Is it possible to pass a struct's member variable as parameter - c++

Question

I am trying to associate a struct's member variable with a class. So that when I create a new class, I can specify that it is associated with this member variable in a struct. For example:
struct A {
int a;
int b;
};
static A a[2];
a[0].a = 1;
a[0].b = 2;
a[1].a = 3;
a[1].b = 4;
class foo {
public:
foo(int index, ???) {
c = a[index].???; //Is it possible to define the 2nd parameter as a getter of struct A's member? So this line could resolve to either a[index].a or a[index].b?
}
private:
int c;
};
So that:
new foo(0, ???) would set c to 1 given ??? refer to A::a
new foo(0, ???) would set c to 2 given ??? refer to A::b
new foo(1, ???) would set c to 3 given ??? refer to A::a
new foo(1, ???) would set c to 4 given ??? refer to A::b

Answer 1

Yes, it is possible, you need to pass a data member pointer:
#include <iostream>
struct A
{
int a;
int b;
};
static A a[2]
{
1, 2
, 3, 4
};
class foo
{
public: int c;
public:
foo(int const index, int A::* const p_field)
{
c = a[index].*p_field;
}
};
int main()
{
foo const f1(0, &A::a);
::std::cout << f1.c << ::std::endl;
foo const f2(0, &A::b);
::std::cout << f2.c << ::std::endl;
foo const f3(1, &A::a);
::std::cout << f3.c << ::std::endl;
foo const f4(1, &A::b);
::std::cout << f4.c << ::std::endl;
return 0;
}
Check this code at online compiler

Answer 2

You have a couple options. If you just want the integer (like you have in your code you've posted), then just take an integer as a parameter to the constructor and pass it the right number.
class foo {
public:
foo(int val) {
c = val
}
private:
int c;
};
int main() {
foo f(a[0].b);
}
Or you could take a reference to an integer. This way if one changes, the other will as well:
class foo {
public:
foo(int &val) : c(val) { } //need to use an initialization list for this one
private:
int &c;
};
int main() {
foo f(a[0].b);
a[0].b = -1; //f.c will be -1 now as well
}

Answer 3

Using a data member pointer as in VTT's answer is the most direct solution but I often find member pointers and member function pointer syntax a bit cumbersome and I believe it is hard for the compiler to optimize.
For these kind of things I prefer to use a stateless lambda. You can pass a lambda to a function template and then the compiler can easily optimize it away:
#include <iostream>
struct A {
int a;
int b;
};
static A a[2]{{1, 2}, {3, 4}};
class foo {
public:
int c;
public:
template<typename F>
foo(int index, F getter) { c = getter(a[index]); }
};
int main() {
auto agetter = [](const A& a){ return a.a; };
auto bgetter = [](const A& a){ return a.b; };
foo const f1(0, agetter);
std::cout << f1.c << "\n";
foo const f2(0, bgetter);
std::cout << f2.c << "\n";
foo const f3(1, agetter);
std::cout << f3.c << "\n";
foo const f4(1, bgetter);
std::cout << f4.c << "\n";
}