c++ - Namespaces confusion -

i new namespaces , trying c++ primer

#include<iostream> namespace jill  {  double bucket;  double fetch;  struct hill{ }; }  double fetch;  int main() {  using namespace jill;  hill thrill;  double water = bucket;   //double fetch; //<<<<<<<<<<<<//  std::cin>> fetch;  std::cin>> ::fetch;  std::cin>> jill::fetch;  std::cout<<"fetch "<<fetch;  std::cout<<"::fetch "<< ::fetch;  std::cout<<"jill::fetch "<< jill::fetch; }  int foom() {  jill::hill top;  jill::hill crest; } 

when line marked //<<<<<<<<<<<<// not commented expected results. i.e. local variable hides global , jill::fetch. when comment out, there 2 fetch left . global fetch , jill::fetch. , compiler gives error

namespacetrial1.cpp:17:13: error: reference ‘fetch’ ambiguous namespacetrial1.cpp:9:8: error: candidates are: double fetch namespacetrial1.cpp:5:9: error:                 double jill::fetch namespacetrial1.cpp:20:26: error: reference ‘fetch’ ambiguous namespacetrial1.cpp:9:8: error: candidates are: double fetch namespacetrial1.cpp:5:9: error:                 double jill::fetch 

my question why the compiler confused lead ambiguity? why not assume fetch jill::fetch , since have added using namespace jill @ start of main()

if use declarative using jill::fetch; @ start of main, issue gets solved. because using jill::fetch makes if has been declared @ location. so, there local fetch variable. [am correct?] why using declaration behave if variable declared @ location , using directive doesnt?

the using directive modifies name lookup in way isn't intuitive programmers. standard says in [namespace.udir]p2:

during unqualified name lookup (3.4.1), names appear if declared in nearest enclosing namespace contains both using-directive , nominated namespace.

this wording means names namespace not appear in current scope, but in outer scope. in example using directive in function in global namespace, , jill in global namespace, names jill appears if in global namespace. (as joachim said, names aren't introduced there, don't conflict existing names , you ambiguous lookups when use them.)

this simple case , compiler gives error, good. can more complicated that.

namespace outer {   namespace mid1 { int = 1; }   namespace mid2 {     namespace tricky {       int = 2;       namespace inner {         void f() {           using namespace mid1;           std::cout << i;         }       }     }   } } 

this output 2, not 1, though had using directive right next line referring i. nearest enclosing namespace contains both mid1 , using directive outer, mid1::i acts if declared in outer. if had outer::i shadowed tricky::i, , mid1::i fares no better.

an easy solution ban using directives , use using declarations , namespace aliases. they're far more intuitive.