How do I properly use "using namespace foo?"

Question

I am slightly confused by the using namespace x in c++. Why would it be incorrect in this context? Does "using namespace" only applicable to the other files we are #including?

#include <iostream>
using namespace A;


namespace A {
    void print() {

std::cout << "From namespace A" << std::endl;
    }
}

namespace B {
    void printB() {
        std::cout << "From namespace B" << std::endl;
    }
}


int main() {
    print();
    printB(); 
}

related/dupe: https://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice — NathanOliver, Oct 13 '17 at 20:01

score 2 · Accepted Answer · answered Oct 13 '17 at 20:07

As the error messages tell you here these functions aren't declared within your current scope.
everything you call with an unspecified namespace is considered to be found in the global namespace as ::print, ::printB.

You need to use the namespace scope operator (::) like follows:

A::print();
B::printB();

or a using statement:

using A::print;
using B::printB;

score 1 · Answer 2 · answered Oct 13 '17 at 20:03

1

Using namespaces would allow you to have both the functions called print. You would use them as A::print() and B::print() rather than having to rename one of them printB()

answered Oct 13 '17 at 20:03

Martin Beckett

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How do I properly use "using namespace foo?"

2 Answers2