c++ - SGI STL 中的绑定(bind)好友模板

Question

我知道SGI STL和我一样老，但我还是想弄明白。

在stl_stack.h ，有一些这样的代码:

template <class T, class Sequence = Deque<T> >
class Stack {
    friend bool operator== __STL_NULL_TMPL_ARGS (const Stack&, const Stack&);
    friend bool operator< __STL_NULL_TMPL_ARGS (const Stack&, const Stack&);
protected:
    Sequence c;
};

template <class T, class Sequence>
bool operator== (const Stack<T, Sequence>& x, const Stack<T, Sequence>& y){
    return x.c == y.c;
}

template <class T, class Sequence>
bool operator< (const Stack<T, Sequence>& x, const Stack<T, Sequence>& y){
    return x.c < y.c;
}

在stl_config.h , __STL_NULL_TMPL_ARGS 定义如下:

# ifdef __STL_CLASS_PARTIAL_SPECIALIZATION
#   define __STL_TEMPLATE_NULL template<>
# else
#   define __STL_TEMPLATE_NULL
# endif

但是当我尝试用 G++ 4.9.2 编译它时，编译器是这样说的:

In file included from stack.cpp:1:0:
stack.h:13:22: error: declaration of ‘operator==’ as non-function
  friend bool operator== __STL_NULL_TMPL_ARGS (const Stack&, const Stack&);
                      ^
stack.h:13:22: error: expected ‘;’ at end of member declaration
In file included from iterator.h:3:0,
                 from deque.h:6,
                 from stack.h:5,
                 from stack.cpp:1:
stl_config.h:111:31: error: expected unqualified-id before ‘<’ token
 # define __STL_NULL_TMPL_ARGS <>
                               ^
stack.h:13:25: note: in expansion of macro ‘__STL_NULL_TMPL_ARGS’
  friend bool operator== __STL_NULL_TMPL_ARGS (const Stack&, const Stack&);

我不知道为什么完全相同的代码不能在我的电脑上编译，这段代码现在是非法代码还是什么？

非常感谢!!!

Answer 1

stl_stack.h有一个错误，当代编译器一定已经注意到了。提operator== <>是违法的之前operator==已声明为模板。

要解决此问题，请声明 operator==在 stack 的定义之前.但是如果你在那个时候声明它，你也可以定义它。并且声明将需要前向声明 stack :

template <class T, class Sequence>
class stack; // Forward declare for sake of operator== declaration.

template <class T, class Sequence>
bool operator==(const stack<T, Sequence>& x, const stack<T, Sequence>& y) {
  return x.c == y.c;
}

#ifndef __STL_LIMITED_DEFAULT_TEMPLATES
template <class T, class Sequence = deque<T> >
#else
…

(当然，将 operator== 定义添加到顶部后，您将从底部删除它。)