Template specialisation for templated base class

This is a discussion on Template specialisation for templated base class within the C++ Programming forums, part of the General Programming Boards category; Hi all, I have a template that I want to specialise for a specific class and it's sub classes i.e. ...

  1. #1
    semi-colon generator ChaosEngine's Avatar
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    Template specialisation for templated base class

    Hi all,
    I have a template that I want to specialise for a specific class and it's sub classes i.e.
    Code:
    template <typename T>
    class MyTemplate
    {
    public:
        static void DoStuff(void)
        {
             cout << "generic template" << endl;
        }
    };
    
    class Base
    {
    };
    
    class Derived : public Base
    {
    };
    
    template <>
    class MyTemplate<Base>
    {
    public:
        static void DoStuff(void)
        {
             cout << "base template" << endl;
        }
    };
    
    int main()
    {
    	MyTemplate<int>::DoStuff();        // OK - prints "generic template"
    	MyTemplate<Base>::DoStuff();    // OK - prints "base template"
    
    	MyTemplate<Derived>::DoStuff(); // BAH! - prints "generic template" 
    }
    anyone have any ideas? If I can solve this than I'll worry about the next problem (Base is also a template!!)
    "I saw a sign that said 'Drink Canada Dry', so I started"
    -- Brendan Behan

    Free Compiler: Visual C++ 2005 Express
    If you program in C++, you need Boost. You should also know how to use the Standard Library (STL). Want to make games? After reading this, I don't like WxWidgets anymore. Want to add some scripting to your App?

  2. #2
    ZuK
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    Code:
    	MyTemplate<int>::DoStuff();        // OK - prints "generic template"
    	MyTemplate<Base>::DoStuff();    // OK - prints "base template"
    
    	MyTemplate<Derived>::DoStuff(); // BAH! - prints "generic template"
    you have not specialized the template for Derived so it uses the generic function.
    Kurt

  3. #3
    Cat without Hat CornedBee's Avatar
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    Boost's enable_if plus is_base_and_derived from Boost.Type_Traits can sort of accomplish what you want. But it's not as simple as just specializing the class.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
    - Flon's Law

  4. #4
    semi-colon generator ChaosEngine's Avatar
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    Quote Originally Posted by ZuK
    Code:
    	MyTemplate<int>::DoStuff();        // OK - prints "generic template"
    	MyTemplate<Base>::DoStuff();    // OK - prints "base template"
    
    	MyTemplate<Derived>::DoStuff(); // BAH! - prints "generic template"
    you have not specialized the template for Derived so it uses the generic function.
    Kurt
    I know WHY it doesn't work! I'm trying to find a way around it!

    CornedBee, I thought about is_base_of but couldn't figure out a way to get it to work, but enable_if looks promising. I'll give it a shot, cheers!
    "I saw a sign that said 'Drink Canada Dry', so I started"
    -- Brendan Behan

    Free Compiler: Visual C++ 2005 Express
    If you program in C++, you need Boost. You should also know how to use the Standard Library (STL). Want to make games? After reading this, I don't like WxWidgets anymore. Want to add some scripting to your App?

  5. #5
    ZuK
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    Sorry. I should read your posts more carefully. Didn't see the "and it's sub classes"
    Kurt

  6. #6
    semi-colon generator ChaosEngine's Avatar
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    ok I tried CornedBee's idea and it works!
    Code:
    #include <boost/type_traits.hpp>
    #include <boost/utility.hpp>
    #include <boost/mpl/or.hpp>
    
    template <typename T, typename Enable = void>
    class MyTemplate
    {
    public:
        static void DoStuff(void)
        {
             cout << "generic template" << endl;
        }
    };
    
    class Base
    {
    };
    
    class Derived : public Base
    {
    };
    
    template <typename T>
    class MyTemplate<T, 
    					  typename boost::enable_if<
    							boost::mpl::or_<
    								boost::is_base_of<Base, T>,
    								boost::is_same<Base, T>
    							>
    						>::type>
    {
    public:
        static void DoStuff(void)
        {
             cout << "base template" << endl;
        }
    };
    
    int main()
    {
    	MyTemplate<int>::DoStuff();        // OK - prints "generic template"
    	MyTemplate<Base>::DoStuff();    // OK - prints "base template"
    
    	MyTemplate<Derived>::DoStuff(); // Woohoo! - prints "base template" 
    }
    god, I love boost! now I just need to figure out how to make it work with a templated Base class...
    "I saw a sign that said 'Drink Canada Dry', so I started"
    -- Brendan Behan

    Free Compiler: Visual C++ 2005 Express
    If you program in C++, you need Boost. You should also know how to use the Standard Library (STL). Want to make games? After reading this, I don't like WxWidgets anymore. Want to add some scripting to your App?

  7. #7
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    havent worked with this type of stuff in a while, but would 'virtual' help at all here?

  8. #8
    Cat without Hat CornedBee's Avatar
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    No, doesn't have anything to do with the issue.


    ChaosEngine, what exactly do you mean by templated base class? That there is a template parameter which names the type on which special handling is taken? I think you'll need a metafunction for that.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
    - Flon's Law

  9. #9
    semi-colon generator ChaosEngine's Avatar
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    Quote Originally Posted by CornedBee
    No, doesn't have anything to do with the issue.


    ChaosEngine, what exactly do you mean by templated base class? That there is a template parameter which names the type on which special handling is taken?
    not quite. It's a bit of an abuse of the type system really.

    if we have a class hierarchy like this
    Code:
    template <typename T>
    class Base
    {
    };
    
    class D1 : public Base<short>
    {
    };
    
    class D2 : public Base<int>
    {
    };
    D1 and D2 have no inheritance relationship as Base<int> and Base<short> are essentially 2 different classes. I've actually soved it using a private "grandfather" class
    Code:
    template <typename T, typename U = void, typename Enable = void>
    class MyTemplate
    {
    public:
        static void DoStuff(void)
        {
    		 cout << "generic template: " << typeid(T).name() << endl;
        }
    };
    
    
    class BaseBase {};
    
    template <typename T>
    class Base : private BaseBase
    {
    };
    
    class Derived : public Base<short>
    {
    };
    
    class D2 : public Base<int>
    {
    };
    
    template <typename T>
    class MyTemplate<T,
                                  typename boost::enable_if<boost::is_base_of<BaseBase, T> >::type
                                 >
    {
    public:
        static void DoStuff(void)
        {
    		 cout << "base template: " << typeid(T).name() << endl;
        }
    };
    
    int main()
    {
    	MyTemplate<int>::DoStuff();     // OK - prints "generic template"
    	MyTemplate<Base<int> >::DoStuff();    // OK - prints "base template"
    
    	MyTemplate<Derived>::DoStuff(); // Woohoo! - prints "base template" 
    	MyTemplate<D2>::DoStuff(); // Woohoo! - prints "base template" 
    }
    it's a bit of a hack, but it works.

    all this work is actually to define a metafunction to handle specialised Scalar math, but it works now. Thanks everyone, especially CornedBee.
    "I saw a sign that said 'Drink Canada Dry', so I started"
    -- Brendan Behan

    Free Compiler: Visual C++ 2005 Express
    If you program in C++, you need Boost. You should also know how to use the Standard Library (STL). Want to make games? After reading this, I don't like WxWidgets anymore. Want to add some scripting to your App?

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