Google Mock for NonVirtual and Private Functions - c++

Question

I'm attempting to write Mocks for Private / Non Virtual / Static functions and come across a way to do the same.
Here is how it looks like..
Lets assume that I have a class A which needs to be mocked and used inside class UsingA. The definition of both classes looks like
class A
{
friend class UsingA;
int privateFn() {}
public:
int nonVirtual() {}
};
// The UsingA class
class UsingA {
A &a1;
public:
UsingA(A & _a1) : a1(_a1) {}
int CallFn() {
return a1.nonVirtual();
}
int CallFn2() {
return a1.privateFn();
}
};
I know that Mocks are meant for generating the behavior of the class and while creating Mocks, we need to derive from the original class.
However, to Mock the behavior I decided not to derive from the original class, instead comment the class A and generate a Mock class with the same Name i.e class A.
Here is how my mock class looks like
// Original class A is commented / header file removed
class A {
public:
MOCK_METHOD0(nonVirtual, int());
MOCK_METHOD0(privateFn, int());
};
And my tests are usual mock tests
TEST(MyMockTest, NonVirtualTest) {
A mstat;
UsingA ua(mstat);
EXPECT_CALL(mstat, nonVirtual())
.Times(1)
.WillOnce(Return(100));
int retVal = ua.CallFn();
EXPECT_EQ(retVal,100);
}
TEST(MyMockTest, PrivateTest) {
A mstat;
UsingA ua(mstat);
EXPECT_CALL(mstat, privateFn())
.Times(1)
.WillOnce(Return(100));
int retVal = ua.CallFn2();
EXPECT_EQ(retVal,100);
}
And everything works fine and I'm able to test UsingA by this mock.
Question is.
This looks easier and serves the purpose, still I haven't seen this kind of examples while browsing for google mock examples. Is there anything that would go wrong if I do this?
Honestly, I didn't find any.
NOTE: Folks, I'm using friend for demonstration only. My actual use case is totally different. Thanks

Answer 1

The wrong is that you are not testing real code, because of that:
comment the class A
generate a Mock class with the same name
These operations alter the code under test.
An example what can go wrong:
Change return type: long nonVirtual in Mock - previously was int
Test that on, let say, nonVirtual() == 0xFF'FFFF'FFFF (which is bigger than INTMAX) some action is being done
Forget to change in real A - so real UsingA have branch that is tested but never reachable in real code
An example code:
class A {
public:
MOCK_METHOD0(nonVirtual, long()); // change
MOCK_METHOD0(privateFn, int());
};
void UsingA::processA()
{
if (a.nonVirtual() > VERY_BIG_NUMBER)
{
throw runtime_error("oops");
}
}
TEST_F(UsingATest, throwOnVeryBigNumber)
{
EXPECT_CALL(aMock, nonVirtual()).WillOnce(Return(VERY_BIG_NUMBER + 1));
ASSERT_THROW(objectUndertTest.processA());
}
But real A did not change - so we test non reachable code in UsingA class:
class A {
public:
int nonVirtual(); // not changed
...
};
The best solution is (in order):
To test in isolation you have to isolate classes - so to use dependency injection (virtual functions etc, base interfaces, etc...) - this is sometimes called London School of TDD
Test both classes A and UsingA w/o any stubbing - test them together in one testcase - thus you test real code - this is called Detroit Shool of TDD
Separate by template code with good restriction on interface - this approach is most similar to yours:
Regarding 3 - you might use something like this:
template <class T = A>
class UsingA {
T &a1;
public:
UsingA(T & _a1) : a1(_a1) {}
long CallFn() {
using ANonVirtualResult = std::invoke_result_t<&T::nonVirtual>;
static_assert(std::is_same<long, ANonVirtualResult>::value);
return a1.nonVirtual();
}
...
};
And in test:
class UsingATest : public ::testing::Test
{
protected:
StrictMock<AMock> aMock;
using ClassUnderTest = UsingA<AMock>;
ClassUnderTest objectUnderTest{aMock};
};
TEST_F(UsingATest, useNonVirtual)
{
const auto VALUE = 123456;
EXPECT_CALL(aMock, nonVirtual()).WillOnce(Return(VALUE));
ASSERT_EQ(VALUE, objectUnderTest.CallFn());
}
You might note that some assumption about A might be tested during compilation as static_assert or via some SFINAE technics (more complicated).
Actually, there are examples with template code in googlemock as workaround for mocking classes w/o virtual functions.

Answer 2

We use your type of using mocks inside a few of our test projects to check callbacks on a larger class that we pass along using dependency injection. In our case, the methods are declared virtual.
In your case, they are not. Your mock implementation would hide the original implementation - if there was any. So I don't think there's an issue here.