c++ - 模板化友元函数查找

Question

下面的简单代码可以正常编译

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;
  }
};

但是如果把 friend 做成模板

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;
  }
};

查找规则改变并且 clang 提示说错误，但是 gcc 和 icpc 没有。谁是对的(C++11)？以及如何为 clang 修复代码？

Answer 1

解决方法是将声明和定义分开:

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;
}