• Custom Type Based on Type Traits

#include <iostream>
#include <string>
enum class SimpleType {
    ST_INVALID, ST_INT64, ST_UINT64, ST_DOUBLE, ST_STRING
};
#define SIMPLE_TYPE_MACRO(MY_MACRO, ...)                                MY_MACRO(SimpleType::ST_INT64,  int64_t,     ##__VA_ARGS__);        MY_MACRO(SimpleType::ST_UINT64, uint64_t,    ##__VA_ARGS__);        MY_MACRO(SimpleType::ST_DOUBLE, double,      ##__VA_ARGS__);        MY_MACRO(SimpleType::ST_STRING, std::string, ##__VA_ARGS__)
template<SimpleType stype>
struct SimpleType2BuiltinType {
    struct InvalidType {};
    typedef InvalidType BuiltinType;
};
#define SimpleType2BuiltinTypeTraits(stype, btype, ...)     template<>                                              struct SimpleType2BuiltinType<stype> {                      typedef btype BuiltinType;                          }
SIMPLE_TYPE_MACRO(SimpleType2BuiltinTypeTraits);
#undef SimpleType2BuiltinTypeTraits
class Object {
public:
    Object(SimpleType stype)
        : type_(stype) {}
    ~Object() {}
public:
    virtual SimpleType type() const {
        return type_;
    }
    virtual void type(SimpleType stype) {
        type_ = stype;
    }
private:
    SimpleType type_;
};
template<class T>
class BasicType : public Object {
public:
    BasicType(SimpleType stype)
        : Object(stype) {}
    ~BasicType() {}
public:
    virtual void value(const T& value) {
        value_ = value;
    }
    virtual const T& value() const {
        return value_;
    }
private:
    T value_;
};
typedef BasicType<std::string> String;
int main(int argc, char *argv[]) {
    std::string v("foo");
    String foo(SimpleType::ST_STRING);
    foo.value(v);
    Object* object = &foo;
    switch (object->type()) {
    #define GET_VALUE(stype, ...)                                                           case stype: {                                                                       typedef SimpleType2BuiltinType<stype>::BuiltinType Type;                        BasicType<Type>* typed = static_cast<BasicType<Type>*>(object);                 const Type& v = typed->value();                                                 std::cout << v << std::endl;                                                    break;                                                                      }
    SIMPLE_TYPE_MACRO(GET_VALUE, object);
    #undef GET_VALUE
    default:
        break;
    }
    return 0;
}