Automatically tracks which member of the union was last set

union_watcher.cpp

template <typename T, typename E, E SYMBOL>
class Proxy {
public:
    Proxy(T& value, E& which) : ref(value), which(which) {}
    Proxy& operator=(const T& value) {
        ref = value;
        which = SYMBOL;
    }
    operator T() { return ref; }
protected:
    T& ref;
    E& which;
};

class Multi {
    union {
        int integer;
        char letter;
        double number;
    } shared;

public:
    enum Which {
        INTEGER,
        LETTER,
        NUMBER,
    } which;

    Proxy<int, Which, INTEGER> integer{shared.integer, which};
    Proxy<char, Which, LETTER> letter{shared.letter, which};
    Proxy<double, Which, NUMBER> number{shared.number, which};
};

#include <iostream>

int main() {
    Multi m;
    m.integer = 42;
    std::cout << m.which << std::endl;
    m.letter = 'c';
    std::cout << m.which << std::endl;
    m.number = 42.3216;
    std::cout << m.which << std::endl;
}