invalid use of incomplete type

Question

I m trying to use a typedef from a subclass in my project  I ve isolated my problem in the example below   Does anyone know where I m going wrong   template lt typename Subclass gt  class A       public            Why doesn t it like this          void action typename Subclass  mytype var                 static cast lt Subclass  gt  this  - gt do action var                 class B   public A lt B gt        public          typedef int mytype           B               void do action mytype var                   Do stuff               int main int argc  char   argv        B myInstance      return 0      This is the output I get   sean SEAN-PC   Documents LucadeStudios experiments  g   -o test test cpp test cpp  In instantiation of    A lt B gt      test cpp 10    instantiated from here test cpp 5  error  invalid use of incomplete type    class B    test cpp 10  error  forward declaration of    class B

User · Answer

You can get around this by using a traits class:
It requires you set up a specialsed traits class for each actuall class you use.

template<typename SubClass>
class SubClass_traits
{};

template<typename Subclass>
class A {
    public:
        void action(typename SubClass_traits<Subclass>::mytype var)
        {
                (static_cast<Subclass*>(this))->do_action(var);
        }
};


// Definitions for B
class B;   // Forward declare

template<> // Define traits for B. So other classes can use it.
class SubClass_traits<B>
{
    public:
        typedef int mytype;
};

// Define B
class B : public A<B>
{
    // Define mytype in terms of the traits type.
    typedef SubClass_traits<B>::mytype  mytype;
    public:

        B() {}

        void do_action(mytype var) {
                // Do stuff
        }
};

int main(int argc, char** argv)
{
    B myInstance;
    return 0;
}

User · Answer

The reason is that when instantiating a class template  all its declarations  not the definitions  of its member functions are instantiated too  The class template is instantiated precisely when the full definition of a specialization is required  That is the case when it is used as a base class for example  as in your case    So what happens is that A lt B gt  is instantiated at   class B   public A lt B gt    at which point B is not a complete type yet  it is after the closing brace of the class definition   However  A lt B gt   action s declaration requires B to be complete  because it is crawling in the scope of it   Subclass  mytype   What you need to do is delaying the instantiation to some point at which B is complete  One way of doing this is to modify the declaration of action to make it a member template   template lt typename T gt  void action T var         static cast lt Subclass  gt  this  - gt do action var       It is still type-safe because if var is not of the right type  passing var to do action will fail

User · Answer

You need to use a pointer or a reference as the proper type is not known at this time the compiler can not instantiate it   Instead try   void action const typename Subclass  mytype  amp var                 static cast lt Subclass  gt  this  - gt do action

User · Answer

You derive B from A lt B gt   so the first thing the compiler does  once it sees the definition of class B is to try to instantiate A lt B gt   To do this it needs to known B  mytype for the parameter of action  But since the compiler is just in the process of figuring out the actual definition of B  it doesn t know this type yet and you get an error   One way around this is would be to declare the parameter type as another template parameter  instead of inside the derived class   template lt typename Subclass  typename Param gt  class A       public          void action Param var                     static cast lt Subclass  gt  this  - gt do action var                 class B   public A lt B  int gt

User · Answer

Not exactly what you were asking  but you can make action a template member function   template lt typename Subclass gt  class A       public            Why doesn t it like this          template lt class V gt  void action V var                     static cast lt Subclass  gt  this  - gt do action                  class B   public A lt B gt        public          typedef int mytype           B               void do action mytype var                       Do stuff               int main int argc  char   argv        B myInstance      return 0

[c++] invalid use of incomplete type

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