Question

It works when I do:

namespace UnitTesting {class UnitTest_1;}

namespace NLa
{
    class Search
    {
       friend class UnitTesting::UnitTest_1;
    }
}

But what if class UnitTest_1 is not in a namespace ?

Answer 1

Use the :: prefix to access the global / unnamed namespace

friend class ::UnitTest_1;

Your original sample could just as easily be written as

friend class ::UnitTesting::UnitTest_1;