function as template parameters - c++

Question

I am trying to avoid this repetitive code by writing a template function.
#include <algorithm>
class X {
public:
void get_amin(double *a){}
void set_amin(double a){}
void get_bmin(double *b){}
void set_bmin(double b){}
//...many pairs like above
};
int main(){
X *x1 = new X;
X *x2 = new X;
//code that will be repeated
{
double x1_amin;
x1->get_amin(&x1_amin);
double x2_amin;
x2->get_amin(&x2_amin);
x1->set_amin(std::min(x1_amin, x2_amin));
}
//repeatation
{
double x1_bmin;
x1->get_bmin(&x1_bmin);
double x2_bmin;
x2->get_bmin(&x2_bmin);
x1->set_bmin(std::min(x1_bmin, x2_bmin));
}
//
delete x1;
delete x2;
}
Now my attempts are below. It seems I am able to write the template but not able to use it. Other posts at stack overflow mostly focus on how to write the template. Also I could not find an example where a class member function is used.
#include <algorithm>
#include <functional>
class X {
public:
void get_amin(double *a){}
void set_amin(double a){}
void get_bmin(double *b){}
void set_bmin(double b){}
//...many pairs like above
};
template <typename F11,typename F12, typename F2>
void templatisedFunction(F12 f11,F12 f12,F2 f2)
{
double x1_amin;
f11(&x1_amin);
double x2_amin;
f12(&x2_amin);
f2(std::min(x1_amin, x2_amin));
}
int main(){
X *x1 = new X;
X *x2 = new X;
//templatisedFunction(x1->get_amin,x2->get_amin,x1->set_amin);
//templatisedFunction(x1->get_amin(double*),x2->get_amin(double*),x1->set_amin(double));
//templatisedFunction<x1->get_amin(double*),x2->get_amin(double*),x1->set_amin(double)>();
//templatisedFunction<x1->get_amin,x2->get_amin,x1->set_amin>();
std::function<void(X*)> memfun(&X::get_amin);//not sure here
//templatisedFunction<x1->get_amin,x2->get_amin,x1->set_amin>();
//
delete x1;
delete x2;
}

Answer 1

void (X::*getf)(double *) and void (X::*setf)(double) are the function signatures for the two pointer to member function that you need.
Using C++11:
int main()
{
X x1;
X x2;
auto lamb = [&](void (X::*getf)(double *), void (X::*setf)(double))
{
double x1_amin;
(x1.*getf)(&x1_amin);
double x2_amin;
(x2.*getf)(&x2_amin);
(x1.*setf)(std::min(x1_amin, x2_amin));
};
lamb(&X::get_amin, &X::set_amin);
lamb(&X::get_bmin, &X::set_bmin);
return 0;
}

Answer 2

You can use pointers to member functions to reduce repetition:
void set_min(X &x1, X &x2, void (X::*get_min)(double *), void (X::*set_min)(double)) {
double x1_amin;
(x1.*get_min)(&x1_amin);
double x2_amin;
(x2.*get_min)(&x2_amin);
(x1.*set_min)(std::min(x1_amin, x2_amin));
}
to be used like this:
set_min(*x1, *x2, &X::get_amin, &X::set_amin);
set_min(*x1, *x2, &X::get_bmin, &X::set_bmin);
If you have many pairs you could go even further and use a loop:
std::pair<void (X::*)(double *), void (X::*)(double)> get_set_pairs[] = {
{&X::get_amin, &X::set_amin},
{&X::get_bmin, &X::set_bmin},
};
for (auto &get_set_pair : get_set_pairs){
set_min(*x1, *x2, get_set_pair.first, get_set_pair.second);
}