Double free or corruption after queue push

Question

include  lt queue gt  using namespace std   class Test      int  myArray           public      Test            myArray   new int 10               Test            delete   myArray              int main        queue lt Test gt  q     Test t      q push t       After I run this  I get a runtime error  double free or corruption   If I get rid of the destructor content  the delete  it works fine  What s wrong

User · Answer

You need to define a copy constructor  assignment  operator     class Test      Test const Test  amp that     Copy constructor    Test amp  operator   const Test  amp rhs     assignment operator     Your copy that is pushed on the queue is pointing to the same memory your original is   When the first is destructed  it deletes the memory   The second destructs and tries to delete the same memory

User · Answer

Um  shouldn t the destructor be calling delete  rather than delete

User · Answer

Let s talk about copying objects in C     Test t   calls the default constructor  which allocates a new array of integers   This is fine  and your expected behavior   Trouble comes when you push t into your queue using q push t   If you re familiar with Java  C   or almost any other object-oriented language  you might expect the object you created earler to be added to the queue  but C   doesn t work that way   When we take a look at std  queue  push method  we see that the element that gets added to the queue is  initialized to a copy of x   It s actually a brand new object that uses the copy constructor to duplicate every member of your original Test object to make a new Test   Your C   compiler generates a copy constructor for you by default  That s pretty handy  but causes problems with pointer members  In your example  remember that int  myArray is just a memory address  when the value of myArray is copied from the old object to the new one  you ll now have two objects pointing to the same array in memory   This isn t intrinsically bad  but the destructor will then try to delete the same array twice  hence the  double free or corruption  runtime error   How do I fix it   The first step is to implement a copy constructor  which can safely copy the data from one object to another   For simplicity  it could look something like this   Test const Test amp  other       myArray   new int 10       memcpy  myArray  other myArray  10        Now when you re copying Test objects  a new array will be allocated for the new object  and the values of the array will be copied as well   We re not completely out trouble yet  though  There s another method that the compiler generates for you that could lead to similar problems - assignment   The difference is that with assignment  we already have an existing object whose memory needs to be managed appropriately   Here s a basic assignment operator implementation   Test amp  operator   const Test amp  other       if  this     amp other            memcpy  myArray  other myArray  10              return  this      The important part here is that we re copying the data from the other array into this object s array  keeping each object s memory separate   We also have a check for self-assignment  otherwise  we d be copying from ourselves to ourselves  which may throw an error  not sure what it s supposed to do    If we were deleting and allocating more memory  the self-assignment check prevents us from deleting memory from which we need to copy

User · Answer

You are getting double free or corruption because first destructor is for object q in this case the memory allocated by new will be free Next time when detructor will be called for object t at that time the memory is already free  done for q  hence when in destructor delete   myArray  will execute it will throw double free or corruption  The reason is that both object sharing the same memory so define  copy  assignment  and equal operator as mentioned in above answer

User · Answer

You can also try to check null before delete such that  if myArray    delete   myArray  myArray   NULL      or you can define all delete operations ina safe manner like this    ifndef SAFE DELETE  define SAFE DELETE p    if p    delete  p    p    NULL       endif   ifndef SAFE DELETE ARRAY  define SAFE DELETE ARRAY p    if p    delete    p    p    NULL       endif   and then use   SAFE DELETE ARRAY myArray

User · Answer

The problem is that your class contains a managed RAW pointer but does not implement the rule of three  five in C  11   As a result you are getting  expectedly  a double delete because of copying   If you are learning you should learn how to implement the rule of three  five   But that is not the correct solution to this problem  You should be using standard container objects rather than try to manage your own internal container  The exact container will depend on what you are trying to do but std  vector is a good default  and you can change afterwords if it is not opimal     include  lt queue gt   include  lt vector gt   class Test      std  vector lt int gt  myArray       public      Test    myArray 10                 int main        queue lt Test gt  q     Test t      q push t       The reason you should use a standard container is the separation of concerns  Your class should be concerned with either business logic or resource management  not both   Assuming Test is some class you are using to maintain some state about your program then it is business logic and it should not be doing resource management  If on the other hand Test is supposed to manage an array then you probably need to learn more about what is available inside the standard library

[c++] Double free or corruption after queue::push

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