CODEWITHSUNDEEP
c++using-directivesname-lookup
choxsword
choxsword

Reputation: 3359

Why does the name lookup does not stop when it finds the entity implicitly declared by using directive?

Here is the code example:

#include<iostream>
using namespace std;
namespace B
{
  int ohoh=2;
}

namespace A
{
  int ohoh=666;
  namespace C 
  {
      //using B::ohoh;(as if declared by using directive) //why does the lookup not stops here?
      int foo()
      {
        using namespace B;
        cout<<ohoh<<endl;
      }
  }
}

int main()
{
    A::C::foo();
}

The output is 666 but not 2. Why?

Quoted from cppref

For an unqualified name, that is a name that does not appear to the right of a scope resolution operator ::, name lookup examines the scopes as described below, until it finds at least one declaration of any kind, at which time the lookup stops and no further scopes are examined. (Note: lookup from some contexts skips some declarations, for example, lookup of the name used to the left of :: ignores function, variable, and enumerator declarations, lookup of a name used as a base class specifier ignores all non-type declarations)

For the purpose of unqualified name lookup, all declarations from a namespace nominated by a using directive appear as if declared in the nearest enclosing namespace which contains, directly or indirectly, both the using-directive and the nominated namespace.

From the quoted paragraph above ,the name lookup should stop at nearest namespace C ,Where I've commented in the code.Why does it does not stop and find A::ohoh ?

By the way,I think I should use the using directive as little as possible.

Upvotes: 0

Views: 76

Answers (1)

Miles Budnek
Miles Budnek

Reputation: 30494

For the purpose of unqualified name lookup, all declarations from a namespace nominated by a using directive as if declared in the nearest enclosing namespace which contains [...] both the using-directive and the nominated namespace.

In this case, the nearest namespace that contains both B and the using-directive is the global namespace. Therefore, all names from B appear inside A::C::foo as if they were declared in the global namespace. When searching for the name ohoh, A is searched before the global namespace, so A::ohoh is the first declaration found and name lookup stops there.

Upvotes: 6

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