clang 3.3中的模板默认Arg替换失败

Question

This is regarding a test case which involves default argument deduction/substitution. The test case can be summarized as :

template <class T, class = typename T::I>h(T){}

template <class T, class = typename T::I>j(T){}

class A
{
   typedef int I;
   friend void h<A>(A);

};

int main()
{
   A a;
   h(a);
   j(a);
}

gcc-4.8.1 throws error for function j, since it has been not been declared friend nor it is private to class A and hence violates access rule for private member I(which is valid). gcc does not throw error for function h since it has been declared as friend to class A and hence can access private member I.

Clang throws error for both the functions. Error for function j (not declared friend is valid and as expected), but it throws error even for friend function h (error : deduction of default arg failed since I is a private member of class A). This violates the accessibility of the friend function.

I checked the code path. While clang is able to deduce the default argument, it checks for access rules before doing any substitution, and gives error. Can someone please provide guidance as to how should this be fixed?

Answer 1

You forgot the return type for the template functions.

This should solve the problem :

template <class T, class = typename T::I> void h(T){}

template <class T, class = typename T::I> void j(T){}

---

After fixing above errors, I still got errors, because

• you declared the typedef in the private section. You need to make it public
• you declared function friend wrongly. It has two template arguments (A::I will not work, since A is not complete type)

Fully compilable problem is here :

#include <iostream>

// Type your code here, or load an example.
template <class T, class = typename T::I> void h(T t){std::cout<<t.a<<std::endl;}

template <class T, class = typename T::I> void j(T t){std::cout<<t.a<<std::endl;}

class A
{
   friend void h<A,int>(A);
   friend void j<A,int>(A);
public :
   typedef int I;
  private :
  int a;

};

int main()
{
   A a;
   h(a);
   j(a);
}