Question

I have tried to implement a “template template template” – template class to fullfill my needs ( I am quite new in using template metaprogramming). Unfortunately, I have found the following topic too late:
Template Template Parameters

Nevertheless, I need to implement something like listed below.

According to the compiler the last typedef is not working. I am not sure, but I think this is due to the limitation of 3x template restriction. Is there any possibility to bypass a 3xtemplate definition in this simple example?

template < typename TValueType >
class ITTranslator
{
public:
    ITTranslator() = 0;
    virtual ~ITTranslator() = 0;
    virtual void doSomething() = 0;
}

template < typename TValueType >
class TConcreteTranslator1 : public ITTranslator<TValueType>
{
public:
    TConcreteTranslator1(){}
    ~TConcreteTranslator1(){}
    void doSomething() {}
}

template < typename TValueType >
class TConcreteTranslator2 : public ITTranslator<TValueType>
{
public:
    TConcreteTranslator2(){}
    ~TConcreteTranslator2(){}
    void doSomething() {}
}

template < 
    typename TValueType, 
    template < typename TValueType > class TTranslatorValueType 
    >
class ITClassifier
{
public:
    ITClassifier() = 0;
    virtual ~ITClassifier() = 0;
}

template < 
    typename TValueType, 
    template < typename TValueType > class TTranslatorValueType 
    >
class TConcreteClassifier1 : public ITClassifier<TValueType,TTranslatorValueType >
{
public:
    TConcreteClassifier1() {}
    ~TConcreteClassifier1() {}
    void dodo(){}
}


template < 
    typename TValueType,
    template <typename TValueType> class TTranslatorValueType,
    template <template<typename TValueType> class TTranslatorValueType> class TClassifierValueType
 >
class ITAlgorithm
{
public:
    ITAlgorithm()=0;
    virtual ~TAlgorithm()=0;
    virtual run() = 0;
}


template < 
    typename TValueType,
    template <typename TValueType> class TTranslatorValueType,
    template <template<typename TValueType> class TTranslatorValueType> class TClassifierValueType
 >
class TConcreteAlgorithm1 : public ITAlgorithm<TValueType,TTranslatorValueType,TTranslatorValueType>
{
public:
    TConcreteAlgorithm1 (){}
    ~TConcreteAlgorithm1 (){}
    run()
    {
        TClassifierValueType< TTranslatorValueType>* l_classifier_pt = new TClassifierValueType< TTranslatorValueType>( );
        // add this object to a internal list...
    }
}



int main()
{
    typedef TConcreteTranslator1< cvbase::uint32_t > translator_t;
    typedef TConcreteClassifier1< cvbase::uint32_t, TConcreteTranslator1> classifier_t;
    typedef TConcreteAlgorithm1 < cvbase::uint32_t, TConcreteTranslator1, TConcreteClassifier1> algorithm_t; // not possible
    return 0;
}

Thanks a lot, I really appreciate any help!

EDIT:
I have extended my listing (I am pretty sure it will not compile :)) to show the motivation why I am using my weird concept 🙂

Answer 1

There is really no need to pass template template parameter around
here. Usually you can just take a normal template argument and provide
a reasonable default:

template<typename ValueType>
struct translator {};

template<typename ValueType, typename Translator = translator<ValueType>>
struct Classifier {};

template<typename ValueType, 
         typename Translator = translator<ValueType>, 
         typename Classifier = classifier<ValueType, Translator>
         >
struct Algorithm {};

This is done the same way for allocator aware containers.

And please do away with the horrible hungarian-notation prefixes.

NB: It seems from your usage of constructors and destructors that you
don’t really have a grasp of basic C++. You might want to stay away
from templates before you have understood easier concepts.