Question

我为多个模板化类重载operator<<，例如map，pair（见下文），list。它们工作得很好，而且用途广泛。每当我需要更具体的内容时，我都会定义其他优先级operator<<。

但我不能为迭代器做同样的事情。我得到编译错误。我想处理operator<<的事实是不可靠的，但重点在于使用迭代器。

我做错了什么，或者我想做什么是不可能的？

我现在的代码（在Brian的回答之后）如下所示。请注意，functiov定义中的每一行都会简要记录错误，并在代码下方详细说明。

#include <iostream>
#include <map>
#include <string>
using namespace std;

typedef pair<string, char> pair_t;
typedef map<pair_t::first_type, pair_t::second_type> map_t;
typedef pair<map_t::iterator, bool> retval_insert_t;

// Templated form of overloading << for a pair
template<typename first_class, class second_class>
ostream& operator<<(ostream& os, const pair<first_class, second_class>& p);
// Templated form of overloading << for an iterator ... does not work
template <class T, class = typename iterator_traits<T>::value_type>
ostream& operator<<(ostream& os, T iterator);

int main(void) {
    pair_t p2;
    p2 = make_pair(string("Fer"), 'C');

    map_t grade_list;
    retval_insert_t retval = grade_list.insert(p2);
    cout << retval.first << endl;  // Does not work with a templated <<

    return 0;
}

//**********************   Overloaded << functions   ***************************************

// ***** pair
// Templated form of overloading << for a pair
template<class first_class, class second_class>
ostream& operator<<(ostream& os, const pair<first_class, second_class>& p)
{
    os << "(" << p.first << ", " << p.second << ")";  // Compilation error: Ambiguous overload for the first <<
    return os;
}

template <class T, class = typename iterator_traits<T>::value_type>
ostream& operator<<(ostream& os, T iterator)
{
    os << iterator;  // No compilation error, but segmentation fault
    os << *iterator;  // Compilation error if uncommenting: Cannot bind
    //cout << "Testing" << endl;  // Compilation error if uncommenting: Ambiguous overload for the first <<
    cout << 1 << endl;
    return os;
}

*Cannot bind*错误：

g++ -Wall -Wunused -Wuninitialized -Wextra -std=gnu++11 -g -g3 -std=gnu++11 -c -o test_iters.o test_iters.cpp
test_iters.cpp: In instantiation of ‘std::ostream& operator<<(std::ostream&, T) [with T = std::_Rb_tree_iterator<std::pair<const std::basic_string<char>, char> >; <template-parameter-1-2> = std::pair<const std::basic_string<char>, char>; std::ostream = std::basic_ostream<char>]’:
test_iters.cpp:20:17:   required from here
test_iters.cpp:31:8: error: cannot bind ‘std::ostream {aka std::basic_ostream<char>}’ lvalue to ‘std::basic_ostream<char>&&’
     os << *iterator;  // Compilation error if uncommenting: Cannot bind
        ^
In file included from /usr/include/c++/4.8/iostream:39:0,
                 from test_iters.cpp:1:
/usr/include/c++/4.8/ostream:602:5: error:   initializing argument 1 of ‘std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&&, const _Tp&) [with _CharT = char; _Traits = std::char_traits<char>; _Tp = std::pair<const std::basic_string<char>, char>]’
     operator<<(basic_ostream<_CharT, _Traits>&& __os, const _Tp& __x)
     ^
make: *** [test_iters.o] Error 1

*Ambiguous override*错误：

g++ -Wall -Wunused -Wuninitialized -Wextra -std=gnu++11 -g -g3 -std=gnu++11 -c -o test_iters.o test_iters.cpp  
test_iters.cpp: In function ‘std::ostream& operator<<(std::ostream&, T)’:
test_iters.cpp:32:7: error: ambiguous overload for ‘operator<<’ (operand types are ‘std::ostream {aka std::basic_ostream<char>}’ and ‘const char [8]’)
  cout << "Testing" << endl;  // Compilation error if uncommenting: Ambiguous overload for the first <<
       ^
test_iters.cpp:32:7: note: candidates are:
In file included from /usr/include/c++/4.8/iostream:39:0,
                 from test_iters.cpp:1:
/usr/include/c++/4.8/ostream:174:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(bool) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
       operator<<(bool __n)
       ^
/usr/include/c++/4.8/ostream:245:7: note: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(const void*) [with _CharT = char; _Traits = std::char_traits<char>; std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
       operator<<(const void* __p)
       ^
test_iters.cpp:28:10: note: std::ostream& operator<<(std::ostream&, T) [with T = const char*; <template-parameter-1-2> = char; std::ostream = std::basic_ostream<char>]
 ostream& operator<<(ostream& os, T iterator)
          ^
...

旧代码（仅用于文档目的）：

#include <iostream>
#include <map>
#include <list>
#include <string>
using namespace std;

typedef pair<string, char> pair_t;
typedef map<pair_t::first_type, pair_t::second_type> map_t;
typedef pair<map_t::iterator, bool> retval_insert_t;

// ***** pair
// Templated form of overloading << for a pair
template<typename first_class, class second_class>
ostream& operator<<(ostream& os, const pair<first_class, second_class>& p);
// ***** iterator
// Templated form of overloading << for a const_iterator
template<class T>
ostream& operator<<(ostream& os, typename T::const_iterator it);
// Templated form of overloading << for an iterator ... does not work
template<class T>
ostream& operator<<(ostream& os, typename T::iterator it);
// Explicit form of overloading << for an iterator
ostream& operator<<(ostream& os, map_t::iterator it);  // Compile error if commenting this line
// ***** return value of insert (a map)
// Explicit form of overloading << for the return value of insert
ostream& operator<<(ostream& os, const retval_insert_t& retval);


int main(void) {
    pair_t p2;
    p2 = make_pair(string("Fer"), 'C');

    map_t grade_list;
    retval_insert_t retval = grade_list.insert(p2);
    cout << retval.first << endl;  // Does not work with a templated operator<<

    return 0;
}

//**********************   Overloaded << functions   ***************************************
//************************  Explicit and templated  ****************************************

// ***** pair
// Templated form of overloading << for a pair
template<class first_class, class second_class>
ostream& operator<<(ostream& os, const pair<first_class, second_class>& p)
{
    os << "(" << p.first << ", " << p.second << ")";
    return os;
}

// ***** iterator
// Templated form of overloading << for a const_iterator
template<class T>
ostream& operator<<(ostream& os, typename T::const_iterator it)
{
    os << *it;
    return os;
}
// Templated form of overloading << for an iterator ... does not work
template<class T>
ostream& operator<<(ostream& os, typename T::iterator it)
{
    os << *it;
    return os;
}
// Explicit form of overloading << for an iterator
ostream& operator<<(ostream& os, map_t::iterator it)
{
    os << *it;
    return os;
}

// ***** return value of insert (a map)
// Explicit form of overloading << for the return value of insert
ostream& operator<<(ostream& os, const retval_insert_t& retval)
{
    os << "retval : <" << retval.first << ", " << retval.second << ">";
    return os;
}

Answer 1

T中的typename T::iterator是一个非推导的上下文，因此当您尝试调用T时，编译器无法确定operator<<应该是什么迭代器类型。

编写适用于迭代器的重载的一种方法是使用SFINAE来检测给定类型是否为迭代器：

template <class T, class = typename std::iterator_traits<T>::value_type>
std::ostream& operator<<(std::ostream& os, T iterator);
//                                         ^ This T can be deduced

不能重载运算符＆lt;＆lt;对于具有模板化函数的迭代器

1 个答案: