Insert member variable at compile-time

[mar11]

Hi all,

consider the following case:

Given a various number of child classes which inherited form basic class. I define an app-class which contains a template member function. Depending on the use case I want to add specific number of child classes to the app classes each time I call the member function of the app class. However, within the app class, only the basic class is declared.

Is there any way to declare/add instances of child classes at compile time for EACH instance of app classes?

A non-compilable example shall demonstrate my question. It could contain syntax errors.

class a{}; class b: public a {}; class c: public a {}; class d: public a {};  

class app
{
template<class ...arg> void insert(arg... Arg)
{
// expand the parameter pack
// insert the objects of child classes in the app classes
}

a* A;

};

int ()
{
app App;
b B;
c C;
d D;

App.insert(1, B, C ); // 1 Use Case 
...
App.insert(1, D ); // e Use Case 
..


}

[Disch]

I'm not entirely sure I understand, but it sounds to me like you could just have a vector. No need for any templates since the common base class abstracts all the children anyway:

class app
{
private:
  std::vector<A*> items;

public:
  void insert(A* toinsert)
  {
    items.push_back( toinsert );
  }
};

//...

int main()
{
  app App;

  App.insert( new B ); // works
  App.insert( new C ); // also works
  // ...etc
}

Of course with this, you'll have to remember to delete each item in the vector. One way to make sure this happens is like this:

class app
{
//...
public:
  ~app()
  {
    while( !items.empty() )
    {
      delete items.back();
      items.pop_back();
    }
  }

private:
  app(const app&);  // make it non-copyable
  void operator = (const app&);  // make it non-assignable
};

[mar11]

@Disch: But if you want to access a value of items in app. You need to cast it statically and that should be hard coded in the app classes.

[JLBorges]

> Depending on the use case I want to add specific number of child classes to the app classes
> each time I call the member function of the app class.

Can't you just make the app class a variadic template, with a std::tuple<> to store the members? And for each use-case, instantiate a different app class?

struct app_base { /* common stuff */ } ;

template< typename... T > struct app : app_base
{

    app( T... args ) : members(args...) {}

    // ....

    std::tuple< T... > members ;

    // ...
};

int main()
{
    app< int, short > use_case_1_app( 23, 7 ) ;

    app< std::string, double, void* > use_case_2_app( "hello", 3.456, nullptr ) ;
}

Perhaps, also use CRTP?

The type of each object stored in a std::tuple<> can be determined at compile time.

[mar11]

Is there any way to access/ check the type of child classes stored in vector "items" in the "App" class at compile time??

[Disch]

mar11:

You shouldn't need to. whatever you need should be able to be accomplished with virtual function calls:

http://cplusplus.com/doc/tutorial/polymorphism/