googletest - mocking abstract class - c++

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