了解'using'关键字：C ++

Question

Can someone please explain below output:

#include <iostream>

using namespace std;

namespace A{
    int x=1;
    int z=2;
    }

namespace B{
    int y=3;
    int z=4;
    }

void doSomethingWith(int i) throw()
{
    cout << i ;
    }

void sample() throw()
{
    using namespace A;
    using namespace B;
    doSomethingWith(x);
    doSomethingWith(y);
    doSomethingWith(z);

    }

int main ()
{
sample();
return 0;
}

Output:

$ g++ -Wall TestCPP.cpp -o TestCPP
TestCPP.cpp: In function `void sample()':
TestCPP.cpp:26: error: `z' undeclared (first use this function)
TestCPP.cpp:26: error: (Each undeclared identifier is reported only once for each function it appears in.)

Answer 1

I have another error:

error: reference to 'z' is ambiguous

Which is pretty clear for me: z exists in both namespaces, and compiler don't know, which one should be used. Do you know? Resolve it by specifying namespace, for example:

doSomethingWith(A::z);

Answer 2

using keyword is used to

1. shortcut the names so you do not need to type things like std::cout

2. to typedef with templates(c++11), ie template<typename T> using VT = std::vector<T>;

In your situation, namespace is used to prevent name pollution, which means two functions/variables accidently shared the same name. If you use the two using together, this will led to ambiguous z . My g++ 4.8.1 reported the error:

abc.cpp: In function ‘void sample()’:
abc.cpp:26:21: error: reference to ‘z’ is ambiguous
     doSomethingWith(z);
                     ^
abc.cpp:12:5: note: candidates are: int B::z
 int z=4;
     ^
abc.cpp:7:5: note:                 int A::z
 int z=2;
     ^

which is expected. I am unsure which gnu compiler you are using, but this is an predictable error.

Answer 3

You get a suboptimal message. A better implementation would still flag error, but say 'z is ambiguous' as that is the problem rather than 'undeclared'.

At the point name z hits multiple things: A::z and B::z, and the rule is that the implementation must not just pick one of them. You must use qualification to resolve the issue.