nass nass - 2 months ago 16
C++ Question

variadic templates with template function names for c++11

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? Can this be done in a one-liner so as to avoid alot of copying using c++11 (i.e. without using generic lambdas as is done here)? How would the template parameters look like if I where to move a working
runAll()
into file "sensor.t.hpp" ?

Thank you.

Answer

As long as the functions to be called are two, you can use a dedicated structure and rely on overloading to solve it.
It follows a minimal, working example:

#include<iostream>

struct Executor {
    template<typename T>
    static void execute(int, T &t) {
        t.update();
    }

    template<typename T>
    static void execute(char, T &t) {
        t.printStats();
    }
};

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 T>
    void execute() {
        int arr[] = { (Executor::execute(T{}, static_cast<SensorType&>(*this)), 0)..., 0 };
        (void)arr;
    }

public:
    void update() {
        execute<int>();
    }

    void printStats() {
        execute<char>();
    }
};

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

In case you have more than two functions to be called, I guess the choice trick applies well here.

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