GMOCK base class methods in derived class

Question

I have a class that is inherited from another class as mentioned below:

class A
{
public:
    virtual void Show(){}
};

class B : public A
public:
    void BMethod1(){Show()}
};

Now I am writing test cases for class B - so I have mocked class A :

class MockA : public A
{
    MOCK_METHOD0(Show, void());
};

Below is my Google Test framework test case:

TEST(BTEST , ShowMethod)
{
    B bobj;
    MockA aobj;
    EXPECT_CALL(aobj , Show());
    bobj.BMethod1(); // updated as from bobj.METHOD0()
}

But the test cases is calling the actual A::Show() implementation - how can call the Mocked version of MockA::Show() in such a case?

====================UPDATE=================================

Well the below implementation did the trick for me:

class A
{
public:
    virtual void Show(){}
};

class B : public A
public:
    void BMethod1(){Show()}
};

class BMock : public B
{
public:
   MOCK_METHOD0(Show, void());
};

Now use BMock object to test you class B required methods

I can if it fixes the issue and would also like to verify if some other mechanism exists — Programmer, Dec 02 '17 at 06:23

user0042 · Answer 1 · 2017-12-02T08:48:43.550

First off:

    bobj.METHOD0();

is totally wrong. METHOD0 is no valid symbol to access a mocked function.

What you want to test is that B::BMethod1() calls A::Show(), right?
So change that line to

    bobj.BMethod1();

You can change class B to take it's base class implementation as a template parameter:

class A
{
public:
    virtual void Show(){}
 // ^^^^^^^ See note 1
};

template<class Base>
class B : public Base
public:
    void BMethod1(){Show()}
};

Then you use

B<A> bobj;
bobj.BMethod1();

in your production code, and

B<MockA> bobj;
EXPECT_CALL(bobj , Show());
bobj.BMethod1(); // <<< trigger the expectation

in your unit test code.

As a side node:

It's a well known pattern to inject base classes through template type parameters called Mixin.
Though Mixins are mainly meant to provide piecewise implementations of interfaces, which are mainly meant to compose public interface implementations.

However, hitting such case when trying to unit test a class regarding those manners, would automatically lead me to the question, if I have a design flaw there:

Is B really an A, or is it rather meant to use an instance of A as an owned or referenced member variable?

If I'm coming up with the need to test such in a unit test, I'd consider refactoring class B to something like

class B {
    A& a_;
public:
    B(A& a) a_(a) {}
    void BMethod1() { a_.Show(); }
};

And adapt the test scenarios accordingly:

TEST(BTEST , ShowMethod)
{
    MockA aobj;
    B bobj(aobj);
    EXPECT_CALL(aobj , Show());
    bobj.BMethod1();
}

As for your comment

The intention is that the test case can return mocked values - EXPECT_CALL(mMockA, show()) .WillOnce(Return(false));

This should work with the example above:

EXPECT_CALL(bobj, Show()) .WillOnce(Return(false));

But requires that Show() is declared like

virtual bool Show();

in class A and like

MOCK_METHOD0(Show, bool());

in the MockA class.

¹⁾_{The virtual declaration is necessary in order to allow MockA to override the original implementation.}

A's methods has virtual declaration. B class is created from A. Thanks but how now I can write in my test case that an Show call of MocakA is expected - like EXPECT_CALL(mMockA, show()) where B is created like B - where MockA is a class and mMockA is an object of type MockA. I mean how can we write EXPECT_CALL for MockA's method in my test cases. The intention is that the test case can return mocked values - EXPECT_CALL(mMockA, show()) .WillOnce(Return(false)); — Programmer, Dec 02 '17 at 07:33
Thanks - but I know have a separate issue which occurs during compilation - which I believe is due to templating class A. In actual classA is defined in A.hpp and its implementation in A.cpp and same for B - B.hpp and B.cpp - now if I refer in B.cpp as like B even though I have #include "A,hpp" in it I get undefined reference to A's constructor and methods during compilation - though I am not sure but my analysis leads to that currently - is that we need templated definitions and declarations in same file — Programmer, Dec 02 '17 at 08:30
@Prakash Sure making `B` a template, [requires to move all the implementation to appear in the header](https://stackoverflow.com/questions/495021/why-can-templates-only-be-implemented-in-the-header-file). I seriously recommend to consider refactoring class `B` to use a reference to `A` provided at construction, instead of inheritance design. — user0042, Dec 02 '17 at 08:34
@Prakash I have updated my answer again. It smells more and more that you have detected a design flaw, when trying to unit test your code. — user0042, Dec 02 '17 at 08:41
I found an alternative to the solution as mentioned below - update the question — Programmer, Dec 02 '17 at 12:45

score 0 · Answer 2 · answered Dec 02 '17 at 06:34

0

You have to make Show() virtual to get MockA::Show() instead of A::Show().

answered Dec 02 '17 at 06:34

Nayfe

2,130
13
18

I tried that but still the same issue - can you please provide the method implementation in details – Programmer Dec 02 '17 at 06:36
Be carefull to put `public` keyword in MockA class too – Nayfe Dec 02 '17 at 06:46
something like this ? `class A { public: virtual void Show(){} }; class MockA : public A { public: MOCK_METHOD0(Show, void()); };` – Nayfe Dec 02 '17 at 07:02
@Nayfe Your answer doesn't really solves the OP's problem, but states a necessary fact to enable `MockA` to override that function. – user0042 Dec 02 '17 at 07:04

score 0 · Answer 3 · answered Dec 02 '17 at 12:44

Well the below implementation did the trick for me:

class A
{
public:
    virtual void Show(){}
};

class B : public A
public:
    void BMethod1(){Show()}
};

class BMock : public B
{
public:
   MOCK_METHOD0(Show, void());
};

Now use BMock object for your test cases

score 0 · Answer 4 · answered Dec 04 '17 at 07:04

You can use Isolator++ to create the test without the setup:

class A
{
public:
    void Show() {}
};

class B : public A
{
public:
    void BMethod1() { Show(); }
};

TEST(BTEST, ShowMethod)
{
    B bobj;
    auto aobj = FAKE<A>();

    bobj.BMethod1();

    ASSERT_WAS_CALLED(aobj->Show());
}

GMOCK base class methods in derived class

4 Answers4