiler flags  preferably for g     How do you avoid these problems

User · Answer

As far as I know  can t you just make it abstract   class parent   public    virtual void handle event int something  const   0          boring default code          I thought I read on www parashift com that you can actually implement an abstract method  Which makes sense to me personally  the only thing it does is force subclasses to implement it  no one said anything about it not being allowed to have an implementation itself

User · Answer

As far as I know  can t you just make it abstract   class parent   public    virtual void handle event int something  const   0          boring default code          I thought I read on www parashift com that you can actually implement an abstract method  Which makes sense to me personally  the only thing it does is force subclasses to implement it  no one said anything about it not being allowed to have an implementation itself

User · Answer

Your compiler may have a warning that it can generate if a base class function becomes hidden   If it does  enable it   That will catch const clashes and differences in parameter lists   Unfortunately this won t uncover a spelling error   For example  this is warning C4263 in Microsoft Visual C

User · Answer

Since g   4 7 it does understand the new C  11 override keyword   class child   public parent       public           force handle event to override a existing function in parent          error out if the function with the correct signature does not exist       void handle event int something  override

User · Answer

Your compiler may have a warning that it can generate if a base class function becomes hidden   If it does  enable it   That will catch const clashes and differences in parameter lists   Unfortunately this won t uncover a spelling error   For example  this is warning C4263 in Microsoft Visual C

User · Answer

Something like C  s override keyword is not part of C     In gcc  -Woverloaded-virtual warns against hiding a base class virtual function with a function of the same name but a sufficiently different signature that it doesn t override it  It won t  though  protect you against failing to override a function due to mis-spelling the function name itself

User · Answer

In MSVC  you can use the CLR override keyword even if you re not compiling for CLR   In g    there s no direct way of enforcing that in all cases  other people have given good answers on how to catch signature differences using -Woverloaded-virtual  In a future version  someone might add syntax like   attribute     override   or the equivalent using the C  0x syntax

User · Answer

Make the function abstract  so that derived classes have no other choice than to override it    Ray Your code is invalid   class parent   public    virtual void handle event int something  const   0          boring default code          Abstract functions cannot have bodies defined inline  It must be modified to become  class parent   public    virtual void handle event int something  const   0      void parent  handle event  int something        do w e you want here

User · Answer

Your compiler may have a warning that it can generate if a base class function becomes hidden   If it does  enable it   That will catch const clashes and differences in parameter lists   Unfortunately this won t uncover a spelling error   For example  this is warning C4263 in Microsoft Visual C

User · Answer

Make the function abstract  so that derived classes have no other choice than to override it    Ray Your code is invalid   class parent   public    virtual void handle event int something  const   0          boring default code          Abstract functions cannot have bodies defined inline  It must be modified to become  class parent   public    virtual void handle event int something  const   0      void parent  handle event  int something        do w e you want here

User · Answer

In MSVC   you can use keyword override  class child   public parent   public    virtual void handle event int something   lt b gt override lt  b gt           new exciting code          override works both for native and CLR code in MSVC

User · Answer

I would suggest a slight change in your logic   It may or may not work  depending on what you need to accomplish   handle event   can still do the  boring default code  but instead of being virtual  at the point where you want it to do the  new exciting code  have the base class call an abstract method  i e  must-be-overridden  method that will be supplied by your descendant class   EDIT  And if you later decide that some of your descendant classes do not need to provide  new exciting code  then you can change the abstract to virtual and supply an empty base class implementation of that  inserted  functionality

User · Answer

Something like C  s override keyword is not part of C     In gcc  -Woverloaded-virtual warns against hiding a base class virtual function with a function of the same name but a sufficiently different signature that it doesn t override it  It won t  though  protect you against failing to override a function due to mis-spelling the function name itself

User · Answer

Make the function abstract  so that derived classes have no other choice than to override it    Ray Your code is invalid   class parent   public    virtual void handle event int something  const   0          boring default code          Abstract functions cannot have bodies defined inline  It must be modified to become  class parent   public    virtual void handle event int something  const   0      void parent  handle event  int something        do w e you want here

User · Answer

As far as I know  can t you just make it abstract   class parent   public    virtual void handle event int something  const   0          boring default code          I thought I read on www parashift com that you can actually implement an abstract method  Which makes sense to me personally  the only thing it does is force subclasses to implement it  no one said anything about it not being allowed to have an implementation itself

User · Answer

C  11 override keyword when used with the function declaration inside the derived class  it forces the compiler to check that the declared function is actually overriding some base class function  Otherwise  the compiler will throw an error   Hence you can use override specifier to ensure dynamic polymorphism  function overriding    class derived  public base  public    virtual void func name int var name  override          statement

User · Answer

In MSVC  you can use the CLR override keyword even if you re not compiling for CLR   In g    there s no direct way of enforcing that in all cases  other people have given good answers on how to catch signature differences using -Woverloaded-virtual  In a future version  someone might add syntax like   attribute     override   or the equivalent using the C  0x syntax

User · Answer

I would suggest a slight change in your logic   It may or may not work  depending on what you need to accomplish   handle event   can still do the  boring default code  but instead of being virtual  at the point where you want it to do the  new exciting code  have the base class call an abstract method  i e  must-be-overridden  method that will be supplied by your descendant class   EDIT  And if you later decide that some of your descendant classes do not need to provide  new exciting code  then you can change the abstract to virtual and supply an empty base class implementation of that  inserted  functionality

User · Answer

I would suggest a slight change in your logic   It may or may not work  depending on what you need to accomplish   handle event   can still do the  boring default code  but instead of being virtual  at the point where you want it to do the  new exciting code  have the base class call an abstract method  i e  must-be-overridden  method that will be supplied by your descendant class   EDIT  And if you later decide that some of your descendant classes do not need to provide  new exciting code  then you can change the abstract to virtual and supply an empty base class implementation of that  inserted  functionality

User · Answer

Make the function abstract  so that derived classes have no other choice than to override it    Ray Your code is invalid   class parent   public    virtual void handle event int something  const   0          boring default code          Abstract functions cannot have bodies defined inline  It must be modified to become  class parent   public    virtual void handle event int something  const   0      void parent  handle event  int something        do w e you want here

User · Answer

Something like C  s override keyword is not part of C     In gcc  -Woverloaded-virtual warns against hiding a base class virtual function with a function of the same name but a sufficiently different signature that it doesn t override it  It won t  though  protect you against failing to override a function due to mis-spelling the function name itself

User · Answer

C  11 override keyword when used with the function declaration inside the derived class  it forces the compiler to check that the declared function is actually overriding some base class function  Otherwise  the compiler will throw an error   Hence you can use override specifier to ensure dynamic polymorphism  function overriding    class derived  public base  public    virtual void func name int var name  override          statement

User · Answer

Your compiler may have a warning that it can generate if a base class function becomes hidden   If it does  enable it   That will catch const clashes and differences in parameter lists   Unfortunately this won t uncover a spelling error   For example  this is warning C4263 in Microsoft Visual C

User · Answer

Since g   4 7 it does understand the new C  11 override keyword   class child   public parent       public           force handle event to override a existing function in parent          error out if the function with the correct signature does not exist       void handle event int something  override

User · Answer

In MSVC   you can use keyword override  class child   public parent   public    virtual void handle event int something   lt b gt override lt  b gt           new exciting code          override works both for native and CLR code in MSVC

User · Answer

I would suggest a slight change in your logic   It may or may not work  depending on what you need to accomplish   handle event   can still do the  boring default code  but instead of being virtual  at the point where you want it to do the  new exciting code  have the base class call an abstract method  i e  must-be-overridden  method that will be supplied by your descendant class   EDIT  And if you later decide that some of your descendant classes do not need to provide  new exciting code  then you can change the abstract to virtual and supply an empty base class implementation of that  inserted  functionality

User · Answer

As far as I know  can t you just make it abstract   class parent   public    virtual void handle event int something  const   0          boring default code          I thought I read on www parashift com that you can actually implement an abstract method  Which makes sense to me personally  the only thing it does is force subclasses to implement it  no one said anything about it not being allowed to have an implementation itself

User · Answer

Something like C  s override keyword is not part of C     In gcc  -Woverloaded-virtual warns against hiding a base class virtual function with a function of the same name but a sufficiently different signature that it doesn t override it  It won t  though  protect you against failing to override a function due to mis-spelling the function name itself

User · Answer

In MSVC  you can use the CLR override keyword even if you re not compiling for CLR   In g    there s no direct way of enforcing that in all cases  other people have given good answers on how to catch signature differences using -Woverloaded-virtual  In a future version  someone might add syntax like   attribute     override   or the equivalent using the C  0x syntax

User · Answer

In MSVC  you can use the CLR override keyword even if you re not compiling for CLR   In g    there s no direct way of enforcing that in all cases  other people have given good answers on how to catch signature differences using -Woverloaded-virtual  In a future version  someone might add syntax like   attribute     override   or the equivalent using the C  0x syntax

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