c++ - Why does scope resolution fail in presence of decltype? -

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it understanding decltype used query type of objects/variables , on.

from examples present on wikipedia, such following:

int i; decltype(i) x3; // type int

i assumed this:

class { public:     int a, b; };  template<typename t> struct isclass {     enum { yes = std::is_class<t>::value };     enum { no = !yes }; };  std::vector<a> v; auto = v.begin(); isclass<decltype(it)::value_type>::yes

because after line legal:

isclass<std::vector<a>::iterator::value_type>::yes

alas wouldn't compile, citing following: error c2039: 'value_type' : not member of 'global namespace''`

any ideas why scope resolution made behave way in presence of decltype?

p.s: if makes difference i'm using msvc2012 (without nov ctp)

this known bug in visual c++ compiler. has not yet been fixed of visual c++ 2013 preview. can work around issue using std::common_type:

isclass<std::common_type<decltype(it)>::type::value_type>::yes         ^^^^^^^^^^^^^^^^^            ^^^^^^^

(std::common_type single template argument yields argument type; it's standardized c++11 equivalent of identity template has long been used in metaprogramming.)

you can find public bug report on microsoft connect: cannot use decltype before scope operator. if issue important you, please consider upvoting bug report.


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