C++/templates: Can I selectively disable a function of a class at compile time?

Question

I need to write 2 constructors for a quaternion class that basically contains nothing but 4 elements of a numerical type T. I have 2 constructors in conflict at compile time (see below). Is it possible to tell the compiler to ignore the constructor from an iterator if I want to call the constructor that takes 4 ints (my attempt commented out - it seems the compiler sees my attempt as all or nothing to instantiate the whole class). The (clang) compiler complaint is "ambiguous conversion", and it lists these 2 constructors when trying to do Quaternion<float> x(1);

With my commented out attempt, clang tells me:

error: no type named 'value_type' in 'std::__1::iterator_traits<float>'
      typename std::enable_if<!std::is_same<typename std::iterator_traits<T>::value_type, void>::value>::type>

The code:

template <typename T>
class Quaternion {
  public:
   template<typename T1>
   Quaternion(T1 a = 0, T1 b = 0, T1 c = 0, T1 d = 0)
   : _a(static_cast<T>(a)),
     _b(static_cast<T>(b)),
     _c(static_cast<T>(c)),
     _d(static_cast<T>(d)) { }

   template <typename It>
   //typename std::enable_if<!std::is_same<typename std::iterator_traits<T>::value_type, void>::value>::type>
   Quaternion(It it)
   : _a(static_cast<T>(++it)),
     _b(static_cast<T>(++it)),
     _c(static_cast<T>(++it)),
     _d(static_cast<T>(++it))
   {}

  private:
    T _a, _b, _c, _d;
};

I got it - I'm using "T" in the commented out line when I should use "It" :-( I'm going to remove this question. — Frank, Dec 18 '15 at 22:52
By the way, see [this](http://stackoverflow.com/questions/9628974/why-cant-the-compiler-deduce-the-template-type-from-default-arguments), why you should have `template` for the first overload, if you want to default-construct it. — LogicStuff, Dec 18 '15 at 22:54
`++it` should be `it++`? Otherwise you skip the first item.. — M.M, Dec 18 '15 at 23:14

score 1 · Accepted Answer · answered Dec 18 '15 at 23:06

You could define a custom trait is_iterator:

template<typename T, typename = void>
struct is_iterator {
   static bool const value = false;
};

template<typename T>
struct is_iterator<T, typename std::enable_if<!std::is_same<typename std::iterator_traits<T>::value_type, void>::value>::type> {
   static bool const value = true;
};

And SFINAE out your constructors with a defaulted template argument in the following way:

template<typename T1, typename = typename std::enable_if<!is_iterator<T1>::value>::type>
Quaternion(T1 a = 0, T1 b = 0, T1 c = 0, T1 d = 0)
:_a(static_cast<T>(a)),
 _b(static_cast<T>(b)),
 _c(static_cast<T>(c)),
 _d(static_cast<T>(d)) 
 {}

template <typename It, typename = typename std::enable_if<is_iterator<It>::value>::type>
Quaternion(It it)
:_a(static_cast<T>(*(++it))),
 _b(static_cast<T>(*(++it))),
 _c(static_cast<T>(*(++it))),
 _d(static_cast<T>(*(++it)))
{}

Live Demo

C++/templates: Can I selectively disable a function of a class at compile time?

1 Answers1