Initializing Base class and member using the same pointer without UB

Question

While reviewing some testing code, I have come across this situation:

Suppose we have these two classes:

class Project
{
public:
    Project(std::string path);
    ~Project;
    // [many more functions]
};

class ResourceManager
{
public:
    ResourceManager(Project &proj);
    virtual ~ResourceManager();
    // [many more functions]
    // [notably no getProject()]
};

In the test code, there exists a 'fake' version of the ResourceManager:

class FakeResourceManager : public ResourceManager
{
    std::unique_ptr<Project> project_;

    Project* makeProject()
    {
        project_= std::make_unique<Project>("/some/path"); // oops, accessing non-initialized member
        return project_.get();
    };

public:
    FakeResourceManager()
        : ResourceManager(*makeProject())
    {}

    virtual ~FakeResourceManager(); // defaulted in cpp

    void markResourceAsDirty(); // 'extension' of base functionality for testing
};

This of course doesn't work, since the project_ member is not initialized yet when makeProject is called, which the address sanitizer catches at runtime.

I reworked this initialization code, but I am unsure if it's now free of any UB and does the expected thing:

class FakeResourceManager : public ResourceManager
{
    std::unique_ptr<Project> project_;

    FakeResourceManager(std::unique_ptr<Project> project)
        : ResourceManager(*project.get())
        , project_(std::move(project)) //(1)
    {}

public:
    FakeResourceManager()
        : FakeResourceManager(std::make_unique<Project>("/some/path"))
    {}

    virtual ~FakeResourceManager(); // defaulted in cpp

    void markResourceAsDirty(); // 'extension' of base functionality for testing
}

Points I considered:

The std::move (1) of the unique_ptr should not invalidate the pointer previously returned by get(). The moved-to member should correctly delete the Project on destruction.
If the ResourceManager destructor uses the passed project reference it would be UB, since that object already got deleted by the derived class dtor. Since it doesn't, it should be ok to have a dangling reference here (or is this still UB?)
The FakeResourceManager needs to be a derived class, since it accesses protected members in the 'extension' method.
A static factory function would work much the same way, but I see no benefit over this version.

Does this solution still contain UB (or some other logic error), or is this safe?

Similar, but "reverse order" using-storing-derived-member-in-derived-class-with-base-class-that-stores-base-member — Jarod42
– Jarod42, Commented Jul 14 at 14:27
@Jarod42 Indeed similar, I suppose in my case this would be akin to having a second delegating ctor (FRM() -> FRM(unique_ptr) -> FRM(unique_ptr, Project&)) — perivesta
– perivesta, Commented Jul 14 at 14:40
Construction should be safe, destruction might not be (Project reference is dangling in ResourceManager's destructor) — Jarod42
– Jarod42, Commented Jul 14 at 16:01

Yakk - Adam Nevraumont · Accepted Answer · 2025-07-14 14:21:02Z

*project.get() is unsafe to me (and can just be *project).

To fix your problem, I'd do this:

struct HasProject {
  std::unique_ptr<Project> project_;

  [[nodiscard]] Project& makeProject(std::string_view path) {
    assert(!project_);
    project_= std::make_unique<Project>(path);
    return *project_;
  };
};

class FakeResourceManager : private HasProject, public ResourceManager
{
public:
  FakeResourceManager()
    : ResourceManager(makeProject("/some/path"))
  {}

  virtual ~FakeResourceManager(); // defaulted in cpp

  void markResourceAsDirty(); // 'extension' of base functionality for testing
};

we move the storage into an earlier than ResourceManager base class, and that base class is fully initialized by the time we construct ResourceManager.

We can handle failures of project construction within makeProject (throwing or whatever), and don't have a method that takes a unique_ptr but fails catastrophically on an nullptr.

That's a neat trick. Even if the get() were safe, this also avoids problems with using the project reference in the base dtor

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