I am trying to write a generic reverse wrapper for containers that use bidirectional iterators using std::reverse_iterator.
However, it would seem that when the compiler looks for begin(...) or end(...), it'll say it can't find a matching function call to reverse_wrapper<CONTAINER>::begin(container) because the candidate expects 0 arguments, but 1 provided.
I would have guessed this is because std::begin(myArray&) and std::end(myArray&) don't exist. Forcing them into the std namespace didn't work (and is not advisable anyway). I've also tried to remove the std:: prefixes from my reverse_wrapper but that didn't work and would also break the working std container implementation.
This seems to be a scope resolution problem, but I can't seem to get the fix. What am I doing wrong?
Code:
#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
}
Errors:
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&)'
In response to the comments, let me write some workaround for the lack of automatically deduced return type.
In summary, the problem is that you are using a namespace-qualified call to begin and end when you actually simply one which ever is the best fit, while using the std:: implementation as a backup.
Since the solutions I've proposed in the comments don't work, you could try this.
In the header containing reverse_wrapper, you can add this method
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
};