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Virtual Classes

Jul 11, 2011 at 3:58am
closed account (S6k9GNh0)
I'm somewhat new to the concept. It's a feature of C++ I've been avoiding for a _long_ time for reasons less than good (other than the constant vtable overhead slander). I wanted to use pure virtual classes to hold a plugin interface. However, I'm rather confused as to how to make a decent plugin. Observe the following code:

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#include <iostream>
#include <list>
#include <memory>
 
struct IPlugin
{
        virtual void plugin_func() =0;
        virtual void plug_func() =0;
};
 
struct Plugin : public IPlugin
{
        virtual void plugin_func() { std::cout << "101" << std::endl; }
        virtual void plug_func() { std::cout << "test" << std::endl; } 
};
 
struct AnotPlugin : public IPlugin
{
        virtual void plugin_func() { std::cout << "LoL" << std::endl; }
        virtual void plug_func() { std::cout << "blahahblah" << std::endl; }
};
 
struct PluginHolder
{
        template <class PluginClass> 
        void register_plugin()
        {
                plugList.push_back(std::shared_ptr<IPlugin>(new PluginClass));
        }
    
        void plugin_func() 
        { 
                for (auto iter = plugList.begin(); iter != plugList.end(); ++iter)
                {
                        (*iter)->plugin_func();
                }
        }
 
        void plug_func()
        {
                for (auto iter = plugList.begin(); iter != plugList.end(); ++iter)
                {
                        (*iter)->plug_func();
                }
        }
 
        std::list<std::shared_ptr<IPlugin> > plugList;
};
 
int main(void)
{
        PluginHolder ph;
        ph.register_plugin<Plugin>();
        ph.register_plugin<AnotPlugin>();
        ph.plugin_func();
        ph.plug_func();
}
Edit & run on cpp.sh


There are some problems with this obviously. I was also rather confused as to the benefits of a factory over something like this as I was told it would be most beneficial (nor do I see how it would work for a plugin system). Am I taking a wrong approach? Could I get advice on how to go about this?

EDIT: Sorry for the poor example. It was meant to be basic. Also, to compile, it requires a C++11 capable compiler. Should compile with VC++, GCC (with --std=gnu++0x), clang.
Last edited on Jul 11, 2011 at 4:02am
Jul 11, 2011 at 7:08am
screw (145)
Hi,

Your solution works at compiled time as well. If you want to use it at run time you can change it as the following:

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void register_plugin(IPlugin * ipn) //at run time
{
	plugList.push_back(std::shared_ptr<IPlugin>(ipn));
}
...
ph.register_plugin(new Plugin);


So the whole code goes like this:
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#include <iostream>
#include <list>
#include <memory>
#include <tr1/memory>
using namespace std::tr1;

 
struct IPlugin
{
        virtual void plugin_func() =0;
        virtual void plug_func() =0;
};
 
struct Plugin : public IPlugin
{
        virtual void plugin_func() { std::cout << "101" << std::endl; }
        virtual void plug_func() { std::cout << "test" << std::endl; } 
};
 
struct AnotPlugin : public IPlugin
{
        virtual void plugin_func() { std::cout << "LoL" << std::endl; }
        virtual void plug_func() { std::cout << "blahahblah" << std::endl; }
};
 
struct PluginHolder
{
        template <class PluginClass> 
        void register_plugin() // at compiled time
        {
                plugList.push_back(std::shared_ptr<IPlugin>(new PluginClass));
        }
    
        void register_plugin(IPlugin * ipn) //at run time
	{
		plugList.push_back(std::shared_ptr<IPlugin>(ipn));
	}
	
        void plugin_func() 
        { 
                for (auto iter = plugList.begin(); iter != plugList.end(); ++iter)
                {
                        (*iter)->plugin_func();
                }
        }
 
        void plug_func()
        {
                for (auto iter = plugList.begin(); iter != plugList.end(); ++iter)
                {
                        (*iter)->plug_func();
                }
        }
 
        std::list<std::shared_ptr<IPlugin> > plugList;
};
 
int main(void)
{
        PluginHolder ph;
        ph.register_plugin<Plugin>();
        ph.register_plugin<AnotPlugin>();
	ph.register_plugin(new Plugin);
        ph.plugin_func();
        ph.plug_func();
}
Edit & run on cpp.sh

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