FRIEND_TEST in Google Test - possible circular dependency?

Question

I am trying to figure out how FRIEND_TEST works in Google Tests. https://github.com/google/googletest/blob/master/googletest/docs/advanced.md#testing-private-code

I am looking at the following item, trying to implement it in my code:

// foo.h
#include "gtest/gtest_prod.h"

// Defines FRIEND_TEST.
class Foo {
  ...
 private:
  FRIEND_TEST(FooTest, BarReturnsZeroOnNull);
  int Bar(void* x);
};

// foo_test.cc
...
TEST(FooTest, BarReturnsZeroOnNull) {
  Foo foo;
  EXPECT_EQ(0, foo.Bar(NULL));
  // Uses Foo's private member Bar().
}

In the code above, the piece that I can't see, is that foo_test.cc must include foo.h, in order to have access to Foo and Bar(). [Perhaps it works differently for Google ? in my code, I must include it]

That will result in circular dependency...

Am I missing something ?

Edit: code sample: (re-edited after fixing - solution being changing test file from *.h to *.cpp):

Project ppppp - file myfile.h:

class INeedToBeTested
{
public:
  extern friend class blah_WantToTestThis_Test;
  INeedToBeTested();
  INeedToBeTested(INeedToBeTested* item);
  INeedToBeTested(OtherClass* thing, const char* filename);
  ~INeedToBeTested();
  bool Testable();
  std::string MoreTestable();

private:
  int WantToTestThis();
};

Project ppppp_gtest, file myFile_gtest.cpp:

#pragma once
#include "gtest/gtest.h"
#include "myfile.h" //property sheet adds include locations
#include "otherclass.h"

  class blah: public ::testing::Test{
  // declarations, SetUp, TearDown to initialize otherclass thing, std::string filename
  }
  TEST_F(blah, WantToTestThis)
      {
        INeedToBeTested item(thing, filename.c_str());
        item.WantToTestThis();   // inaccessible when this content is in header file
      }

During my efforts to get this to work, I also experimented with creating a wrapper class (this also works only if in a cpp, not in header); while it requires changing private to protected, it doesn't require additional declarations inside tested code which each new test:

// option: create wrapper (change private to protected first) 
  class INeedToBeTestedWrapper:public INeedToBeTested 
      {
      public:
         INeedToBeTestedWrapper(OtherClass* thing, std::string filename):
            INeedToBeTested(OtherClass* thing, filename.c_str());
      public:
         using INeedToBeTested::WantToTestThis;
      };

      TEST_F(blah, WantToTestThis)
      {
        INeedToBeTestedWrapper item(thing, filename);
        item.WantToTestThis();   
      }

Answer 1

There shouldn't be a problem here.

FRIEND_TEST in this case simply defines

friend class FooTest_BarReturnsZeroOnNull_Test;

which is the class ultimately defined by using the TEST macro. There's no need to link gtest or any of your test code to the foo library/exe. You only need to #include "gtest/gtest_prod.h" as you have done.

In foo_test.cc, you need to #include "foo.h" since it's using an actual instance of a Foo object. You also need to link your foo library to the test executable, or if foo isn't a library, you need to compile in the foo sources.

So in summary, foo doesn't need any test code with the exception of the tiny gtest_prod.h header, but the test needs linked to the foo code.