Accessing protected member functions from test code in C++

Question

I've been racking my brain trying to think of the best way to access a protected member function from some test code in C++, here's my problem:

//in Foo.h 
Class Foo
{
protected:
    void DoSomething(Data data);
}

//in Blah.h
Class Blah
{
public:
    Foo foo;
    Data data; 
};

//in test code...
Blah blah;
blah.foo.DoSomething(blah.data); // Here's my problem!

Some possible solutions so far:

Make the test code class a friend of Foo, but this pollutes Foo with test code

Make DoSomething a public function

I've looked at creating a test wrapper for Foo, as suggested in this post, however this won't work as Blah contains the instance of Foo.

All advice/insight/opinions are most welcome!

Thanks

Answer 1

There is a way which is completely allowed by the Standard.

//in Foo.h 
class Foo
{
protected:
    void DoSomething(Data data);
};

//in Blah.h
class Blah
{
public:
    Foo foo;
    Data data; 
};

//in test code...
struct FooExposer : Foo {
  using Foo::DoSomething;
};

Blah blah;
(blah.foo.*&FooExposer::DoSomething)(blah.data);

Read the Hidden features of C++ entry for an explanation.

---

You may write a macro for your convenience (the parenthesis are there so that you can use this macro also for types that have a comma, like vector<pair<A, B>>):

#define ACCESS(A, M, N) struct N : get_a1<void A>::type { using get_a1<void A>::type::M; }

template<typename T> struct get_a1;
template<typename R, typename A1> struct get_a1<R(A1)> { typedef A1 type; };

The matter now becomes

ACCESS((Foo), DoSomething, GetDoSomething);
Blah blah;
(blah.foo.*&GetDoSomething::DoSomething)(blah.data);