c++ - Is this a bug in GCC? -


edit: not bug, me not knowing dependent name lookups in templated base classes (which msvc "helpfully" resolves without errors).

i wrote functor implementation while back, , simple "event" wrapper uses it. compiles fine under msvc, gcc gives error member variable in base class, subscribers, not being declared; changing subscribers this->subscribers resolves issue(!). appears happen curiously recurring template pattern, , partial template specialization.

simplified source (sorry mind-bending template usage...):

#include <vector>  template<typename tevent> struct eventbase { protected:         std::vector<int> subscribers; };  template<typename targ1 = void, typename targ2 = void> struct event : public eventbase<event<targ1, targ2> > {         void trigger(targ1 arg1, targ2 arg2) const         {                 // error on next line                 auto = subscribers.cbegin();         } };  template<typename targ1> struct event<targ1, void> : public eventbase<event<targ1> > {         void trigger(targ1 arg1) const         {                 // using `this` fixes error(?!)                 auto = this->subscribers.cbegin();         } };  template<> struct event<void, void> : public eventbase<event<> > {         void trigger() const         {                 // no error here without `this`, reason!                 auto = subscribers.cbegin();         } };  int main() {         return 0; }

am invoking undefined behaviour somewhere? syntax somehow wrong? bug in gcc? perhaps known bug? insight appreciated!

more details: compiled using g++ -std=c++11 main.cpp. i'm using gcc version 4.7.2. exact error message:

main.cpp: in member function ‘void event<targ1, targ2>::trigger(targ1, targ2) const’: main.cpp:17:15: error: ‘subscribers’ not declared in scope

this bug in msvc instead. names dependent base classes have "thisambiguated".

the reason unqualified lookup of dependent names proceeds in 2 phases. during first phase, base class not yet known , compiler cannot resolve name. msvc not implement two-phase name lookup , delays lookup until second phase.

the full specialization 

template<> struct event<void, void> : public eventbase<event<> > {         void trigger() const         {                 // no error here without `this`, reason!                 auto = subscribers.cbegin();         } };

does not suffer problem, because both class , base regular classes, not class templates, , there no template dependency begin with.

when porting c++ code msvc gcc/clang, dependent name lookup disambiguation , template keyword disambiguation (i.e. calling member function template using ::template, ->template or .template syntax) 2 of subtleties have deal (empty base optimization one). standards compliance rhetoric, never fixed reasons of backwards compatibility.


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