templated friend function lookup

问题

The following simple code compiles fine

class A {
  int x[3];
public:
  A() { x[0]=1; x[1]=2; x[2]=3; }
  friend int const&at(A const&a, unsigned i) noexcept
  {
    return a.x[i];
  }
  friend int foo(A const&a, unsigned i) noexcept
  {
    int tmp = at(a,i);
    return tmp*tmp;
  }
};

but if the friends are made templates

class A {
  int x[3];
public:
  A() { x[0]=1; x[1]=2; x[2]=3; }

  template<unsigned I>
  friend int const&at(A const&a) noexcept
  {
    static_assert(I<3,"array boundary exceeded");
    return a.x[I];
  }

  template<unsigned I>
  friend int foo(A const&a) noexcept
  {
    int tmp = at<I>(a);   // <- error: use of undeclared identifier 'at'
    return tmp*tmp;
  }
};

the look-up rules change and clang complains with said error, but gcc and icpc don't. Who is right (C++11)? and how to get the code fixed for clang?

回答1:

The fix is to separate declaration and definition:

class A {
  int x[3];
public:
  A() { x[0]=1; x[1]=2; x[2]=3; }

  template<unsigned I>
  friend int const&at(A const&a) noexcept;

  template<unsigned I>
  friend int foo(A const&a) noexcept;
};

template<unsigned I>
int const&at(A const&a) noexcept
{
  static_assert(I<3,"array boundary exceeded");
  return a.x[I];
}

template<unsigned I>
int foo(A const&a) noexcept
{
  int tmp = at<I>(a);
  return tmp*tmp;
}

来源：https://stackoverflow.com/questions/15745776/templated-friend-function-lookup