Please help me with C++ Namespace

Question

#include <iostream>
#include <cstdlib>

namespace A {
    int a = 10;
    void get_value(){ std::cout << "a = " << a << std::endl; }
}

namespace B {
    int b;
    void get_value(){ std::cout << "b =" << b << std::endl; }
}

void set_B();

int main(){

    using namespace A; 
    get_value(); 
    set_B();

    system("PAUSE");
    return 0;
}

void  set_B(){
    using namespace B;
    b = 15;
    get_value();    // Why call to get_value() is ambiguous, error is not generated here ?
 }

Why call to get_value() inside set_B() function is not ambiguous ( A::get_value() or B::get_value()) as both appear as ::get_value() inside set_B().

@juanchopanza as using namespace B; statement inside set_B() introduces B::get_value() into global namespace and statement using namespace A; inside main() introduces A::get_value() into global namespace. — curious_kid, Aug 15 '14 at 13:00
Please read: http://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice — Matt, Aug 15 '14 at 13:03
@matt thanks for the link . I know using namespace std; is not a good practice. That's why I have used std::cout in the above code. I am beginner in C++ that's why I'm playing with the code. :) — curious_kid, Aug 15 '14 at 13:11
OK where you go wring is "introduces ... *into global namespace*". The `using` only have effect in their enclosing scope. — juanchopanza, Aug 15 '14 at 13:24
@juanchopanza So It means that using namespace A; inside main() has no effect inside the set_B() function. So get_value() inside set_B() only refer to B::set_value(). Hence no error. — curious_kid, Aug 15 '14 at 13:29

Potatoswatter · Accepted Answer · 2014-08-15T22:56:24.747

3

using namespace A; isn't active inside set_B because it only appears inside main. It is limited to the scope in which it appears: the body of main.

edited Aug 15 '14 at 22:56

answered Aug 15 '14 at 12:55

Potatoswatter

134,909
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But moving it to global namespace scope is causing call to get_value() is ambiguous compile time error . – curious_kid Aug 15 '14 at 13:06
@curious_kid Ah, that's right, `using namespace` does not import the namespace into the scope it appears. But its effect is limited to that scope. Updated. – Potatoswatter Aug 15 '14 at 22:57

score 0 · Answer 2 · answered Aug 15 '14 at 13:00

0

because A::get_value and B::get_value arn't normally visible in the global scope. the using namespace statement makes the declarations in that namespace visible inside set_B.

answered Aug 15 '14 at 13:00

programmerjake

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Please help me with C++ Namespace

2 Answers2