How can I simplify this redundant C++ code?

问题:

There are two existing classes, one is SrcField which returns the concrete type value, and the other is a union DSTField, that defines the corresponding data type.

class SrcField
{
public:
    signed char     GetInt8();
    unsigned char   GetUInt8();
    short           GetInt16();
    unsigned short  GetUInt16();
    int             GetInt32();
    unsigned int    GetUInt32();
    float           GetFloat();
    double          GetDouble();
    bool            GetBool();
    DataType        GetType();
private:
    DataType m_type;
    DSTField m_data;
};

union DSTField
{
    signed char    m_int8;
    unsigned char  m_uint8;
    short          m_int16;
    unsigned short m_uint16;
    int            m_int32;
    unsigned int   m_uint32;
    float          m_float;
    double         m_double;
    bool           m_bool;
};

When I use both classes, the application is as below. It's very redundant; is there any good way to simplify it, such as templates, generic programming, etc?

int main()
{
    SrcField sf;
    DSTField df;

    switch(sf.GetType())
    {
    case TYPE_INT8:
        df.m_int8 = sf.GetInt8();
        break;
    case TYPE_UINT8:
        df.m_uint8 = sf.GetUInt8();
        break;
    case TYPE_INT16:
        df.m_int16 = sf.GetInt16();
        break;
    case TYPE_UINT16:
        df.m_uint16 = sf.GetUInt16();
        break;
    case TYPE_INT32:
        df.m_int32 = sf.GetInt32();
        break;
    case TYPE_UINT32:
        df.m_uint32 = sf.GetUInt32();
        break;
    case TYPE_FLOAT:
        df.m_float = sf.GetFloat();
        break;
    case TYPE_DOUBLE:
        df.m_double = sf.GetDouble();
        break;
    case TYPE_BOOL:
        df.m_bool = sf.GetBool();
        break;
    default:
        break;
    }
}

回答1:

Using std::variant your code would look like this:

#include <iostream>
#include <variant>

typedef std::variant<
    signed char,
    unsigned char,
    short,
    unsigned short,
    int,
    unsigned int,
    float,
    double,
    bool
> SrcField, DSTField;

int main()
{
    SrcField sf(97.0f);
    DSTField df;

    df = sf;

    if(auto pval = std::get_if<float>(&df))
      std::cout << "variant value: " << *pval << '\n'; 
    else 
      std::cout << "failed to get value!" << '\n';
}

---

Note: Since it's c++17, for previous versions I recommend to use boost::variant, boost::any or a header-only implementation of Any class (for example I use one based on this in my project)

回答2:

You said that you cannot change SrcField, therefore a good solution could be the use of a visitor. The redundant code is still there, but it is present only once. See this:

template<typename Visitor>
constexpr void
visitField(Visitor&& visitor, SrcField& field)
{
    switch(field.GetType())
    {
    case TYPE_INT8:
        visitor(field.GetInt8());
        break;
    case TYPE_UINT8:
        visitor(field.GetUInt8());
        break;
    ....
    default:
        throw std::runtime_error("invalid type");
}

In this way you are able to use the values in a simple way:

int main()
{
    SrcField field;
    visitField([](auto value) {
        if constexpr(std::is_same<decltype(value), double>::value)
            std::cout << "Hey, double here!\n";
        else if constexpr(std::is_same<decltype(value), bool>::value)
            std::cout << "True or false?\n";
        else
            std::cout << "Other types\n";
        std::cout << value << '\n';
    }, field);
}

In this case I used the if constexpr capability from C++17. Another possibility use a lambda overload

You can find a more complete example here on godbolt

Note: As you can see, I did not use DSTField at all. If you really need to use DSTField, you can use a similar approach:

template<typename T>
constexpr void
setField(DSTField& dstField, T value)
{
    static_assert(std::is_arithmetic<T>::value,
                  "value must be an arithmetic type");

    if constexpr(std::is_same<T, signed char>::value)
        dstField.m_int8 = value;
    else if constexpr(std::is_same<T, unsigned char>::value)
        dstField.m_uint8 = value;
    ...
}

which can be used with something like

DSTField dest;
setField(dest, 4.f);

Other note: I marked the visitField function as constexpr, but I cannot be sure if you can use in that way. Indeed, if SrcField::GetType can only be executed at runtime, visitField will never be executed at compile time.

Other other note: I don't know if this could depend on your code or not, but you have to keep in mind that you cannot be sure that signed char is a std::int8_t (as for most of the other types, obviously). You should use fixed width integer types if you want to make your code work as expected on foreign architectures.