共用体をラップしたクラス。あまり役に立ちそうにない。http://jumble-note.blogspot.jp/2013/12/c.html

union.cpp

#include <iostream>

#define def_t(n)    \
    typename T ## n = __type_util::_void

#define def_m(n)    T ## n t ## n;
#define check(n)    typename __type_util::is_void<T ## n, T>::type* = 0

#define def_a(n)    \
    template<typename T>    \
    inline T _get(const typename __type_util::guard<T ## n, n>::type, check(n)) const { return t ## n; }    \
    template<typename T>    \
    inline void _set(const typename __type_util::guard<T ## n, n>::type &t, check(n)) { t ## n = t; _id = n; }

namespace __type_util
{
    struct _void { };

    template<typename S, typename T>
    struct is_void
    {
        typedef T type;
    };

    template<typename T>
    struct is_void<_void, T>
    { };

    template<typename T, int N>
    struct guard
    {
        typedef T type;
    };

    template<int N>
    struct guard<_void, N>
    {
        typedef guard<_void, N> type;
    };
}

template<def_t(1), def_t(2), def_t(3), def_t(4), def_t(5), def_t(6), def_t(7), def_t(8)>
class Union
{
private:
    int _id;
    union
    {
        def_m(1)
        def_m(2)
        def_m(3)
        def_m(4)
        def_m(5)
        def_m(6)
        def_m(7)
        def_m(8)
    };

    def_a(1)
    def_a(2)
    def_a(3)
    def_a(4)
    def_a(5)
    def_a(6)
    def_a(7)
    def_a(8)
public:
    Union() : _id(0) { }

    template<typename T>
    inline Union(const T &t) : _id(0) { _set<T>(t); }

    template<typename T>
    inline operator T() const { return _get<T>(T()); }

    template<typename T>
    inline T get() const { return _get<T>(T()); }

    template<typename T>
    inline T operator =(const T &t) { _set<T>(t); return t; }

    inline int which() const { return _id; }
};

#undef def_a
#undef check
#undef def_m
#undef def_t

int main(int argc, const char * argv[])
{
    Union<int, double> x = 100;

    // check size
    printf("size of x: %lu\n", sizeof(x));

    // get int value
    const int v = x;
    printf("x(type: %d): %d\n", x.which(), v);

    // set double value
    x = 123.456;

    printf("x(type: %d): %f\n", x.which(), x.get<double>());

    return 0;
}