# Assigning a subclass to a variable defined as the base class:

**URL:** <https://forums.swift.org/t/assigning-a-subclass-to-a-variable-defined-as-the-base-class/68228>\
**Category:** C++ interoperability\
**Created:** [November 3, 2023, 4:58am UTC](https://forums.swift.org/t/assigning-a-subclass-to-a-variable-defined-as-the-base-class/68228 "2023-11-03T04:58:25Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Peter\_Z](https://avatars.discourse-cdn.com/v4/letter/p/91b2a8/32.png) [@Peter\_Z](https://forums.swift.org/u/Peter_Z)\
**Post date:** [November 3, 2023, 4:58am UTC](https://forums.swift.org/t/assigning-a-subclass-to-a-variable-defined-as-the-base-class/68228/1 "2023-11-03T04:58:25Z")

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I have been experimenting a bit with C++ interoperability and have come across a snag that I thought was fixed. It is related to issue:  
[https://github.com/apple/swift/issues/66323](https://github.com/apple/swift/issues/66323) - ‘Finalize Swift 5.9 rules that determine when inherited members that shadow other members can be introduced to the Swift type that represents the C++ structure or class #66323.’

I downloaded the latest toolchain that has the fix and created the following C++ class ( as mentioned in the link ):

```swift
class Plant {
public:
  void water(float amount) { moisture += amount; }
private:
  float moisture = 0.0;
};

class Fern: public Plant {
public:
  void water(float amount);
};

```

What I would like to do is create a variable of type Plant but then later, use the same variable to hold a Fern. However, I can’t get the following to compile:

```swift
var plant: Plant = Plant()

…. later

plant = Fern() <— ‘Cannot assign value of type 'Fern' to type 'Plant’’

```

I also tried:

```swift
var plant: Plant = Plant()
var fern: Fern = Fern()
    
plant = fern as! Plant <——— ‘Cast from 'Fern' to unrelated type 'Plant' always fails’

```

Could someone let me know what I might be doing wrong? Thanks!

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<div class="post-metadata">

**Author:** ![John\_McCall](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/john_mccall/32/1351_2.png) [@John\_McCall](https://forums.swift.org/u/John_McCall)\
**Post date:** [November 3, 2023, 6:54am UTC](https://forums.swift.org/t/assigning-a-subclass-to-a-variable-defined-as-the-base-class/68228/2 "2023-11-03T06:54:49Z")

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Well, there’s two levels of problem here.

The first is that we don’t yet provide implicit conversions in Swift for what would be derived-to-base conversions in C++. I do think this is something we could generally support for types we import as reference types, although multiple inheritance does make it tricky.

The second is that, because these are _not_ being imported as reference types, but instead are value types, you’re actually requesting something called an “object slice”. The `Plant` class, taken as a value type, is not polymorphic: variables of that type always just store the fields associated with `Plant` and not anything associated with its subclasses. If you did this assignment in C++, it would just copy the portion of the source object that belongs to `Plant` and ignore the rest of the object; this is generally considered to be bad. You need some kind of indirection / reference semantics (in C++ terms, a pointer or reference) to get polymorphism like this in C++.

Swift’s C++ interop is naturally going to duplicate that conception of the C++ types in Swift by default. If you want `Plant` to be a polymorphic type, you need to tell Swift to import it as a reference type. And then someday we’ll support derived-to-base conversions on it in Swift.

I don’t think we would ever want to do derived-to-base conversions on value types.

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<div class="post-metadata">

**Author:** ![Peter\_Z](https://avatars.discourse-cdn.com/v4/letter/p/91b2a8/32.png) [@Peter\_Z](https://forums.swift.org/u/Peter_Z)\
**Post date:** [November 3, 2023, 9:55am UTC](https://forums.swift.org/t/assigning-a-subclass-to-a-variable-defined-as-the-base-class/68228/3 "2023-11-03T09:55:40Z")

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Ok, thanks for the explanation. Since I’m not that great with C++, it looks like I might have to stick with the working Objective C++ wrapper route I already use.

Just out of interest, what I am trying to do is basically like in the example I showed. The user has an option to select what exact type of C++ subclass they want to do some work. There is no multiple inheritance. Basically like the Swift equivalent.

I had a bit of a closer look at the docs regarding Mapping C++ Types to Swift Reference Types and it looks like it could be possible, but my C++ isn’t up to it. So until there is a lot more simple examples available I’ll have to stick with the Objective C++ wrapper.