How to write destructor for union-like class

Question

I\'m trying to use an union (C++) that has some non-primitive variables, but I\'m stuck trying to create the destructor for that class. As I have read, it is not possible to

Answer 1

If you want to use std::string in a union in C++11, you have to explicitly call its destructor, and placement new to construct it. Example from cppreference.com:

#include <iostream>
#include <string>
#include <vector>
union S {
    std::string str;
    std::vector<int> vec;
    ~S() {} // needs to know which member is active, only possible in union-like class 
}; // the whole union occupies max(sizeof(string), sizeof(vector<int>))

int main()
{
    S s = {"Hello, world"};
    // at this point, reading from s.vec is UB
    std::cout << "s.str = " << s.str << '\n';
    s.str.~basic_string<char>();
    new (&s.vec) std::vector<int>;
    // now, s.vec is the active member of the union
    s.vec.push_back(10);
    std::cout << s.vec.size() << '\n';
    s.vec.~vector<int>();
}

Answer 2

This grouping (union + enum value for discriminating type) is called a discriminated union.

It will be up to you to call any construction/destruction, because the union itself cannot (if it could, it would also be able to discriminate for initialized/non-initialized types within the union, and you would not need the enum).

Code:

class LuaVariant // no public access to the raw union
{
public:
    LuaVariant() : type(VARIANT_NONE) { }
    ~LuaVariant() { destroy_value(); }

    void text(std::string value) // here's a setter example
    {
        using std::string;
        destroy_value();
        type = VARIANT_TEXT;
        new (&value.text) string{ std::move(value) };
    }
private:

    void destroy_value()
    {
        using std::string;
        switch(type)
        {
        case VARIANT_TEXT:
            (&value.text)->string::~string(); 
            break;
        case VARIANT_POSITION:
            (&value.pos)->Position::~Position(); 
            break;
        case VARIANT_NUMBER:
            value.number = 0;
            break;
        default:
            break;
        }
    }

    LuaVariantType_t type;
    union {
        std::string text;
        Position pos;
        uint32_t number;
    } value;
};