嵌套模板类的C++非成员函数

我已经编写了几个包含嵌套迭代器类的类模板,需要对这些类进行相等比较。因为我认为这是相当典型的,所以比较的对象是非会员(和非朋友)。 operator== 功能。在执行此操作时,我的编译器(我使用带有标志的mingw32 gcc 4.4 -O3 -g -Wall )找不到这个函数,我已经没有可能的原因了。

在下面相当大的代码块中,有三个类:基类、包含基对象的组合类和与组合类相同的嵌套类,只是嵌套在外部类中。非成员 运算符== 每个都有功能。这些类采用模板化和非模板化形式(在各自的名称空间中),后者相当于前者专门用于无符号整数。

在 main ,比较每个类的两个相同对象。对于未模板化的情况没有问题,但是对于模板化的情况,编译器找不到 运算符== . 发生什么事?

#include <iostream>

namespace templated {

template<typename T>
class Base {
  T t_;
public:
  explicit Base(const T& t) : t_(t) {}

  bool
  equal(const Base& x) const {
    return x.t_==t_;
  }
};

template<typename T>
bool
operator==(const Base<T> &x, const Base<T> &y) {
  return x.equal(y);
}

template<typename T>
class Composed {
  typedef Base<T> Base_;
  Base_ base_;
public:
  explicit Composed(const T& t) : base_(t) {}
  bool equal(const Composed& x) const {return x.base_==base_;}
};

template<typename T>
bool
operator==(const Composed<T> &x, const Composed<T> &y) {
  return x.equal(y);
}

template<typename T>
class Outer {
public:
  class Nested {
    typedef Base<T> Base_;
    Base_ base_;
  public:
    explicit Nested(const T& t) : base_(t) {}
    bool equal(const Nested& x) const {return x.base_==base_;}
  };
};

template<typename T>
bool
operator==(const typename Outer<T>::Nested &x,
    const typename Outer<T>::Nested &y) {
  return x.equal(y);
}

} // namespace templated

namespace untemplated {

class Base {
  unsigned int t_;
public:
  explicit Base(const unsigned int& t) : t_(t) {}

  bool
  equal(const Base& x) const {
    return x.t_==t_;
  }
};

bool
operator==(const Base &x, const Base &y) {
  return x.equal(y);
}

class Composed {
  typedef Base Base_;
  Base_ base_;
public:
  explicit Composed(const unsigned int& t) : base_(t) {}
  bool equal(const Composed& x) const {return x.base_==base_;}
};

bool
operator==(const Composed &x, const Composed &y) {
  return x.equal(y);
}

class Outer {
public:
  class Nested {
    typedef Base Base_;
    Base_ base_;
  public:
    explicit Nested(const unsigned int& t) : base_(t) {}
    bool equal(const Nested& x) const {return x.base_==base_;}
  };
};

bool
operator==(const Outer::Nested &x,
    const Outer::Nested &y) {
  return x.equal(y);
}

} // namespace untemplated

int main() {
  using std::cout;
  unsigned int testVal=3;
  { // No templates first
    typedef untemplated::Base Base_t;
    Base_t a(testVal);
    Base_t b(testVal);

    cout << "a=b=" << testVal << "\n";
    cout << "a==b ? " << (a==b ? "TRUE" : "FALSE") << "\n";

    typedef untemplated::Composed Composed_t;
    Composed_t c(testVal);
    Composed_t d(testVal);

    cout << "c=d=" << testVal << "\n";
    cout << "c==d ? " << (c==d ? "TRUE" : "FALSE") << "\n";

    typedef untemplated::Outer::Nested Nested_t;
    Nested_t e(testVal);
    Nested_t f(testVal);

    cout << "e=f=" << testVal << "\n";
    cout << "e==f ? " << (e==f ? "TRUE" : "FALSE") << "\n";
  }
  { // Now with templates
    typedef templated::Base<unsigned int> Base_t;
    Base_t a(testVal);
    Base_t b(testVal);

    cout << "a=b=" << testVal << "\n";
    cout << "a==b ? " << (a==b ? "TRUE" : "FALSE") << "\n";

    typedef templated::Composed<unsigned int> Composed_t;
    Composed_t c(testVal);
    Composed_t d(testVal);

    cout << "c=d=" << testVal << "\n";
    cout << "d==c ? " << (c==d ? "TRUE" : "FALSE") << "\n";

    typedef templated::Outer<unsigned int>::Nested Nested_t;
    Nested_t e(testVal);
    Nested_t f(testVal);

    cout << "e=f=" << testVal << "\n";
    cout << "e==f ? " << (e==f ? "TRUE" : "FALSE") << "\n";
    // Above line causes compiler error:
    // error: no match for 'operator==' in 'e == f'
  }

  cout << std::endl;
  return 0;
}