C++, Googlemock - testing local object

Question

I started to use googletest and googlemock libraries and I have a problem which I cannot solve. I have a code something like this:

class Painter
{
  public:
  void DrawSomething();
};

void Painter::DrawSomething()
{
  Turtle turtle;
 turtle.doSomething();
}

main()
{
  Painter p;
  p.DrawSomething();
}

I have mocked the Turtle class but how can I test doSomething() method (for example with EXPECT_CALL) when the object of turtle is created locally? Is it possible without modifying Painter class?

Thank you for answers.

Answer 1

I have mocked the Turtle class ...

How exactly did you mock it?

... but how can I test doSomething() method (for example with EXPECT_CALL ) when the object of turtle is created locally? Is it possible without modifying Painter class?
_{(Emphasis mine)}

The straightforward answer is: No .

You cannot magically inject a mock instead of a real instance used in another class without decoupling via an interface.

---

You should have something like the following code instead:

struct ITurtle {
    virtual void PenUp() = 0;
    virtual void PenDown() = 0;
    virtual void TurnLeft(double degrees) = 0;
    virtual void Move(double distance) = 0;
    // ...
    virtual ~ITurtle() {}
};

---

struct TurtleMock : ITurtle {
    // Mock method declarations
    MOCK_METHOD0(PenUp, void ());
    MOCK_METHOD0(PenDown, void ());
    MOCK_METHOD1(TurnLeft, void (double));
    MOCK_METHOD1(Move, void (double));
};

---

class Turtle : public ITurtle {
public:
    void PenUp();
    void PenDown();
    void TurnLeft(double degrees);
    void Move(double distance);
};

Provide the real implementation for the above declarations in a separate translation unit.

---

class Painter {
public:
    Painter(ITurtle& turtle) : turtle_(turtle) {}   
    void DrawSomething();
private:
    ITurtle& turtle_;
};

void Painter::DrawSomething() {
    turtle_.PenDown();
    turtle_.TurnLeft(30.0);
    turtle_.Move(10.0);
    turtle_.TurnLeft(30.0);
    turtle_.Move(10.0);
    // ...
}

You can alternatively pass the ITurtle interface to the DrawSomething() function:

class Painter {
public:
    void DrawSomething(ITurtle& turtle);
};

void Painter::DrawSomething(ITurtle& turtle) {
    turtle.PenDown();
    turtle.TurnLeft(30.0);
    turtle.Move(10.0);
    turtle.TurnLeft(30.0);
    turtle.Move(10.0);
    // ...
}

---

int main() {
     NiceMock<TurtleMock> turtle;
     Painter p(turtle);
     // Painter p; <<< for the alternative solution

     EXPECT_CALL(turtle,PenDown())
         .Times(1);
     EXPECT_CALL(turtle,TurnLeft(_))
         .Times(2);
     EXPECT_CALL(turtle,Move(_))
         .Times(2);

     p.DrawSomething();
     //  p.DrawSomething(turtle); <<< for the alternative solution

}

Answer 2

I have written a wrapper class to mock production code without changing it. Please let me know if there is any flaw in this.

#include "gtest/gtest.h"
#include "src/gtest-all.cc"
#include "src/gmock-all.cc"
#include "src/gmock_main.cc"
#include <iostream>
#include <string>
#include <vector>

using ::testing::An;
using ::testing::AtLeast;
using ::testing::DoAll;
using ::testing::NotNull;
using ::testing::Return;
using ::testing::ReturnRef;
using ::testing::SetArgReferee;

using namespace std;

class Student
{
    int iAge;

    public:
    Student(int _iAge) : iAge(_iAge)
    {
    }

    virtual void PrintDetails()
    {
        cout<<"Age:"<<iAge<<endl;
    }
    virtual bool CheckGrade(int iGrade)
    {
        return (iGrade - 5) == iAge;
    }
};

class StudentFaker
{
    static Student* internalObject;

    public:

    static void FakerSetObject(Student* object) {
        internalObject = object;
    }

    StudentFaker(int _iAge){
    }

    void PrintDetails()  {
        internalObject->PrintDetails();
    }

    bool CheckGrade(int iGrade)  {
        return internalObject->CheckGrade(iGrade);
    }
};

Student* StudentFaker::internalObject = NULL;

class StudentMock : public Student
{
    public:
    StudentMock(int _iAge) : Student(_iAge) { }

    MOCK_METHOD0(PrintDetails,void());
    MOCK_METHOD1(CheckGrade,bool(int));
};

#define UNITTEST

bool ProductionCode();

TEST(STUDENT,TEST)
{
    StudentMock stMock(8);

    EXPECT_CALL(stMock, PrintDetails())
        .Times(AtLeast(1))
        .WillOnce(Return());

    EXPECT_CALL(stMock, CheckGrade(5))
        .Times(AtLeast(1))
        .WillOnce(Return(true));

    StudentFaker::FakerSetObject(&stMock);

    EXPECT_TRUE(ProductionCode());
}

//Production code
#ifdef UNITTEST
#define Student StudentFaker
#endif

bool ProductionCode()
{
    Student st(8);
    st.PrintDetails();
    if(st.CheckGrade(5))
        return true;
    else
        return false;
}

//Production code