link error while compiling this "simple" program with clang and g++

Question

I am working on a project, and it seems that clang is unable to generate a valid bytecode (as the linker fail to link, some static constexpr in a template class is not found at link time) I can fix it with a static getter in the class, but this lead to some really ugly/overloaded code.

Here is a "minimal" sample of code that makes the bug (is it a bug ?) appear. Unfortunately, here, g++ will produce the same link error.

I'm asking if this is a compiler bug, or I am simply doing something wrong, and if there's a solution to avoid this error. (And if I am doing something wrong, why the same construct works in my bigger project ??)

NOTE: The bigger project is named yaggler on github, but is still at the early beginning of its life

#include <type_traits>
#include <iostream>

// exemple vector classes
struct vector2
{
  constexpr vector2(unsigned int _x = 0, unsigned int _y = 0) : x(_x), y(_y) {} // for clang

  unsigned int x;
  unsigned int y;
};
struct vector3 : public vector2 // uh ;)
{
  constexpr vector3(unsigned int _x = 0, unsigned int _y = 0, unsigned int _z = 0) : vector2(_x, _y), z(_z) {} // for clang
  unsigned int z;
};

// simple templated generic vector type
// we could make a more generic one, but this would require something like a tuple.
template<unsigned int... Vals>
struct vector
{
    static_assert(!(sizeof...(Vals) + 1), "[...]");
};
template<unsigned int X>
struct vector<X>
{
  using vec_type = unsigned int;
  static constexpr unsigned int value = X;
};
template<unsigned int X, unsigned int Y>
struct vector<X, Y>
{
  using vec_type = vector2;
  static constexpr vector2 value = vector2(X, Y);
};
template<unsigned int X, unsigned int Y, unsigned int Z>
struct vector<X, Y, Z>
{
  using vec_type = vector3;
  static constexpr vector3 value = vector3(X, Y, Z);
};

// a simple wrapper
template<typename V>
struct some_wrapper
{
  static constexpr typename V::vec_type value = V::value;
};

// a dummy function that print something to stdout.
void do_something(int32_t id, const vector3 &value)
{
  std::cout << id << " " << value.z << std::endl;
}
void do_something(int32_t id, const vector2 &value)
{
  std::cout << id << " " << value.y << std::endl;
}
void do_something(int32_t id, int value)
{
  std::cout << id << " " << value << std::endl;
}

// the class used to create the error
template< typename... Args>
class exemple
{
  private:
    // an initialisation that recurse over the Args... template arguments
    template<typename Current, typename... Others>
    void __rec_init() const
    {
      do_something(0, Current::value);
      __rec_init<Others...>();
    }

    // end of recursion
    template<size_t = 0>
    void __rec_init() const {}

    // launch the recursion
    void tpl_init() const
    {
      __rec_init<Args...>();
    }

  public:
    exemple()
    {
      tpl_init();
    }
};

int main()
{
  // and here, we get a linker error.
  exemple<some_wrapper<vector<4, 4, 5>>, some_wrapper<vector<4, 1>>, some_wrapper<vector<9>>>();
}

EDIT: just to mention gcc and clang versions: gcc 4.7.3/4.8.2 and clang 3.2/3.3

Answer 1

The specializations of vector class template for 2 and 3 template arguments have got static constexpr data member value of literal type (vector2 and vector3, respectively) that don't have namespace scope definitions.

You'll need them because you odr-use value when it binds to the reference parameter when passed to do_something function.

§9.4.2/3 [class.static.mfct]

If a non-volatile const static data member is of integral or enumeration type, its declaration in the class definition can specify a brace-or-equal-initializer in which every initializer-clause that is an assignment- expression is a constant expression (5.19). A static data member of literal type can be declared in the class definition with the constexpr specifier; if so, its declaration shall specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression. [Note: In both these cases, the member may appear in constant expressions. —end note ] The member shall still be defined in a namespace scope if it is odr-used (3.2) in the program and the namespace scope definition shall not contain an initializer.

EDIT: Correcting myself, it's actualy some_wrapper<T>::value that needs this definition (for the reason mentioned above, nonetheless). So what you need is this in namespace scope after the definition of some_wrapper:

template<typename V>
constexpr typename V::vec_type some_wrapper<V>::value;

After that, your code compiles and runs.