Help me with "template parameters not used in partial specialization"

I've been struggling with NOT C++0x code, but regular C++. Don't ask me why I have to use regular C++, it's just some kind of silly requirement.

So here's the thing : I need to get a value in an enum to be 1 or 0, regarding that some statement is true or false. So of course, I templated a struct containing 0 in an enum, specializated it with the second statement where the enum contains 1 instead of 0.

Seems quite legit to me, however, it tells me that I should use the parameters of the specialization. Which is kinda strange, because I tried to use it in all possible ways and it just kept popping out this error.

Here's the code :

  template<typename T>
  struct CanPrint
  {
    template<size_t>
    struct Value               { enum { val = 0 }; };

    template<size_t>
    struct Value<sizeof(True)> { enum { val = 1 }; };

    enum
    { value = Value<sizeof(IsTrue<T>(0))>::val };
  };

I bet this would work if it wasn't a partial specialization, but explicit ones can't be at namespace-sc开发者_如何学Cope. And I obviously can't specialize a template within a template without specializing both of them. Can I ?

---

template<>  //<---- leave it empty
struct Value<sizeof(True)> { enum { val = 1 };

By the way, its not partial specialization if True isn't template argument. It is full specialization .

And since this is full specialization, you cannot define it inside the class, i.e at class-scope. Full specialization can be defined only at namespace-scope. So define Value , primary well as the specialization, at namespace scope.

Or, you can do this instead:

template<typename T>
struct CanPrint
{
    //modified
    template<typename U, size_t N = sizeof(U)> 
    struct Value { enum { val = 0 }; };

    //modified - now its partial specialization
    template<typename U>    
    struct Value<U, sizeof(True)> { enum { val = 1 }; };

    enum { value = Value<IsTrue<T> >::val }; //modified here as well
};

See the online demo : http://www.ideone.com/MSG5X

---

template<> // note the empty <>
struct Value<sizeof(True)> { enum { val = 1 }; };

You only list parameters for partial specializations:

template< typename T, typename U> 
struct X;

template<typename U> 
struct X<char,U> {...};

template<typename Z, typename U> 
struct X<std::vector<Z>, U> {...};

Not for full specializations:

template<> 
struct X<double,int> {...};