Re: [Cppcms-users] Plugins and templates

[Julian P. <ju...@wh...>]

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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.
The problem that occurs is the following:

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

So what I need is basically the following:

1. A way to use something like dynamic_pointer_cast with a type that is
known only at runtime (could be queried from the plugin)
2. Or: The template compiler should not write a constructor expecting
content::hello_world but instead from a
booster::shared_ptr<content::master> which is then
dynamic_pointer_cast'ed to the required pointer to a
content::hello_world. Assumption for the compiler would be, that the
base class is always content::master, so that the information from <%
view uses content::hello_world %> would be used to construct this:

content::hello_world &content;

hello_world(booster::shared_ptr<content::master> ptr) {
	booster::shared_ptr<content::hello_world> hptr =
booster::dynamic_pointer_cast<content::hello_world,content::master> (ptr);
	content = *hptr.get();
}

Any ideas on that matter?

Thank you,
Julian
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