10

I am trying to append a blank object to a list using the push_back method.

main.cpp

    vector<FacialMemory> facial_memory;

    printf("2\n");

    // Add people face memories based on number of sections
    for (int i = 0; i < QuadrantDict::getMaxFaceAreas(); i++)
    {
            printf("i %d\n", i);
            FacialMemory n_fm;
            facial_memory.push_back(n_fm);  // NOTE: Breaks here
    }

At the push_back method call, the program crashes with a segmentation fault. I have looked around at similar questions and they point to the solution I have here. I have also tried to pass FacialMemory() into the push_back call but still the same issue.

The FacialMemory class is defined as such: FacialMemory.h

class FacialMemory
{
private:
        vector<FaceData> face_memory;
public:
        FacialMemory();
        ~FacialMemory();
        void pushData(FaceData face);
        bool isEmpty();
        vector<FaceData> getFaces();
        FaceData getRecent();
};

Constructor and destructor

FacialMemory::FacialMemory()
{
}


FacialMemory::~FacialMemory()
{
       delete[] & face_memory;
}
DSchana
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  • What's in your FacialMemory class' constructor? Also your code is inefficient because you're copying objects everywhere. Consider using move semantics or heap-allocated objects. – Dai Jan 18 '17 at 06:03
  • Ok, I've added the constructor and destructor for the FacialMemory – DSchana Jan 18 '17 at 06:14
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    Why do you delete `face_memory` with `delete []` ? – pSoLT Jan 18 '17 at 06:16
  • Ok, removing the `delete[]` line fixes the error. Still not sure why though? – DSchana Jan 18 '17 at 06:18
  • You would likely have a similar crash even if you removed the `facial_memory.push_back(n_fm);` statement since each time through the loop the destructor for `n_fm` is called. – Michael Burr Jan 18 '17 at 06:26

3 Answers3

14

When you push_back an item into a vector, the item is copied. Sometimes this triggers more work as the vector is resized: Its current contents are copied, and the now-copied elements that used to belong to the vector are destroyed. destruction invokes the destructor.

Unfortunately, FacialMemory's destructor contains a fatal error: 

FacialMemory::~FacialMemory()
{
       delete[] & face_memory; <<== right here
}

It tries to delete[] data that was not allocated by new[], and whatever is managing the program's memory threw a fit because the expected book-keeping structures that keep track of dynamically allocated storage (memory allocated with new or with new[]) for the storage being returned were not found or not correct.

Further, face_memory is a std::vector, an object designed to look after its memory for you. You can create, copy, resize, and delete a vector without any intervention in most cases. The most notable counter case is a vector of pointers where you may have to release the pointed-at data when removing the pointer from the vector.

The solution is to do nothing in the FacialMemory class destructor. In fact, the Rule of Zero recommends that you not have a destructor at all because FacialMemory has no members or resources that require special handling. The compiler will generate the destructor for you with approximately zero chance of a mistake being made.

While reading the link for the Rule of Zero, pay attention to the Rules of Three and Five because they handle the cases where a class does require special handling and outline the minimum handling you should provide.

user4581301
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  • I have this exact issue but if I remove the destructor or add a copy constructor and copy assignment I still have the same problem. – Wilhelmina Lohan Apr 16 '21 at 22:55
  • @WilliamLohan Sounds different enough to be worth a new question.. At the very least, I'd need to see code to be of much help, and code and comments don't mix well. – user4581301 Apr 16 '21 at 23:08
  • I found a work around, I don't understand why but pointerToObject->appendChild(pointerToChild) threw a segment fault but valueOfObject.appendChild(pointerToChild) worked (where appendChild calls vector.push_back() ) but the code has completely changed to so I'd have nothing to make a new question with. – Wilhelmina Lohan Apr 17 '21 at 00:24
  • Could be that as `vector`s are resized pointers to elements in the `vector` are rendered invalid. No way to say anything with certainty though. – user4581301 Apr 17 '21 at 00:27
1

One of the reason to occur Segmentation fault is when ever you access a memory part which is invalid and In your program you are deallocating memory(delete keyword in your destructor) which is not allocated by new keyword.

sitaram chhimpa
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-5

Please refer to vector-push-back

Try adding a meaningful copy constructor to FacialMemory class

Community
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Rishi
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