Re: [Cppcms-users] Plugins and templates

[Artyom <art...@ya...>]

> Just another  question:
> The plugins currently return their own content class derived from  a
> master class containing common properties such as a page title.
> Because  C++ doesn't allow to redefine return types when overloading
> virtual methods,  I use a booster::shared_ptr<content::master> for
> passing the content  class.

Actually you can redefine. But not for shared pointer but rather for
ordinary pointers and references. i.e.

class A{}; class B: public A{}

class foo {
public:
   virtual A &get();
};

class bar {
public:
   virtual B &get();
};

Is legal as B derived from A. This is what is called co-variant types.
So you may do this.


> Example:
> Plugin  HelloWorld has a content class content::hello_world derived  from
> content::master which is returned on calling handleRequest() wrapped  in
> a booster::shared_ptr<content::master>. This content class now has  to be
> rendered by the main application which does not now which  content
> subclass is wrapped in the pointer / knows it at runtime only, so  that a
> hard-compiled conversion  via
> booster::dynamic_pointer_cast<content::hello_world,content::master>  is
> not possible, because there could be other plugins returning  different
> subclasses of the master than hello_world.
> The plugin tells the  main application to use the view 'hello_world' for
> rendering. This view uses  content::hello_world, because it needs access
> to some fields defined only in  the subclass.
> 
> Now, if I call
> 
> render("hello_world",  *c.get());
> 
> where c is the returned shared_ptr, cppcms catches a  std::bad_cast
> exception occuring in http_context.c, line 132.


First of all `render` function always receives `content::master` and
dynamic casts after-wards in views to correct content. If cast
fails it throws bad_cast.


> If I use  booster::dynamic_pointer_cast beforehand and pass the casted
> shared_ptr,  everything works.

What do you cast? Do you mean that you cast to `content::hello_world` ?

It seems to me this is more shared object issues.

Do you compile your application with -rdynamic or --export-dynamic.
see:   
<http://art-blog.no-ip.info/wikipp/en/page/cppcms_1x_templates_bld#Dynamic+Loading+of+Views>


Because use it looks like this issue.

If you don't compile your applications with -rdynamic, dynamic cast between 
shared object boundaries
may fail.

This is very important to remember.


Artyom