What is the difference between override and virtual?

In Java there is no keyword virtual. 😋

… So I can’t remember having had any problems (…)

In my comment, I am referring entirely to C#.

In Java you do not have an explicit language mark for virtual/override, but can optionally the annotation @Override to let the compiler check if you actually overwrite a method.

In C++ you can read the keyword override use (as in Java optional, since only version 11 is introduced here). If you try to overwrite a method that is not as virtual you only cover it. That is, polymorphic mechanism does not attack as usual.

Example:

#include 

class Base {
  public:
    void doSomething() {
      std::cout << "doSomething (base)" << std::endl;
    }

    virtual void doSomethingVirtual() {
      std::cout << "doSomethingVirtual (base)" << std::endl;
    }
};

class Sub : public Base {
  public:
    void doSomething() {
      std::cout << "doSomething (sub)" << std::endl;
    }

    void doSomethingVirtual() {
      std::cout << "doSomethingVirtual (sub)" << std::endl;
    }
};

int main() {
  Base* sub = new Sub;
  sub->doSomething(); // doSomething (base)
  sub->doSomethingVirtual(); // doSomethingVirtual (sub)

  return 0;
};

If you are override-keyword uses:

class Sub : public Base {
  public:
    void doSomething() override {
      std::cout << "doSomething (sub)" << std::endl;
    }

    /* ... */
};

for this reason you get a compiler error.

error: ‘void Sub::doSomething()’ marked ‘override’, but does not override