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With using namespace I make the whole contents of that namespace directly visible without using the namespace qualifier. This can cause problems if using namespace occurs in widely used headers - we can unintendedly make two namespaces with identical classes names visible and the compiler will refuse to compile unless the class name is prepended with the namespace qualifier.
Can I undo using namespace so that the compiler forgets that it saw it previously?
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No you can't unuse a namespace. The only thing you can do is putting the using namespace -statement a block to limit it's scope.
Thus, removing using namespace std; changes the meaning of those unqualified names, rather than just making the code fail to compile. These might be names from your code; or perhaps they are C library functions.
You should definitely NOT use using namespace in headers for precisely the reason you say, that it can unexpectedly change the meaning of code in any other files that include that header. There's no way to undo a using namespace which is another reason it's so dangerous.
It doesn't affect the runtime performance at all.
No, but you can tell your coworkers that you should never have a using directive or declaration in a header.
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As others said, you can't and the problem shouldn't be there in the first place.
The next-best thing you can do is bring in your needed symbols so that they are preferred by the name look-up:
namespace A { class C {}; } namespace B { class C {}; } using namespace A; using namespace B; namespace D { using A::C; // fixes ambiguity C c; } In some cases you can also wrap the offending includes with a namespace:
namespace offender { # include "offender.h" } If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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