templated friend function lookup

Question

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?

@JoachimPileborg of course (the code here is only a simple example, where no such reason may exist), that's the point of the question. — Walter, Apr 01 '13 at 15:02
`at` needs a declaration in the surrounding namespace. Friend functions without one can only be found via ADL, and that doesn't happen yet when you explicitly specify template arguments, IIRC. — Xeo, Apr 01 '13 at 15:03

score 4 · Accepted Answer · answered Apr 01 '13 at 15:04

4

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;
}

answered Apr 01 '13 at 15:04

Daniel Frey

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@DrewDormann: Because of what Xeo wrote in the comments to the question. (btw, +1) – Andy Prowl Apr 01 '13 at 15:06
Does this imply that clang is correct and the other compiler wrong? – Walter Apr 01 '13 at 15:07
2

BTW, you only need a *declaration* to be present in the namespace, the definition can still be inside the class namespace (although it will not make any difference, other than for readers of the code) – David Rodríguez - dribeas Apr 01 '13 at 15:13

templated friend function lookup

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