C++: Union containing class instances calls wrong virtual function

问题

I came across a strange phenomena upon running the following code:

#include <iostream>    

class Piece {
public:
    class Queen;
    class Knight;
    union Any;
    virtual const char* name() const = 0;
};

class Piece::Queen : public Piece {
public:
    virtual const char* name() const {
        return "Queen";
    }
};

class Piece::Knight : public Piece {
public:
    virtual const char* name() const {
        return "Knight";
    }
};

union Piece::Any {
public:
    Any() {}
    Piece::Queen queen;
    Piece::Knight knight;
};

using namespace std;
int main(int argc, const char* argv[]) {
    Piece::Any any;
    any.queen = Piece::Queen();
    cout << any.queen.name() << endl;

    return 0;
}

The program compiled successfully on the Apple LLVM 3.0 compiler, but the output was "Knight". I was expecting for the output to be "Queen". From my testing I saw that when Piece::Any's default constructor runs, it calls both Piece::Queen and Piece::Knights' constructors, one after another. If I were to declare Piece::Any like this:

union Piece::Any {
public:
    Any() {}
    Piece::Knight knight;
    Piece::Queen queen;
};

(I basically swapped the order of knight and queen) then the output would be Queen. Any help would be appreciated.

Thanks

回答1:

First of all - your constructor seems to initialize none of its members. You should choose one, for example

Piece::Any::Any(): knight() {}

Then according to 9.5.4

In general, one must use explicit destructor calls and placement new operators to change the active member of a union

so correct switching from knight to queen is

any.knight.~Knight();
new(&any.queen) Queen;

If it looks ugly to you (as it does to me), it is clear indication, that keeping objects with non-trivial constructors in union is not a good idea (how about boost::variant?).