googletest - mocking abstract class

Question

I am learning mocking and using googletest I created MockServer class that should mock the abstract class IServer:

    class IServer
{
    virtual void writeData(QString buffer) = 0;
    virtual QByteArray readData() = 0;

protected:
    virtual ~IServer() = default;
};

class MockServer: public:: testing:: Test, public IServer
{
    MOCK_METHOD(void, writeData, (QString buffer), (override));
    MOCK_METHOD(QByteArray, readData, (), (override));
};

And now want to test the other class that uses it, however, I cannot initialize MockServer because it is an abstract class. How to solve this?

TEST_F(Serv_test, start)
{
    ?? MockServer mockServer; // <- how to declare it?
    EXPECT_CALL(mockServer, writeData(testing::_)).Times(1);
    EXPECT_CALL(mockServer, readData()).Return("testing");

    Car car (nullptr, mockServer);
}

Answer 1

You are confusing a few things:

1. You are using a test fixture, but a test fixture needs to be a standalone class. You are mixing it with your mock class. You need to create a separate class for it.

2. The Car class should take a parameter of type mock class (not the test fixture).

3. .Return should be used inside WillOnce or WillRepeatedly among other places. You can't use it directly on EXPECT_CALL.

I rewrote your code as follows:

class IServer
{
  public:
    virtual void writeData(QString buffer) = 0;
    virtual QByteArray readData() = 0;

protected:
    virtual ~IServer() = default;
};

// Test fixture
class Serv_test : public ::testing::Test {
};

// Mock class
class MockServer: public IServer
{
 public:
    MOCK_METHOD(void, writeData, (QString buffer), (override));
    MOCK_METHOD(QByteArray, readData, (), (override));
};

class Car{
  public:
  Car(int*, IServer* server):_server(server){
      _server->writeData("testing");
      _server->readData();
  }

  IServer* _server;
};

TEST_F(Serv_test, start)
{
    MockServer mockServer; // <- how to declare it?
    EXPECT_CALL(mockServer, writeData(testing::_)).Times(1);
    EXPECT_CALL(mockServer, readData())
      .WillOnce(testing::Return("testing"));

    Car car (nullptr, &mockServer);
}

Working example here: https://godbolt.org/z/avsKdh37r