Java polymorphism in C++

Question

This code throws an error:

class A {};
class B : public A {};

int main() {
    A instance = new B();
}

Error message:

void *operator new(unsigned int)
no suitable constructor exists to convert from "B *" to "A"

My first language was Java and I'm really confused about this. Every subclass can be treated as its motherclass because it derives everything, so why shouldn't they add the option to treat B like A?

Or is there a mistake in my syntax? I started learning C++ 3 weaks ago, it could be possible.

I wasn't able to find a solution with google or by searching through my tutorial, so please help me.

You need to read a textbook, unlike java in C++ pointer and variable not the same thing. Problem is not with inheritance, but that `A` and `A*` are different types. — Slava, May 21 '17 at 16:23
Looks closely at the error message, and read up on pointers. `new` results in a `B*` which is different than a `B`. Similarly an `A` is different than an `A*`. — TheUndeadFish, May 21 '17 at 16:25
Did `A instance = new A();` work? That one has no inheritation. — juanchopanza, May 21 '17 at 16:29
_@Timon_ 1st thing you should forget about when switching from Java to C++ is using `new`. That should be one of the rarest operators used. — πάντα ῥεῖ, May 21 '17 at 16:34
I need it for my game. I have a vector of living enemies which treats everything from Skeletons to Slimes as Enemies, their base class. — Timon Paßlick, May 21 '17 at 16:39
@Timon Before start programming a game in C++ you should have a look [here](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) — πάντα ῥεῖ, May 21 '17 at 16:58

score 0 · Accepted Answer · answered May 21 '17 at 17:03

0

This is how your code should look like in C++:

class A {};
class B : public A {};

int main() {
    std::unique_ptr<A> instance = std::make_unique<B>();
}

answered May 21 '17 at 17:03

πάντα ῥεῖ

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Or I could make `instance` `auto`. On Windows, I'd use `unique_ptr instance = new B;` because `std::make_unique` does error checking and throws. This is just on Windows, though. – Timon Paßlick Jul 08 '18 at 11:49

Java polymorphism in C++

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