Cannot assign derived raw pointer to base unique_ptr

Question

I have some code that looks something this:

class Info {
  public:
    virtual bool IsHere() = 0;
    virtual std::wstring GetStr() = 0;
};

class WindowsInfo : public Info {
  public:
    virtual std::wstring GetAnotherStr() = 0;
    bool IsHere() override;
};

class AdvancedWindowsInfo : public WindowsInfo {
  public:
    AdvancedWindowsInfo() {}
    ~AdvancedWindowsInfo() {}

    std::wstring GetAnotherStr() override;
    std::wstring GetStr() override;
};
  
class InfoFactory {
  public:
    static Info* GetInfo();
};
  
class InfoManager {
  public:
    InfoManager();
    //~InfoManager();

    bool IsSomething();

  private:
    std::unique_ptr<Info> info;
};
  
InfoManager::InfoManager() {
  #if WIN
    info = std::make_unique<WindowsInfo>();
  #else  // currently no implementation Linux
    info = nullptr;
  #endif
}
  
bool InfoManager::IsSomething() {
    std::unique_ptr<Info> info = InfoFactory::GetInfo();

    return info && info->IsHere();
}
  
Info* InfoFactory::GetInfo() {
  #if IS_WINDOWS
    return new AdvancedWindowsInfo();
  #else
    return nullptr;
  #endif
}

The entire code is too large (and confidential) to post here, but this snippet sums it up pretty well.

Essentially, I have a base class and some derived classes.

I also have a manager that uses a (smart) pointer to that base class.

And a Factory Method that returns the appropriate Derived object (although the signature returns a Base*).

Unfortunately, I can't get the the assignment (via the Factory Method) to work. I've tried multiple approaches but nothing works.

I tried using unique_ptr and make_unique<raw pointer>() --> it doesn't work with derived classes, only base.

I tried using unique_ptr and raw pointers --> conversion is not possible.

I tried using raw pointers (although I don't want this) and raw pointers --> it tells me that the destructor is called on the base object which is abstract. How can you call a destructor when you haven't instantiated the object (since it's an abstract class)? The compiler is contradicting itself!

Can't you have `InfoFactory::GetInfo()` return a `std::unique_ptr`? That would be more recommended. — Galik, Jan 25 '23 at 17:48
"Doesn't work" and "Can't get it working" are not descriptive. Do you get a compiler error? Does it crash? Do you get unexpected results? If you get an error, what error do you get? — François Andrieux, Jan 25 '23 at 17:52
Also, `std::unique_ptr info = InfoFactory::GetInfo();` creates a _local_variable info which goes out of scope when the function returns. It leaves the member variable info unaffected. Perhaps what you intended instead was `info.reset( InfoFactory::GetInfo() );` — Avi Berger, Jan 25 '23 at 17:59
"How can you call a destructor when you haven't instantiated the object..." Did you give the real `Info` a `virtual ~Info() = default;` destructor? You didn't give the shown `Info` one. See [here](https://stackoverflow.com/questions/461203/when-to-use-virtual-destructors) — Nathan Pierson, Jan 25 '23 at 18:01

Nathan Pierson · Answer 1 · 2023-01-25T17:59:16.083

Let's check the documentation for std::unique_ptr's constructors. The signature of the relevant constructor:

explicit unique_ptr( pointer p ) noexcept; (2)

The converting constructor that converts a raw pointer to a std::unique_ptr is explicit. Among other things, this means it cannot be used for copy initialization of the form

std::unique_ptr<Info> info = InfoFactory::GetInfo();

Instead, you can use direct initialization:

std::unique_ptr<Info> info{InfoFactory::GetInfo()};

Which will allow you to perform that conversion.

While looking at this code, however, I notice that the local variable info in InfoManager::IsSomething is shadowing the class member variable InfoManager::info. If you want to change an existing std::unique_ptr so that it's now managing a new raw pointer, you might want to use reset:

info.reset(InfoFactory::GetInfo());

If you want the conversion at all. There is room for failure between `new AdvancedWindowsInfo();` and safe storage in `info` that would be solved by an end-to-end `unique_ptr` curtesy of [`std::make_unique`](https://en.cppreference.com/w/cpp/memory/unique_ptr/make_unique). — user4581301, Jan 25 '23 at 18:16

score 0 · Accepted Answer · answered Jan 25 '23 at 18:42

Ok, so I did the following:

InfoFactory::GetInfo() now returns a std::unique_ptr<Info>, as indicated by Galik
Added virtual ~Info() = default; as indicated by Nathan Pierson

Now everything seems to be working ok. For now, I will the question unanswered as I still need to run some tests and double check some things, but basically it seems to be ok.

Thank you to everyone who made positive contibutions!

Cannot assign derived raw pointer to base unique_ptr

2 Answers2