nass nass - 2 months ago 13
C++ Question

variadic templates with template function names

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

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

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

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

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