Policy based design - policy implementation has to access members of the host class

Question

I think the best way to explain my question is with a piece of code:

class IncreasingMultiplier {

protected:

    IncreasingMultiplier(int initialMultiplier = 0, int incrementation = 1)

    int getMultiplier() {
        mCurrentMultiplier += mIncrementation;
        return mCurrentMultiplier - mIncrementation;
    }

    void setMultiplier(int multiplier) {
        mCurrentMultiplier = multiplier;
    }

    void setIncrementation(int incrementation) {
        mIncrementation = incrementation;
    }

private:

    int mCurrentMultiplier;
    int mIncrementation;`

}

class ConstMultiplier {

protected:

    int getMultiplier() const {
        return 10;
    }

}


class NumberLogger {

public:

    void log() {

        int currentNumber = getNumber(); // How can I call this method?

        std::cout << "Current number is " << currentNumber << std::endl;

    }

}



template<

    class MultiplierPolicy,

    class LoggingPolicy

>
class Host : public MultiplierPolicy, public LoggingPolicy {

public:

    int getNumber() const {
        return mNumber * getMultiplier();

    }

private:


    int mNumber;

}

Basically, one policy may need to access members defined in the host class, which are in turn dependent on other policies supplied to the host class.

Thanks!

Answer 1

The following code compiles with VS2013 (have not tried with GCC):

#include <iostream>

class IncreasingMultiplier {

protected:

    IncreasingMultiplier(int initialMultiplier = 0, int incrementation = 1)
        : mCurrentMultiplier(initialMultiplier)
        , mIncrementation(incrementation)
    {}

    int getMultiplier() {
        mCurrentMultiplier += mIncrementation;
        return mCurrentMultiplier - mIncrementation;
    }

    void setMultiplier(int multiplier) {
        mCurrentMultiplier = multiplier;
    }

    void setIncrementation(int incrementation) {
        mIncrementation = incrementation;
    }

private:

    int mCurrentMultiplier;
    int mIncrementation;
};

class ConstMultiplier {

protected:

    int getMultiplier() const {
        return 10;
    }

};

// Template the logger policy
// Unfortunately - couldn't get host inheritance CRTP pattern
// compiling in Visual Studio 2013 :(
// https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern
template < typename t_Host >
class NumberLogger /*: public t_Host*/ {

public:

    void log() {
        // This part of the CRTP pattern does work in Visual Studio 2013
        int currentNumber = static_cast<t_Host*>(this)->getNumber(); // How can I call this method?

        std::cout << "Current number is " << currentNumber << std::endl;
    }

};

// Template based on a list of policies
template<
    typename PoliciesList
>
class Host : public PoliciesList::MultiplierPolicy, public PoliciesList::LoggingPolicy {

public:

    Host() : mNumber(1) {}

    int getNumber() /*const*/ {
        return mNumber * getMultiplier();
    }

private:

    int mNumber;
};

// Un-templated policies list
// Could create a macro to declare various policy combinations:
class ConcretePoliciesList_Const
{
public:
    typedef Host<ConcretePoliciesList_Const> MyHost;
    typedef ConstMultiplier MultiplierPolicy;
    typedef NumberLogger<MyHost> LoggingPolicy;
};

class ConcretePoliciesList_Increasing
{
public:
    typedef Host<ConcretePoliciesList_Increasing> MyHost;
    typedef IncreasingMultiplier MultiplierPolicy;
    typedef NumberLogger<MyHost> LoggingPolicy;
};

int main()
{
    ConcretePoliciesList_Const::MyHost const_host;
    ConcretePoliciesList_Increasing::MyHost increasing_host;

    std::cout << "Const policy:" << std::endl;
    const_host.log();
    const_host.log();
    const_host.log();

    std::cout << "Increasing policy:" << std::endl;
    increasing_host.log();
    increasing_host.log();
    increasing_host.log();

    return 0;
}

The resulting output is:

Const policy:
Current number is 10
Current number is 10
Current number is 10
Increasing policy
Current number is 0
Current number is 1
Current number is 2