The segmentation fault is resolved, but cannot figure out why

Question

The header file contains:

class WorkScene;
class Mesh;

class Director
{
private:
    WorkScene *scene            = nullptr; // owner
    Mesh      *selected_mesh    = nullptr; // borrower
public:
    Director(WorkScene *scene);
    ~Director();

    const Mesh *get_selected_mesh() const;
    Mesh *get_selected_mesh();
};

The implementation of the get_selected_mesh method is:

cvas::p3de::Mesh *cvas::p3de::Director::get_selected_mesh()
{
    return selected_mesh;
}

However I receive a segmentation fault at this line:

---

The segmentation fault error text is:

The inferior stopped because it received a signal from the operating system.

Signal name : SIGSEGV

Signal meaning : Segmentation fault

---

The segmentation fault is resolved when modifying the code like this:

cvas::p3de::Mesh *cvas::p3de::Director::get_selected_mesh()
{
    //return selected_mesh;
    return nullptr;
}

Well, I can't figure out why the segmentation fault is resolved when modifying the code like above, considering the fact that inside the header file, the selected_mesh identifier was already declared/initialized as nullptr . Can anyone give me a hint?

Answer 1

You seem to have a Director* that is nullptr and then you call get_selected_mesh on that pointer. This is undefined behavior.

In the case where you just return nullptr the compiler does not care and just returns nullptr . In the case where you return selected_mesh the compiler de facto needs to do a return this->selected_mesh . This dereferences the invalid this and thus the access violation.

The error is somewhere in the code that you do not show.