Question

I’m trying to compile the following code under VC2010.

struct CircValRange
{
    double a,b; // range: [a,b)
};

template <struct CircValRange* Range>
class CircVal
{
    // todo
};


const CircValRange SignedDegRange= {-180., 180.};

CircVal<SignedDegRange> x;

I’m getting

error C2970: 'CircVal' : template parameter 'Range' : 'SignedDegRange' : an expression involving objects with internal linkage cannot be used as a non-type argument
1>          d:\4\circval\circval\circval.h(8) : see declaration of 'CircVal'
1>          d:\4\circval\circval\circval.h(13) : see declaration of 'SignedDegRange'

I am trying to define a templated class CircVal that will receive a struct Range as the templated parameter.

I don’t want it to be possible to assign classes with one range to classes with another range (I want them to be different types).

How can I do it?

Answer 1

Someone has recommended a constructor parameter, which I second. But you can still do it as originally desired

struct CircValRange
{
    double a,b; // range: [a,b)
};

template <CircValRange const& Range>
class CircVal
{
    // todo
};


extern const CircValRange SignedDegRange= {-180., 180.};

CircVal<SignedDegRange> x;

But notice that the property that determines the type-identity of CircVal<SignedDegRange> is not the value of SignedDegRange, but the address/identity of it. That is, the following does not work because CircVal<SignedDegRange1> denotes a different type

extern const CircValRange SignedDegRange1 = {-180., 180.};

CircVal<SignedDegRange1> y = x; // error!

As such, an enumeration may be better suited for this

enum RangeKind {
  SignedDegRange,
  UnsignedDegRange
};

const CircValRange Ranges[] = { { -180., -180. }, { 0., 360. } };

template <RangeKind Range>
class CircVal
{
    // todo
};

Or even a traits class with static member functions, similar to a solution someone else had

template <typename Range>
class CircVal
{
    // todo
};

struct SignedDegRange {
  static double min() { return -180.; }
  static double max() { return  180.; }
};

CircVal<SignedDegRange> x;