mock_interface.cpp

class ISmartObject {
 public:
  virtual int Foo() const = 0;
}

class RealObject : public ISmartObject {
 public:
  RealObject() { /* allocate resource and throw exception */ }
  int Foo() const override { /* do real work */ }
}

class MockObject : public ISmartObject {
 public:
  int Foo() const override { /* do nothing */ }
}

class ClientObject {
 public:
  ClientObject();
  UseSmartObject(const ISmartObject& obj); // Use interface not implementation!
}

// Somewhere in test
ClientObject obj;
obj.UseSmartObject(MockObject());