我正在尝试为使用std::reverse_iterator的双向迭代器的容器编写一个通用的反向包装器。
然而，当编译器查找begin(...)或end(...)时，它似乎会说找不到与reverse_wrapper<CONTAINER>::begin(container)匹配的函数调用，因为候选人需要0个参数，但提供1个参数。
我猜这是因为std::begin(myArray&)和std::end(myArray&)不存在。强制它们进入std命名空间并不起作用（无论如何都是不可取的）。我还尝试从我的std::中移除reverse_wrapper前缀，但这些前缀不起作用，也会破坏正在运行的std容器实现。
这似乎是一个范围解决问题，但我似乎无法解决问题。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&)'
答案 0 :(得分：3)
在回复评论时，让我为缺少自动推断的返回类型编写一些解决方法。
总而言之，问题在于，在使用begin时，当您实际上只使用最适合的end和std::时，您正在使用符合命名空间的调用实施作为备份。
由于我在评论中提出的解决方案不起作用，您可以试试这个。
在包含reverse_wrapper的标头中，您可以添加此方法
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
};
答案 1 :(得分：1)
我可以想到三种可能的解决方案，其中一种已在another answer中描述过;我建议使用那个，考虑到你使用的是VS 2013，但我认为我会提供其他两个作为参考。
如果可能，首先想到的解决方案是将成员函数begin()和end()添加到myArray（以及它作为占位符的所有类型）;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()返回适当的迭代器，从而允许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()。同样，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，使其具有成员函数begin()和end()（如果它还没有它们），这可能非常耗时且可能出错 - 俯卧任务。如果从头开始创建容器，我建议使用此方法，但如果使用将begin()和end()定义为独立函数的预先存在的容器，则不建议这样做。
或者，正如注释中指出的KaBoissonneault，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返回类型扣除。
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));
}
};
对于不支持退货类型扣除的编译器，这是不可能的;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.如果使用之前的版本，则需要使用不同的解决方案。
或者，正如KaBoissonneault's answer所解释的那样，您可以隐藏命名空间中的实现细节，即使编译器不支持返回类型推导，也可以获得使用声明的好处。这是最简单的解决方案，需要的更改量最少。由于不需要任何C ++ 14功能，它也是任何给定编译器最有可能支持的。