为什么范围解析不超出 class？

Question

我正在尝试为使用双向迭代器的容器编写一个通用的反向包装器 std::reverse_iterator。

但是，当编译器查找 begin(...) 或 end(...) 时，它会说找不到对 reverse_wrapper<CONTAINER>::begin(container) 的匹配函数调用，因为候选需要 0 个参数，但提供了 1 个。

我猜这是因为 std::begin(myArray&) 和 std::end(myArray&) 不存在。强制他们进入 std 命名空间是行不通的（无论如何都是不可取的）。我还尝试从我的 reverse_wrapper 中删除 std:: 前缀，但这没有用，而且还会破坏正在工作的 std 容器实现。

这似乎是范围解析问题，但我似乎无法解决。我做错了什么？

代码：

#include <iterator>
#include <iostream>
#include <vector>

#define Fn1 0 // std container WORKS
#define Fn2 1 // non-std container DOESN'T WORK

template <typename Container>
struct reverse_wrapper
{
    Container& container;

    auto begin()
        -> std::reverse_iterator< decltype(std::end(container)) >
    {
        return std::reverse_iterator< decltype(std::end(container)) >(std::end(container));
    }

    auto end()
        -> std::reverse_iterator< decltype(std::begin(container)) >
    {
        return std::reverse_iterator< decltype(std::begin(container)) >(std::begin(container));
    }
};

template <typename Container>
auto reverse(Container&& container)
-> reverse_wrapper<Container>
{
    return{ container };
}

struct myArray
{
    int elements[5] = {1,2,3,4,5};
};

int* begin(myArray& array) { return &array.elements[0]; }
int*   end(myArray& array) { return &array.elements[5]; }

#if Fn1
void fn1()
{
    std::vector<int> x = { 1,2,3,4,5 };
    for (auto& ix : reverse(x))
    {
        std::cout << ix << std::endl;
    }
    std::cout << "-----" << std::endl;
    for (auto& ix : x)
    {
        std::cout << ix << std::endl;
    }
}
#endif

#if Fn2
void fn2()
{
    myArray x;
    for (auto& ix : reverse(x))
    {
        std::cout << ix << std::endl;
    }
    std::cout << "-----" << std::endl;
    for (auto& ix : x)
    {
        std::cout << ix << std::endl;
    }
}
#endif

int main()
{
#if Fn1
    fn1();
#endif
#if Fn2
    fn2();
#endif
}

错误：

In instantiation of 'struct reverse_wrapper':
61:30:   required from here
14:56: error: no matching function for call to 'end(myArray&)'
14:56: note: candidates are:
In file included from /usr/include/c++/4.9/string:51:0,
                 from /usr/include/c++/4.9/bits/locale_classes.h:40,
                 from /usr/include/c++/4.9/bits/ios_base.h:41,
                 from /usr/include/c++/4.9/ios:42,
                 from /usr/include/c++/4.9/ostream:38,
                 from /usr/include/c++/4.9/iterator:64,
                 from 1:
/usr/include/c++/4.9/bits/range_access.h:68:5: note: template decltype (__cont.end()) std::end(_Container&)
     end(_Container& __cont) -> decltype(__cont.end())
     ^
/usr/include/c++/4.9/bits/range_access.h:68:5: note:   template argument deduction/substitution failed:
/usr/include/c++/4.9/bits/range_access.h: In substitution of 'template decltype (__cont.end()) std::end(_Container&) [with _Container = myArray]':
14:56:   required from 'struct reverse_wrapper'
61:30:   required from here
/usr/include/c++/4.9/bits/range_access.h:68:5: error: 'struct myArray' has no member named 'end'
 In instantiation of 'struct reverse_wrapper':
61:30:   required from here
/usr/include/c++/4.9/bits/range_access.h:78:5: note: template decltype (__cont.end()) std::end(const _Container&)
     end(const _Container& __cont) -> decltype(__cont.end())
     ^
/usr/include/c++/4.9/bits/range_access.h:78:5: note:   template argument deduction/substitution failed:
/usr/include/c++/4.9/bits/range_access.h: In substitution of 'template decltype (__cont.end()) std::end(const _Container&) [with _Container = myArray]':
14:56:   required from 'struct reverse_wrapper'
61:30:   required from here
/usr/include/c++/4.9/bits/range_access.h:78:5: error: 'const struct myArray' has no member named 'end'
 In instantiation of 'struct reverse_wrapper':
61:30:   required from here
/usr/include/c++/4.9/bits/range_access.h:97:5: note: template _Tp* std::end(_Tp (&)[_Nm])
     end(_Tp (&__arr)[_Nm])
     ^
/usr/include/c++/4.9/bits/range_access.h:97:5: note:   template argument deduction/substitution failed:
14:56: note:   mismatched types '_Tp [_Nm]' and 'myArray'
In file included from /usr/include/c++/4.9/bits/basic_string.h:42:0,
                 from /usr/include/c++/4.9/string:52,
                 from /usr/include/c++/4.9/bits/locale_classes.h:40,
                 from /usr/include/c++/4.9/bits/ios_base.h:41,
                 from /usr/include/c++/4.9/ios:42,
                 from /usr/include/c++/4.9/ostream:38,
                 from /usr/include/c++/4.9/iterator:64,
                 from 1:
/usr/include/c++/4.9/initializer_list:99:5: note: template constexpr const _Tp* std::end(std::initializer_list)
     end(initializer_list __ils) noexcept
     ^
/usr/include/c++/4.9/initializer_list:99:5: note:   template argument deduction/substitution failed:
14:56: note:   'myArray' is not derived from 'std::initializer_list'
20:58: error: no matching function for call to 'begin(myArray&)'
20:58: note: candidates are:
In file included from /usr/include/c++/4.9/string:51:0,
                 from /usr/include/c++/4.9/bits/locale_classes.h:40,
                 from /usr/include/c++/4.9/bits/ios_base.h:41,
                 from /usr/include/c++/4.9/ios:42,
                 from /usr/include/c++/4.9/ostream:38,
                 from /usr/include/c++/4.9/iterator:64,
                 from 1:
/usr/include/c++/4.9/bits/range_access.h:48:5: note: template decltype (__cont.begin()) std::begin(_Container&)
     begin(_Container& __cont) -> decltype(__cont.begin())
     ^
/usr/include/c++/4.9/bits/range_access.h:48:5: note:   template argument deduction/substitution failed:
/usr/include/c++/4.9/bits/range_access.h: In substitution of 'template decltype (__cont.begin()) std::begin(_Container&) [with _Container = myArray]':
20:58:   required from 'struct reverse_wrapper'
61:30:   required from here
/usr/include/c++/4.9/bits/range_access.h:48:5: error: 'struct myArray' has no member named 'begin'
 In instantiation of 'struct reverse_wrapper':
61:30:   required from here
/usr/include/c++/4.9/bits/range_access.h:58:5: note: template decltype (__cont.begin()) std::begin(const _Container&)
     begin(const _Container& __cont) -> decltype(__cont.begin())
     ^
/usr/include/c++/4.9/bits/range_access.h:58:5: note:   template argument deduction/substitution failed:
/usr/include/c++/4.9/bits/range_access.h: In substitution of 'template decltype (__cont.begin()) std::begin(const _Container&) [with _Container = myArray]':
20:58:   required from 'struct reverse_wrapper'
61:30:   required from here
/usr/include/c++/4.9/bits/range_access.h:58:5: error: 'const struct myArray' has no member named 'begin'
 In instantiation of 'struct reverse_wrapper':
61:30:   required from here
/usr/include/c++/4.9/bits/range_access.h:87:5: note: template _Tp* std::begin(_Tp (&)[_Nm])
     begin(_Tp (&__arr)[_Nm])
     ^
/usr/include/c++/4.9/bits/range_access.h:87:5: note:   template argument deduction/substitution failed:
20:58: note:   mismatched types '_Tp [_Nm]' and 'myArray'
In file included from /usr/include/c++/4.9/bits/basic_string.h:42:0,
                 from /usr/include/c++/4.9/string:52,
                 from /usr/include/c++/4.9/bits/locale_classes.h:40,
                 from /usr/include/c++/4.9/bits/ios_base.h:41,
                 from /usr/include/c++/4.9/ios:42,
                 from /usr/include/c++/4.9/ostream:38,
                 from /usr/include/c++/4.9/iterator:64,
                 from 1:
/usr/include/c++/4.9/initializer_list:89:5: note: template constexpr const _Tp* std::begin(std::initializer_list)
     begin(initializer_list __ils) noexcept
     ^
/usr/include/c++/4.9/initializer_list:89:5: note:   template argument deduction/substitution failed:
20:58: note:   'myArray' is not derived from 'std::initializer_list'
 In function 'void fn2()':
61:30: error: invalid initialization of reference of type 'myArray&' from expression of type 'reverse_wrapper'
38:6: note: in passing argument 1 of 'int* begin(myArray&)'
61:30: error: invalid initialization of reference of type 'myArray&' from expression of type 'reverse_wrapper'
39:8: note: in passing argument 1 of 'int* end(myArray&)'

Demo

Answer 1

为了回应评论，让我写一些解决方法来解决缺少自动推导的 return 类型。

总而言之，问题是您使用的是对 begin 和 end 的命名空间限定调用，而实际上您只是使用最合适的调用，而使用 std:: 实施作为备份。

由于我在评论中提出的解决方案不起作用，你可以试试这个。

在包含reverse_wrapper的header中，可以添加这个方法

namespace detail {
    using std::begin;
    using std::end;
    template< typename Container >
    auto impl_begin( Container & c ) -> decltype( begin(c) ) {
         return begin(c);
    }
    // Same for end
}

template< typename Container >
class reverse_wrapper {
    Container& container;
public:
    auto end() -> decltype( detail::impl_begin(container) ) {
        return std::reverse_iterator<decltype( detail::impl_begin(container) )>( detail::impl_begin(container) );
    }
    // ... and the rest
};

Answer 2

我能想到三种可能的解决方案，其中一种已经在中描述过；考虑到您使用的是 VS 2013，我建议您使用那个，但我想我会提供另外两个供参考。

想到的第一个解决方案是，如果可能的话，将成员函数begin()和end()添加到myArray（以及它充当占位符的所有类型） ;最简单的方法是将函数 int* begin(myArray&) 和 int* end(myArray&) 转换为成员函数。这允许 std::begin() 和 std::end() 到 return 适当的迭代器，这反过来又允许 reverse_wrapper::begin() 和 reverse_wrapper::end() 工作。

struct myArray
{
    int elements[5] = {1,2,3,4,5};

    int* begin() { return &elements[0]; }
    int*   end() { return &elements[5]; }
};

// int* begin(myArray& array) { return &array.elements[0]; }
// int*   end(myArray& array) { return &array.elements[5]; }

这是因为for any instance c of container class C, std::begin() will take C& c and return exactly c.begin(). Likewise, std::end() will take C& c and return exactly c.end().

请注意，这将需要修改每个用户定义的容器类型以使其具有成员函数 begin() 和 end()（如果它还没有），这可能很耗时并且可能容易出错的任务。如果从头开始制作容器，我会推荐这种方法，但如果使用将 begin() 和 end() 定义为独立函数的预先存在的容器，我不会推荐这种方法。

或者，正如 KaBoissonneault 在评论中指出的那样，您可以使用 using 声明 using std::begin; 和 using std::end;，前提是您的编译器可以执行 return 类型推导.

template <typename Container>
struct reverse_wrapper
{
    Container& container;

    auto begin()
//        -> std::reverse_iterator< decltype(std::end(container)) >
    {
        using std::end;
        return std::reverse_iterator< decltype(/*std::*/end(container)) >(/*std::*/end(container));
    }

    auto end()
//        -> std::reverse_iterator< decltype(std::begin(container)) >
    {
        using std::begin;
        return std::reverse_iterator< decltype(/*std::*/begin(container)) >(/*std::*/begin(container));
    }
};

这在不支持 return 类型推导的编译器上是不可能的；在最大的 3 个编译器中，它需要 Clang 3.4 或更高版本、GCC 4.9 或更高版本或 Visual Studio 2015。如果使用这些版本之前的版本，则需要使用不同的解决方案。

或者，如所解释的，您可以将实现细节隐藏在命名空间中，即使编译器不支持 return 类型推导，您也可以利用 using 声明的好处.这是最简单的解决方案，需要的更改最少。由于不需要任何 C++14 功能，它也是最有可能被任何给定编译器支持的一种。

为什么范围解析不超出 class？

Why doesn't the scope resolution not go outside of the class?

iterator

c++11

reverse-iterator

visual-studio-2013