How to inject mock object? - c++

Question

I am trying to do a unit test to test the DoLogin method:
CLoginActivity::CLoginActivity()
{
m_pTask = new Task();
}
void CLoginActivity::DoLogin()
{
m_pTask.execute();
}
where the Task is another class that I need to mock.
class MockTask : public Task
{
public:
MOCK_METHOD0(Execute, void());
};
To inject the MockTask task object, I must change my construction:
CLoginActivity::CLoginActivity(Task& task)
{
m_pTask = task;
}
or write a set function:
CLoginActivity::SetTask(Task& task)
{
m_pTask = task;
}
Is there other method to inject instead those two methods? And I use gmock in my unit test project.

Answer 1

Injecting via constructor is the best - keep this design.
But there are few others ways for people that like to complicate simple things.
1) Make your CLoginActivity class template:
template <class TaskImpl>
class CLoginActivityTemplate
{
CLoginActivityTemplate() { m_pTask = new TaskImpl(); }
};
using CLoginActivity = CLoginActivityTemplate<Task>;
In your tests, test this instantion:
using CLoginActivityTestable = CLoginActivityTemplate<TaskMock>;
However it is not always so easy - because it is often hard to get access to this mock to set expentantions on it. But you might define TestMockWrapper class
to ensure access to mock task is easy:
class TestMockWrapper : public TestMock
{
public:
static TestMock* lastCreated;
TestMockWrapper() { lastCreated = this; }
};
using CLoginActivityTestable = CLoginActivityTemplate<TaskMockWrapper>;
2) Inject factory object to your constructor:
CLoginActivity::CLoginActivity(ITaskFactory& taskFactory)
{
m_pTask = taskFactory.create();
}
You need to mock this factory class to ensure mock-factory creates mock-object. Maybe it does not look promissing - but this is just intro to next point.
3) Have special factory function implemented in other file:
CLoginActivity.cpp
#include "TaskCreate.hpp"
CLoginActivity::CLoginActivity()
{
m_pTask = taskCreate();
}
TaskCreate.cpp
// your "real" function here
ITask* createTask() { return new Task(); }
Having such design- create UT test for your CLoginActivity with using only selected files from your projects - simple speaking - replace TaskCreate.cpp with TaskcreateStub.cpp in your UT project:
CloginActivity.cpp
TaskMock.cpp (if exists)
TaskCreateStub.cpp
TaskCreateStub.cpp should return task-mock - not real task. You need also access to this returned mock-object - so you can set expectations on it.
TaskCreateStub.cpp
// your "real" function here
static TaskMock* taskMockForCreateStub = nullptr;
ITask* createTask() { return taskMockForCreateStub ; }
void setTaskMockForCreateTaskStub(TaskMock* taskMock) { taskMockForCreateStub = taskMock; }
I strongly discourage you from using such linker-level mocking. Use it only for testing legacy code where you cannot redesign it and this might be the only way for using mock...