Using Template Metaprogramming
Here is something I came up with to solve your problem (at least partially). By using template metaprogramming, you can leverage the compiler to do most of the job for you. These techniques look weird for those who have never seen such code before, but thankfully most of the complexity can be hidden away in a header and the users only interact with the library in a neat and terse manner.
A Sample Class Definition and its Use
Here is an example of what defining a class would entail on your part:
template <
//Declare your fields here, with types and default values
typename PortNumber = field<int, 100>,
typename PortLetter = field<char, 'A'>
>
struct MyStruct : public const_obj<MyStruct, PortNumber, PortLetter> //Derive from const_obj like this, passing the name of your class + all field names as parameters
{
//Your setters have to be declared like this, by calling the Set<> template provided by the base class
//The compiler needs to be told that Set is part of MyStruct, probably because const_obj has not been instantiated at this point
//in the parsing so it doesn't know what members it has. The result is that you have to use the weird 'typename MyStruct::template Set<>' syntax
//You need to provide the 0-based index of the field that holds the corresponding value
template<int portNumber>
using SetPortNumber = typename MyStruct::template Set<0, portNumber>;
template<int portLetter>
using SetPortLetter = typename MyStruct::template Set<1, portLetter>;
template<int portNumber, char portLetter>
using SetPort = typename MyStruct::template Set<0, portNumber>
::MyStruct::template Set<1, portLetter>;
//You getters, if you want them, can be declared like this
constexpr int GetPortNumber() const
{
return MyStruct::template Get<0>();
}
constexpr char GetPortLetter() const
{
return MyStruct::template Get<1>();
}
};
Using the Class
int main()
{
//Compile-time generation of the type
constexpr auto myObject =
MyStruct<>
::SetPortNumber<150>
::SetPortLetter<'Z'>();
cout << myObject.GetPortNumber() << endl;
cout << myObject.GetPortLetter() << endl;
}
Most of the job is done by the const_obj
template. It provides a mechanism to modify your object at compile time. Much like a Tuple
, the fields are accessed with 0-based indices but this does not stop you from wrapping the setters with friendly names, as is done with SetPortNumber and SetPortLetter above. (They just forward to Set<0> and Set<1>)
About Storage
In the current implementation, after all the setters have been called and the object declared, the fields end up being stored in a compact array of const unsigned char
's named data
in the base class. If you use fields that are not unsigned chars (as in done above with PortNumber for example) the field is divided in big endien unsigned char
's (could be changed to little endien as needed). If you don't need an actual storage that has an actual memory address, you could omit it altogether by modifying the packed_storage
(see full implementation link below) and the values would still be accessible at compile time.
Limitations
This implementation only allows integral types to be used as fields (all flavors of shorts, ints, longs, bool, char). You can still provide setters that act on more than one field though. Example:
template<int portNumber, char portLetter>
using SetPort = typename MyStruct::template Set<0, portNumber>::
MyStruct::template Set<1, portLetter>;
Full Code
The full code for the implementation of this little library can be found here:
Additional Notes
This code has been tested and works with the C++11 implementation of both g++ and clang. It has not been tested for hours and hours so of course there may be bugs but it should provide you with a good base to start with. I hope this helps!