C++ Type Traits

Custom Type Based on Type Traits

#include <stdint.h>
#include <string>
#include <iostream>

enum class SimpleType {
    ST_INVALID,
    ST_INT64,
    ST_UINT64,
    ST_DOUBLE,
    ST_STRING
};

template<SimpleType stype>
struct SimpleType2BuiltinType {
    struct InvalidType {};
    typedef InvalidType BuiltinType;
};

#define SimpleType2BuiltinTypeTraits(stype, btype)      template<>                                          struct SimpleType2BuiltinType<stype> {                  typedef btype BuiltinType;                      }

SimpleType2BuiltinTypeTraits(SimpleType::ST_INT64, int64_t);
SimpleType2BuiltinTypeTraits(SimpleType::ST_UINT64, uint64_t);
SimpleType2BuiltinTypeTraits(SimpleType::ST_DOUBLE, double);
SimpleType2BuiltinTypeTraits(SimpleType::ST_STRING, std::string);

#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<int64_t> Int64;
typedef BasicType<uint64_t> UInt64;
typedef BasicType<double> Double;
typedef BasicType<std::string> String;

#define SIMPLE_VALUE_HELPER(stype, obj)                                     case stype: {                                                               typedef SimpleType2BuiltinType<stype>::BuiltinType Type;                BasicType<Type>* typed = static_cast<BasicType<Type>*>(obj);            const Type& v = typed->value();                                         std::cout << v << std::endl;                                            break;                                                              }

#define SIMPLE_TYPE_MACRO_HEPLER(MY_MACRO, obj)     MY_MACRO(SimpleType::ST_INT64, obj)             MY_MACRO(SimpleType::ST_UINT64, obj)            MY_MACRO(SimpleType::ST_DOUBLE, obj)            MY_MACRO(SimpleType::ST_STRING, obj)

int main(int argc, char *argv[]) {
    std::string v("foo");
    String foo(SimpleType::ST_STRING);
    foo.value(v);
    Object* object = &foo;

    switch (object->type()) {
        SIMPLE_TYPE_MACRO_HEPLER(SIMPLE_VALUE_HELPER, object);
    default:
        break;
    }
    return 0;
}

原文地址：https://www.cnblogs.com/rain-blog/p/cpp-type-traits.html