How to pass member of structure/union array into function - c++

Question

For example, I want to display value of only one member of structure/union array, so I want to pass it as argument into a function that will display only that one member and will also display any other single member that I pass as argument.
#include <iostream>
using namespace std;
union ThreeTypes
{
char letter;
int whole;
double real;
};
void showArr(ThreeTypes[], int); // ?? What parameters to pass?
int main()
{
const int SIZE = 50;
ThreeTypes arr[SIZE];
for (int i = 0; i < SIZE; i++)
arr[i].real = 2.37;
showArr(arr, SIZE, ??? ); // what argument to pass to display member?
return 0;
}
void showArr(ThreeTypes arr[],int size,???) // also, what parameters??
{
for (int i = 0; i < size; i++)
cout << arr[i].?? << endl; // member from argument????
}

Answer 1

One option would be a templated pointer-to-member type. These are usually used with class or struct types, but are also valid with a union type. A pointer-to-member type is declared like MemberType ClassType::*pointer_name, and the name of such a pointer can be used to the right of the .* or ->* operator.
template <typename T>
void showArr(const ThreeTypes arr[], int size, T ThreeTypes::*ptr)
{
for (int i = 0; i < size; ++i)
std::cout << (arr[i].*ptr) << std::endl;
}
And you create a pointer-to-member value with the syntax &ClassType::member_name:
int main()
{
const int SIZE = 50;
ThreeTypes arr[SIZE];
for (int i = 0; i < SIZE; i++)
arr[i].real = 2.37;
showArr(arr, SIZE, &ThreeTypes::real);
}
Another more general option would be to take a callable functor:
template <typename F>
void showArr(const ThreeTypes arr[], int size, const F& func)
{
for (int i = 0; i < size; ++i)
std::cout << func(arr[i]) << std::endl;
}
You can create a functor to access a member using a lambda or std::mem_fn:
void print_reals_twice(const ThreeTypes arr[], int size)
{
showArr(arr, size, [](const ThreeTypes& u) { return u.real; });
// Same effects:
showArr(arr, size, std::mem_fn(&ThreeTypes::real));
}
But defining showArr this way also lets you pass a functor that does something more complicated than just return a member, if you wanted:
void print_sin2x_all(const ThreeTypes arr[], int size)
{
showArr(arr, size, [](const ThreeTypes& u) { return std::sin(2*u.real); });
}

Answer 2

In C++17 you should use std::variant:
using ThreeTypes = std::variant<char, int, double>;
template <std::size_t N>
auto show_arr(std::array<ThreeTypes, N>& arr)
{
for (auto& e : arr)
{
std::visit([](auto e) { std::cout << e << std::endl;}, e);
}
}
auto test()
{
std::array<ThreeTypes, 2> arr = {3.4, 'a'};
show_arr(arr);
}