variadic templates with template function names

Question

following this question , I am trying to avoid copy-pasting some code related to calling all of the same-named methods of the mixins of the class BaseSensor.

in sensor.hpp

struct EdgeSensor //a mixin
{
    void update(){}
    void printStats() {}
};

struct TrendSensor //another mixin
{
    void update(){}
    void printStats() {}
};

template<typename ... SensorType>
class BaseSensor : public SensorType ... //to my BaseSensor class
{
    void update() /*{ what goes in here??? }*/
    void printStats() /*{ what goes in here??? }*/
};

in sensor.t.hpp

template<typename ... SensorType>
void BaseSensor<SensorType...>::update()
{
    int arr[] = { (SensorType::update(), 0)..., 0 };
    (void)arr;
}

template<typename ... SensorType>
void BaseSensor<SensorType...>::printStats()
{
    int arr[] = { (SensorType::printStats(), 0)..., 0 };
    (void)arr;
}

in main.cpp

int main(int , const char **) 
{
    {
        BaseSensor<EdgeSensor,TrendSensor> ets;
        ets.update();
        ets.printStats();
    }
    {
        BaseSensor<EdgeSensor> ets;
        ets.update();
        ets.printStats();
    }
}

The above code executes the update() of all the mixins in turn, before going on to execute all the printStats() from all the mixins as well.

I wonder if it is somehow possible to avoid duplicating the implementation of BaseSensor::update() and BaseSensor::printStats() and create a generic (template) function that accepts the name of the target function to execute across all the mixins:

For example, I could create a method runAll()

template<typename ... SensorType>
class BaseSensor : public SensorType ... //to my BaseSensor class
{
    void update() /*{ what goes in here??? }*/
    void printStats() /*{ what goes in here??? }*/

    template<typename FnName>
    void runAll(FnName f)
    {
        int arr[] = { (SensorType::f(), 0)..., 0 };
        (void)arr;
    }
};

How would I call it then from BaseSensor::update() and BaseSensor::printStats(). I have attempted to use

void update() { runAll<update>(); }
void printStats() { runAll<printStats>(); }

but this does not work (did not expect it to). The problem with passing function name as a function argument (which I see is many other questions such as here is that I do not know how to point to various ::update() functions from BaseSensor::update(). for example

void update() { runAll<update>( update() ); }

is also not correct.

Is it possible to avoid copying in this case? How would the template parameters look like if I where to move a working runAll() into file "sensor.t.hpp" ?

Thank you

Answer 1

You can use a generic lambda and a kind of inversion of control.
It follows a minimal, working example:

#include<iostream>

struct EdgeSensor
{
    void update() { std::cout << "EdgeSensor::update" << std::endl; }
    void printStats() { std::cout << "EdgeSensor::printStats" << std::endl; }
};

struct TrendSensor
{
    void update() { std::cout << "TrendSensor::update" << std::endl; }
    void printStats() { std::cout << "TrendSensor::printStats" << std::endl; }
};

template<typename ... SensorType>
class BaseSensor : public SensorType ...
{
    template<typename F>
    void execute(F &&f) {
        int arr[] = { (f(static_cast<SensorType&>(*this)), 0)..., 0 };
        (void)arr;
    }

public:
    void update() {
        execute([](auto &t) { t.update(); });
    }

    void printStats() {
        execute([](auto &t) { t.printStats(); });
    }
};

int main() {
    BaseSensor<EdgeSensor,TrendSensor> ets;
    ets.update();
    ets.printStats();
}

Answer 2

If you can use c++1z fold expressions would probably be the shortest way:

template<typename ... SensorType>
class BaseSensor : public SensorType ... //to my BaseSensor class
{
public:
    void update() { (SensorType::update(),...); }
    void printStats() { (SensorType::printStats(),...); }
};

Answer 3

Another c++11 way could be to use std::array of pointer to method e.g.:

template<typename ... SensorType>
class BaseSensor : public SensorType ... //to my BaseSensor class
{
   void runAll(std::array<void (BaseSensor::*)(), sizeof...(SensorType)>&& vs) {
      for (auto v: vs) {
         (this->*v)();
      }
   }

public:
    void update() {
       runAll({&SensorType::update...});
    }
    void printStats() {
       runAll({&SensorType::printStats...});
    }
};

Answer 4

Here is another, more compact solution that compiles in C++11:

#include <type_traits>
#include <iostream>

struct EdgeSensor {
    void update() { std::cout << "EdgeSensor::update" << std::endl; }
    void printStats() { std::cout << "EdgeSensor::printStats" << std::endl; }
};

struct TrendSensor {
    void update() { std::cout << "TrendSensor::update" << std::endl; }
    void printStats() { std::cout << "TrendSensor::printStats" << std::endl; }
};

template<typename ... SensorType>
class BaseSensor : public SensorType ... { 
    template <void(SensorType::* ...M)()>
    void run() {
        int arr[] = { 0, ((this->*M)(), 0)... };
        (void)arr;
    }

public:   
    void update() {   
        run<&SensorType::update...>();
    }

    void printStats() {
        run<&SensorType::printStats...>();
    }
};

int main() {
    BaseSensor<EdgeSensor, TrendSensor> bs;
    bs.update();
    bs.printStats();
}

I would be tempted to say that you don't need any structure to support the pack of pointers to member methods.
Anyway, I found that this compiles with clang and it doesn't work with GCC. I'm still trying to figure out if the code is ill-formed or if the problem is in the compiler.

I'd suggest to follow the other question to know if you can use or not this code.

Answer 5

Yet another pure c++11 answer which came to my mind. This one uses tag dispatching and non-type template parameters:

template <class T, void (T::*)()>
struct Method { };

template<typename ... SensorType>
class BaseSensor : public SensorType ... //to my BaseSensor class
{
   template <class T, void(T::*M)()>
   int runSingle(Method<T, M>) {
      (this->*M)();
      return 0;
   }

   template <class... Ts>
   void runAll() {
      int run[sizeof...(Ts)] = { runSingle(Ts{})... };
      (void)run;
   }

public:
    void update() {
       runAll<Method<SensorType, &SensorType::update>...>();
    }
    void printStats() {
       runAll<Method<SensorType, &SensorType::printStats>...>();
    }
};

It has to be stated though that non of these answers apart from the fold expressions (including skypjack's one) would deal with the virtual callee method of a mixin class... However I think skypjack answer could be easily modified to achieve such an effect:

#include<type_traits>

// (...)

template<typename ... SensorType>
class BaseSensor : public SensorType ...
{
    template<typename F>
    void execute(F &&f) {
        int arr[] = { (f(static_cast<SensorType&>(*this)), 0)..., 0 };
        (void)arr;
    }

public:
    void update() {
        execute([](auto &t) { t.std::remove_reference<decltype(t)>::type::update(); });
    }

    void printStats() {
        execute([](auto &t) { t.std::remove_reference<decltype(t)>::type::printStats(); });
    }
};