Unresolved external symbol when linking to static lib with namespace

Question

I ran into a behavior today that I don't completely understand. I jump right into a minimal code example and will explain along the way.

I have 2 Projects: A static c++ library and a console application.

Static Lib Project:

Library.h

#pragma once

namespace foo
{
    int testFunc();

    class StaticLibClass
    {
    public:
        static int testMemberFunc();
    };

}

Library.cpp

#include "Library.h"

using namespace foo;

// just some functions that don't do much
int testFunc()
{
    return 10;
}

int StaticLibClass::testMemberFunc()
{
    return 11;
}

Console Application Project:

main.cpp

#include "library.h"

using namespace foo;

void main()
{
    // calling this function reslts in LNK2019: unresolved external symbol...
    testFunc();

    // this function works just fine
    StaticLibClass::testMemberFunc();
}

As you can see the static member function of a class works just fine. The single testFunc however results in a linker error. Why is this?

The solution to the problem is to not use "using" in the Library.cpp file but also wrap it in the namespace like so:

Changes that fix the problem:

Library.cpp

#include "Library.h"

namespace foo
{

    // just some functions that don't do much
    int testFunc()
    {
        return 10;
    }

    int StaticLibClass::testMemberFunc()
    {
        return 11;
    }
}

Answer 1

You either need to wrap the body of the implementation functions/methods in a namespace statement that matches the original header, or you can use fully qualified names which is probably better C++ style:

#include "Library.h"

// just some functions that don't do much
int foo::testFunc()
{
    return 10;
}

int foo::StaticLibClass::testMemberFunc()
{
    return 11;
}

You don't need a using namespace foo; in this version. You are already in the namespace 'foo' when implementing the body's of those two methods, but it can be convenient depending on other types in that namespace.