I just wonder why my code cannot be compiled. Is the below okay? I'm trying to declare a simple class with Category1 and Category2 typedef-s.
Category1 typedef compiles fine but Category2 one does not.
It seems that Category2 typedef cannot be compiled because class iterator_traits<> is instantiated despite the surrounding class X is not instantiated... Seems very confusing for me.
#include <iterator>
template <class GetterFunType>
struct X {
GetterFunType containerGetterFun;
// works
typedef typename std::iterator_traits<typename GetterFunType::iterator>::iterator_category Category1;
// compile error - std::iterator_traits<> is instantiated with type 'unknown'
typedef typename std::iterator_traits<
decltype(containerGetterFun().begin())>::iterator_category Category2;
X(GetterFunType _containerGetterFun) : containerGetterFun(_containerGetterFun) { }
};
Note that I do not need to instantiate class X to get the following errors (the above is the complete compilation unit).
In Visual Studio 2012 I'm getting this:
1>c:\program files (x86)\microsoft visual studio 11.0\vc\include\xutility(364): error C2146: syntax error : missing ';' before identifier 'iterator_category'
1> c:\data\fsl\apif_src_review\apif_src\systemns.cpp(11) : see reference to class template instantiation 'std::iterator_traits<_Iter>' being compiled
1> with
1> [
1> _Iter=unknown
1> ]
1> c:\data\fsl\apif_src_review\apif_src\systemns.cpp(14) : see reference to class template instantiation 'X<GetterFunType>' being compiled
1>c:\program files (x86)\microsoft visual studio 11.0\vc\include\xutility(364): error C3254: 'std::iterator_traits<_Iter>' : class contains explicit override 'iterator_category' but does not derive from an interface that contains the function declaration
1> with
1> [
1> _Iter=unknown
1> ]
1>c:\program files (x86)\microsoft visual studio 11.0\vc\include\xutility(364): error C2838: 'iterator_category' : illegal qualified name in member declaration
1>c:\program files (x86)\microsoft visual studio 11.0\vc\include\xutility(364): error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
1>c:\program files (x86)\microsoft visual studio 11.0\vc\include\xutility(364): error C2602: 'std::iterator_traits<_Iter>::iterator_category' is not a member of a base class of 'std::iterator_traits<_Iter>'
1> with
1> [
1> _Iter=unknown
1> ]
1> c:\program files (x86)\microsoft visual studio 11.0\vc\include\xutility(364) : see declaration of 'std::iterator_traits<_Iter>::iterator_category'
1> with
1> [
1> _Iter=unknown
1> ]
1>c:\program files (x86)\microsoft visual studio 11.0\vc\include\xutility(364): error C2868: 'std::iterator_traits<_Iter>::iterator_category' : illegal syntax for using-declaration; expected qualified-name
1> with
1> [
1> _Iter=unknown
1> ]
In xutility(364) there is:
template<class _Iter>
struct iterator_traits
{ // get traits from iterator _Iter
typedef typename _Iter::iterator_category iterator_category;
typedef typename _Iter::value_type value_type;
typedef typename _Iter::difference_type difference_type;
typedef difference_type distance_type; // retained
typedef typename _Iter::pointer pointer;
typedef typename _Iter::reference reference;
};
My case is that I want to declare a class that gets a lambda in constructor. The lambda is expected to return a reference to a container. And I need to determine wheter the returned container has a random-access iterator. But I got stuck with this compilation error. Thank you for explanation!
I was able to compile the same code without any errors using gcc 5.3.1, with -std=c++11
Your compiler is a relatively old compiler that does not support the current C++1x standard. Switching to another compiler is the only option I can see here, if you need to use modern C++ features.