CODEWITHSUNDEEP
c++c++11
Ahmed A
Ahmed A

Reputation: 3652

namespace-name not recognized

I have the following two sample c++ files:

File: foo.cc

namespace test_ns {
int x = 100;
}

File: bar.cc

using namespace test_ns;

extern int x;

int main()
{
    x = 200;
    return 0;
}

I must be doing something really silly as I am getting the following compiler error:
bar.cc:2:17: error: 'test_ns' is not a namespace-name bar.cc:2:24: error: expected namespace-name before ';' token

However, my main question (after I resolve the namespace error), with the code as is, should I be able to use "x" in bar.cc, or do I need to use additional qualifiers.

Sorry to bother with such a simple issue, but I am pretty stumped with such a simple program.

Thank you, Ahmed.

Upvotes: 1

Views: 6568

Answers (3)

Angew is no longer proud of SO
Angew is no longer proud of SO

Reputation: 171097

Just like every other name in C++, a namespace name must be declared before it can be accessed. When compiling bar.cc, the compiler doesn't know test_ns is a namespace name - you haven't provided a declaration for it.

Namespaces and using namespace aren't magical tricks that let you get around the necessity to declare anything you want to use. To make this work, create a header file:

foo.hh

namespace test_ns {
  extern int x;
}

Then #include "foo.hh" in bar.cc, and remove extern int x; from bar.cc. That will make these two translation units form a valid program. And after you do that, referring to x in bar.cc will indeed refer to test_ns::x (thanks to the using directive).

As a side note, if you don't remove the extern int x; from bar.cc (after you include the header file), it will declare a new global variable ::x and not refer to test_nest::x. using namespace in a source file allows you to define class members without referring to the class's namespace, but namespace-scope entities (variables and functions) always have to be defined inside the namespace or using a qualified name.

Upvotes: 3

Jay
Jay

Reputation: 59

namespace is defined in a different file. It needs to be defined in a header file which should be included in bar.cc .

Upvotes: 0

Bathsheba
Bathsheba

Reputation: 234635

They are in different compilation units. You need to declare the namespace in a common header; which you can do using:

namespace test_ns {}

Upvotes: 0

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