Structure class to be able to mock private unique_ptr

Question

I have a class defined as:

class Foo {
public:
    Foo();
private:
    std::unique_ptr<Bar> _bar;
}

Now I want to unit test class Foo by mocking the class Bar and EXPECT_CALL some functions there.

Bar is currently initialized as like:

Foo:Foo(...) : _bar( new Bar(...) ) {
    ...
}

Can I somehow inject my mocked class without some preprocessor hacks or structure this code differently to be able to do it?

Answer 1

The short answer is no, you can't do it unless you make some minor changes to your class.

EXPECT_CALL requires access to the mock object:

EXPECT_CALL(mock_object, method_name(matchers...))

So how can you call it without any reference to mock_object when it is a private member without any getters and no friends?

You do need to somehow provide external access to _bar so that EXPECT_CALL can use it to check your calls on _bar.

There are several ways to do this, but they all require changing your code:

1. Make _bar public
2. Dependency injection
3. Add a getter of _bar to Foo
4. Make your test a friend of Foo

It's common to do this by dependency injection. For example, you can add a new constructor like this:

class Foo {
 public:
  Foo() : _bar(new Bar()) {}
  Foo(Bar* bar) : _bar(bar) {}

  int DoSomething(int a) { return _bar->f1(a); }

  Bar *getBar() { return _bar.get(); } // Getter was added.

 private:
  std::unique_ptr<Bar> _bar;
};

And here is an example on how you can do this by adding a getter function to Foo:

class Bar {
 public:
  MOCK_METHOD(int, f1, (int), ());
};

class Foo {
 public:
  Foo() : _bar(new Bar()) {}
  int DoSomething(int a) { return _bar->f1(a); }

 // A getter was added (Albeit not ideal since it exposes the raw pointer).  
 Bar *getBar() { return _bar.get(); } 

 private:
  std::unique_ptr<Bar> _bar;
};

TEST(FooTest, TestBar) {
  Foo foo;
  EXPECT_CALL(*(foo.getBar()), f1(2)).WillOnce(Return(4));

  auto result = foo.DoSomething(2);
  EXPECT_EQ(result, 4);
}

Working example here: https://godbolt.org/z/1cdq7Po1T