Dynamically allocating an array of objects

Question

I have a class that contains a dynamically allocated array  say  class A       int  myArray      A                 myArray   0            A int size                myArray   new int size              A                    Note that as per MikeB s helpful style critique  no need to check against 0          delete    myArray            But now I want to create a dynamically allocated array of these classes  Here s my current code   A  arrayOfAs   new A 5   for  int i   0  i  lt  5    i        arrayOfAs i    A 3       But this blows up terribly  Because the new A object created  with the A 3  call  gets destructed when the for loop iteration finishes  and this means that the internal myArray of that A instance gets delete   -ed   So I think my syntax must be terribly wrong  I guess there are a few fixes that seem like overkill  which I m hoping to avoid    Creating a copy constructor for A  Using vector lt int gt  and vector lt A gt  so I don t have to worry about all this  Instead of having arrayOfAs be an array of A objects  have it be an array of A  pointers    I would think this is just some beginners thing where there s a syntax that actually works when attempting to dynamically allocate an array of things that have internal dynamic allocation    Also  style critiques appreciated  since it s been a while since I did C      Update for future viewers  All of the answers below are really helpful  Martin s is accepted because of the example code and the useful  rule of 4   but I really suggest reading them all  Some are good  succinct statements of what s wrong  and some point out correctly how and why vectors are a good way to go

User · Answer

You need an assignment operator so that   arrayOfAs i    A 3     works as it should

User · Answer

Using the placement feature of new operator  you can create the object in place and avoid copying       placement  3   void  operator new  std  size t size  void  ptr  noexcept       Simply returns ptr  no storage is allocated     Notice though that  if the function is called by a new-expression  the proper initialization will be performed  for class objects  this includes calling its default constructor     I suggest the following   A  arrayOfAs   new A 5     Allocate a block of memory for 5 objects for  int i   0  i  lt  5    i          Do not allocate memory        initialize an object in memory address provided by the pointer     new   amp arrayOfAs i   A 3

User · Answer

I d recommend using std  vector  something like  typedef std  vector lt int gt  A  typedef std  vector lt A gt  AS    There s nothing wrong with the slight overkill of STL  and you ll be able to spend more time implementing the specific features of your app instead of reinventing the bicycle

User · Answer

The constructor of your A object allocates another object dynamically and stores a pointer to that dynamically allocated object in a raw pointer   For that scenario  you must define your own copy constructor   assignment operator and destructor   The compiler generated ones will not work correctly     This is a corollary to the  Law of the Big Three    A class with any of destructor  assignment operator  copy constructor generally needs all 3    You have defined your own destructor  and you mentioned creating a copy constructor   but you need to define both of the other 2 of the big three   An alternative is to store the pointer to your dynamically allocated int   in some other object that will take care of these things for you   Something like a vector lt int gt   as you mentioned  or a boost  shared array lt  gt    To boil this down - to take advantage of RAII to the full extent  you should avoid dealing with raw pointers to the extent possible   And since you asked for other style critiques  a minor one is that when you are deleting raw pointers you do not need to check for 0 before calling delete - delete handles that case by doing nothing so you don t have to clutter you code with the checks

User · Answer

For building containers you obviously want to use one of the standard containers  such as a std  vector   But this is a perfect example of the things you need to consider when your object contains RAW pointers   If your object has a RAW pointer then you need to remember the rule of 3  now the rule of 5 in C  11     Constructor Destructor Copy Constructor Assignment Operator Move Constructor  C  11  Move Assignment   C  11    This is because if not defined the compiler will generate its own version of these methods  see below   The compiler generated versions are not always useful when dealing with RAW pointers   The copy constructor is the hard one to get correct  it s non trivial if you want to provide the strong exception guarantee   The Assignment operator can be defined in terms of the Copy Constructor as you can use the copy and swap idiom internally   See below for full details on the absolute minimum for a class containing a pointer to an array of integers   Knowing that it is non trivial to get it correct you should consider using std  vector rather than a pointer to an array of integers  The vector is easy to use  and expand  and covers all the problems associated with exceptions  Compare the following class with the definition of A below   class A        std  vector lt int gt    mArray      public          A             A size t s   mArray s           Looking at your problem   A  arrayOfAs   new A 5   for  int i   0  i  lt  5    i           As you surmised the problem is on this line      arrayOfAs i    A 3           What is happening         1  A 3  Build your A object  fine         2  A  operator  A const amp   is called to assign the value           onto the result of the array access  Because you did           not define this operator the compiler generated one is           used      The compiler generated assignment operator is fine for nearly all situations  but when RAW pointers are in play you need to pay attention  In your case it is causing a problem because of the shallow copy problem  You have ended up with two objects that contain pointers to the same piece of memory  When the A 3  goes out of scope at the end of the loop it calls delete    on its pointer  Thus the other object  in the array  now contains a pointer to memory that has been returned to the system   The compiler generated copy constructor  copies each member variable by using that members copy constructor  For pointers this just means the pointer value is copied from the source object to the destination object  hence shallow copy    The compiler generated assignment operator  copies each member variable by using that members assignment operator  For pointers this just means the pointer value is copied from the source object to the destination object  hence shallow copy    So the minimum for a class that contains a pointer   class A       size t     mSize      int        mArray      public              Simple constructor destructor are obvious           A size t s   0   mSize s mArray   new int mSize             A                delete    mArray                Copy constructor needs more work          A A const amp  copy                          mSize    copy mSize               mArray   new int copy mSize                    Don t need to worry about copying integers                  But if the object has a copy constructor then                 it would also need to worry about throws from the copy constructor               std  copy  amp copy mArray 0   amp copy mArray c mSize  mArray                            Define assignment operator in terms of the copy constructor             Modified  There is a slight twist to the copy swap idiom  that you can                       Remove the manual copy made by passing the rhs by value thus                       providing an implicit copy generated by the compiler           A amp  operator  A rhs     Pass by value  thus generating a copy                          rhs swap  this      Now swap data with the copy                                   The rhs parameter will delete the array when it                                  goes out of scope at the end of the function              return  this                      void swap A amp  s  noexcept                         using std  swap               swap this mArray s mArray                swap this mSize  s mSize                           C  11          A A amp  amp  src  noexcept                mSize 0                 mArray NULL                          src swap  this                       A amp  operator  A amp  amp  src  noexcept                         src swap  this          You are moving the state of the src object                                      into this one  The state of the src object                                      after the move must be valid but indeterminate                                                                            The easiest way to do this is to swap the states                                      of the two objects                                                                            Note  Doing any operation on src after a move                                       is risky  apart from destroy  until you put it                                       into a specific state  Your object should have                                      appropriate methods for this                                                                             Example  Assignment  operator   should work                                                 std  vector   has clear   which sets                                               a specific state without needing to                                               know the current state               return  this

User · Answer

Use array or common container for objects only if they have default and copy constructors   Store pointers otherwise  or smart pointers  but may meet some issues in this case     PS  Always define own default and copy constructors otherwise auto-generated will be used

User · Answer

Use array or common container for objects only if they have default and copy constructors   Store pointers otherwise  or smart pointers  but may meet some issues in this case     PS  Always define own default and copy constructors otherwise auto-generated will be used

User · Answer

For building containers you obviously want to use one of the standard containers  such as a std  vector   But this is a perfect example of the things you need to consider when your object contains RAW pointers   If your object has a RAW pointer then you need to remember the rule of 3  now the rule of 5 in C  11     Constructor Destructor Copy Constructor Assignment Operator Move Constructor  C  11  Move Assignment   C  11    This is because if not defined the compiler will generate its own version of these methods  see below   The compiler generated versions are not always useful when dealing with RAW pointers   The copy constructor is the hard one to get correct  it s non trivial if you want to provide the strong exception guarantee   The Assignment operator can be defined in terms of the Copy Constructor as you can use the copy and swap idiom internally   See below for full details on the absolute minimum for a class containing a pointer to an array of integers   Knowing that it is non trivial to get it correct you should consider using std  vector rather than a pointer to an array of integers  The vector is easy to use  and expand  and covers all the problems associated with exceptions  Compare the following class with the definition of A below   class A        std  vector lt int gt    mArray      public          A             A size t s   mArray s           Looking at your problem   A  arrayOfAs   new A 5   for  int i   0  i  lt  5    i           As you surmised the problem is on this line      arrayOfAs i    A 3           What is happening         1  A 3  Build your A object  fine         2  A  operator  A const amp   is called to assign the value           onto the result of the array access  Because you did           not define this operator the compiler generated one is           used      The compiler generated assignment operator is fine for nearly all situations  but when RAW pointers are in play you need to pay attention  In your case it is causing a problem because of the shallow copy problem  You have ended up with two objects that contain pointers to the same piece of memory  When the A 3  goes out of scope at the end of the loop it calls delete    on its pointer  Thus the other object  in the array  now contains a pointer to memory that has been returned to the system   The compiler generated copy constructor  copies each member variable by using that members copy constructor  For pointers this just means the pointer value is copied from the source object to the destination object  hence shallow copy    The compiler generated assignment operator  copies each member variable by using that members assignment operator  For pointers this just means the pointer value is copied from the source object to the destination object  hence shallow copy    So the minimum for a class that contains a pointer   class A       size t     mSize      int        mArray      public              Simple constructor destructor are obvious           A size t s   0   mSize s mArray   new int mSize             A                delete    mArray                Copy constructor needs more work          A A const amp  copy                          mSize    copy mSize               mArray   new int copy mSize                    Don t need to worry about copying integers                  But if the object has a copy constructor then                 it would also need to worry about throws from the copy constructor               std  copy  amp copy mArray 0   amp copy mArray c mSize  mArray                            Define assignment operator in terms of the copy constructor             Modified  There is a slight twist to the copy swap idiom  that you can                       Remove the manual copy made by passing the rhs by value thus                       providing an implicit copy generated by the compiler           A amp  operator  A rhs     Pass by value  thus generating a copy                          rhs swap  this      Now swap data with the copy                                   The rhs parameter will delete the array when it                                  goes out of scope at the end of the function              return  this                      void swap A amp  s  noexcept                         using std  swap               swap this mArray s mArray                swap this mSize  s mSize                           C  11          A A amp  amp  src  noexcept                mSize 0                 mArray NULL                          src swap  this                       A amp  operator  A amp  amp  src  noexcept                         src swap  this          You are moving the state of the src object                                      into this one  The state of the src object                                      after the move must be valid but indeterminate                                                                            The easiest way to do this is to swap the states                                      of the two objects                                                                            Note  Doing any operation on src after a move                                       is risky  apart from destroy  until you put it                                       into a specific state  Your object should have                                      appropriate methods for this                                                                             Example  Assignment  operator   should work                                                 std  vector   has clear   which sets                                               a specific state without needing to                                               know the current state               return  this

User · Answer

I d recommend using std  vector  something like  typedef std  vector lt int gt  A  typedef std  vector lt A gt  AS    There s nothing wrong with the slight overkill of STL  and you ll be able to spend more time implementing the specific features of your app instead of reinventing the bicycle

User · Answer

You need an assignment operator so that   arrayOfAs i    A 3     works as it should

User · Answer

The constructor of your A object allocates another object dynamically and stores a pointer to that dynamically allocated object in a raw pointer   For that scenario  you must define your own copy constructor   assignment operator and destructor   The compiler generated ones will not work correctly     This is a corollary to the  Law of the Big Three    A class with any of destructor  assignment operator  copy constructor generally needs all 3    You have defined your own destructor  and you mentioned creating a copy constructor   but you need to define both of the other 2 of the big three   An alternative is to store the pointer to your dynamically allocated int   in some other object that will take care of these things for you   Something like a vector lt int gt   as you mentioned  or a boost  shared array lt  gt    To boil this down - to take advantage of RAII to the full extent  you should avoid dealing with raw pointers to the extent possible   And since you asked for other style critiques  a minor one is that when you are deleting raw pointers you do not need to check for 0 before calling delete - delete handles that case by doing nothing so you don t have to clutter you code with the checks

User · Answer

Use array or common container for objects only if they have default and copy constructors   Store pointers otherwise  or smart pointers  but may meet some issues in this case     PS  Always define own default and copy constructors otherwise auto-generated will be used

User · Answer

Using the placement feature of new operator  you can create the object in place and avoid copying       placement  3   void  operator new  std  size t size  void  ptr  noexcept       Simply returns ptr  no storage is allocated     Notice though that  if the function is called by a new-expression  the proper initialization will be performed  for class objects  this includes calling its default constructor     I suggest the following   A  arrayOfAs   new A 5     Allocate a block of memory for 5 objects for  int i   0  i  lt  5    i          Do not allocate memory        initialize an object in memory address provided by the pointer     new   amp arrayOfAs i   A 3

User · Answer

I d recommend using std  vector  something like  typedef std  vector lt int gt  A  typedef std  vector lt A gt  AS    There s nothing wrong with the slight overkill of STL  and you ll be able to spend more time implementing the specific features of your app instead of reinventing the bicycle

User · Answer

I d recommend using std  vector  something like  typedef std  vector lt int gt  A  typedef std  vector lt A gt  AS    There s nothing wrong with the slight overkill of STL  and you ll be able to spend more time implementing the specific features of your app instead of reinventing the bicycle

User · Answer

You need an assignment operator so that   arrayOfAs i    A 3     works as it should

User · Answer

For building containers you obviously want to use one of the standard containers  such as a std  vector   But this is a perfect example of the things you need to consider when your object contains RAW pointers   If your object has a RAW pointer then you need to remember the rule of 3  now the rule of 5 in C  11     Constructor Destructor Copy Constructor Assignment Operator Move Constructor  C  11  Move Assignment   C  11    This is because if not defined the compiler will generate its own version of these methods  see below   The compiler generated versions are not always useful when dealing with RAW pointers   The copy constructor is the hard one to get correct  it s non trivial if you want to provide the strong exception guarantee   The Assignment operator can be defined in terms of the Copy Constructor as you can use the copy and swap idiom internally   See below for full details on the absolute minimum for a class containing a pointer to an array of integers   Knowing that it is non trivial to get it correct you should consider using std  vector rather than a pointer to an array of integers  The vector is easy to use  and expand  and covers all the problems associated with exceptions  Compare the following class with the definition of A below   class A        std  vector lt int gt    mArray      public          A             A size t s   mArray s           Looking at your problem   A  arrayOfAs   new A 5   for  int i   0  i  lt  5    i           As you surmised the problem is on this line      arrayOfAs i    A 3           What is happening         1  A 3  Build your A object  fine         2  A  operator  A const amp   is called to assign the value           onto the result of the array access  Because you did           not define this operator the compiler generated one is           used      The compiler generated assignment operator is fine for nearly all situations  but when RAW pointers are in play you need to pay attention  In your case it is causing a problem because of the shallow copy problem  You have ended up with two objects that contain pointers to the same piece of memory  When the A 3  goes out of scope at the end of the loop it calls delete    on its pointer  Thus the other object  in the array  now contains a pointer to memory that has been returned to the system   The compiler generated copy constructor  copies each member variable by using that members copy constructor  For pointers this just means the pointer value is copied from the source object to the destination object  hence shallow copy    The compiler generated assignment operator  copies each member variable by using that members assignment operator  For pointers this just means the pointer value is copied from the source object to the destination object  hence shallow copy    So the minimum for a class that contains a pointer   class A       size t     mSize      int        mArray      public              Simple constructor destructor are obvious           A size t s   0   mSize s mArray   new int mSize             A                delete    mArray                Copy constructor needs more work          A A const amp  copy                          mSize    copy mSize               mArray   new int copy mSize                    Don t need to worry about copying integers                  But if the object has a copy constructor then                 it would also need to worry about throws from the copy constructor               std  copy  amp copy mArray 0   amp copy mArray c mSize  mArray                            Define assignment operator in terms of the copy constructor             Modified  There is a slight twist to the copy swap idiom  that you can                       Remove the manual copy made by passing the rhs by value thus                       providing an implicit copy generated by the compiler           A amp  operator  A rhs     Pass by value  thus generating a copy                          rhs swap  this      Now swap data with the copy                                   The rhs parameter will delete the array when it                                  goes out of scope at the end of the function              return  this                      void swap A amp  s  noexcept                         using std  swap               swap this mArray s mArray                swap this mSize  s mSize                           C  11          A A amp  amp  src  noexcept                mSize 0                 mArray NULL                          src swap  this                       A amp  operator  A amp  amp  src  noexcept                         src swap  this          You are moving the state of the src object                                      into this one  The state of the src object                                      after the move must be valid but indeterminate                                                                            The easiest way to do this is to swap the states                                      of the two objects                                                                            Note  Doing any operation on src after a move                                       is risky  apart from destroy  until you put it                                       into a specific state  Your object should have                                      appropriate methods for this                                                                             Example  Assignment  operator   should work                                                 std  vector   has clear   which sets                                               a specific state without needing to                                               know the current state               return  this

User · Answer

You need an assignment operator so that   arrayOfAs i    A 3     works as it should

User · Answer

Why not have a setSize method    A  arrayOfAs   new A 5   for  int i   0  i  lt  5    i        arrayOfAs i  SetSize 3       I like the  copy  but in this case the default constructor isn t really doing anything   The SetSize could copy the data out of the original m array  if it exists    You d have to store the size of the array within the class to do that   OR  The SetSize could delete the original m array    void SetSize unsigned int p newSize          I don t care if it s null because delete is smart enough to deal with that      delete myArray      myArray   new int p newSize       ASSERT myArray

User · Answer

The constructor of your A object allocates another object dynamically and stores a pointer to that dynamically allocated object in a raw pointer   For that scenario  you must define your own copy constructor   assignment operator and destructor   The compiler generated ones will not work correctly     This is a corollary to the  Law of the Big Three    A class with any of destructor  assignment operator  copy constructor generally needs all 3    You have defined your own destructor  and you mentioned creating a copy constructor   but you need to define both of the other 2 of the big three   An alternative is to store the pointer to your dynamically allocated int   in some other object that will take care of these things for you   Something like a vector lt int gt   as you mentioned  or a boost  shared array lt  gt    To boil this down - to take advantage of RAII to the full extent  you should avoid dealing with raw pointers to the extent possible   And since you asked for other style critiques  a minor one is that when you are deleting raw pointers you do not need to check for 0 before calling delete - delete handles that case by doing nothing so you don t have to clutter you code with the checks

User · Answer

Why not have a setSize method    A  arrayOfAs   new A 5   for  int i   0  i  lt  5    i        arrayOfAs i  SetSize 3       I like the  copy  but in this case the default constructor isn t really doing anything   The SetSize could copy the data out of the original m array  if it exists    You d have to store the size of the array within the class to do that   OR  The SetSize could delete the original m array    void SetSize unsigned int p newSize          I don t care if it s null because delete is smart enough to deal with that      delete myArray      myArray   new int p newSize       ASSERT myArray

User · Answer

Use array or common container for objects only if they have default and copy constructors   Store pointers otherwise  or smart pointers  but may meet some issues in this case     PS  Always define own default and copy constructors otherwise auto-generated will be used

User · Answer

Why not have a setSize method    A  arrayOfAs   new A 5   for  int i   0  i  lt  5    i        arrayOfAs i  SetSize 3       I like the  copy  but in this case the default constructor isn t really doing anything   The SetSize could copy the data out of the original m array  if it exists    You d have to store the size of the array within the class to do that   OR  The SetSize could delete the original m array    void SetSize unsigned int p newSize          I don t care if it s null because delete is smart enough to deal with that      delete myArray      myArray   new int p newSize       ASSERT myArray

User · Answer

The constructor of your A object allocates another object dynamically and stores a pointer to that dynamically allocated object in a raw pointer   For that scenario  you must define your own copy constructor   assignment operator and destructor   The compiler generated ones will not work correctly     This is a corollary to the  Law of the Big Three    A class with any of destructor  assignment operator  copy constructor generally needs all 3    You have defined your own destructor  and you mentioned creating a copy constructor   but you need to define both of the other 2 of the big three   An alternative is to store the pointer to your dynamically allocated int   in some other object that will take care of these things for you   Something like a vector lt int gt   as you mentioned  or a boost  shared array lt  gt    To boil this down - to take advantage of RAII to the full extent  you should avoid dealing with raw pointers to the extent possible   And since you asked for other style critiques  a minor one is that when you are deleting raw pointers you do not need to check for 0 before calling delete - delete handles that case by doing nothing so you don t have to clutter you code with the checks

User · Answer

For building containers you obviously want to use one of the standard containers  such as a std  vector   But this is a perfect example of the things you need to consider when your object contains RAW pointers   If your object has a RAW pointer then you need to remember the rule of 3  now the rule of 5 in C  11     Constructor Destructor Copy Constructor Assignment Operator Move Constructor  C  11  Move Assignment   C  11    This is because if not defined the compiler will generate its own version of these methods  see below   The compiler generated versions are not always useful when dealing with RAW pointers   The copy constructor is the hard one to get correct  it s non trivial if you want to provide the strong exception guarantee   The Assignment operator can be defined in terms of the Copy Constructor as you can use the copy and swap idiom internally   See below for full details on the absolute minimum for a class containing a pointer to an array of integers   Knowing that it is non trivial to get it correct you should consider using std  vector rather than a pointer to an array of integers  The vector is easy to use  and expand  and covers all the problems associated with exceptions  Compare the following class with the definition of A below   class A        std  vector lt int gt    mArray      public          A             A size t s   mArray s           Looking at your problem   A  arrayOfAs   new A 5   for  int i   0  i  lt  5    i           As you surmised the problem is on this line      arrayOfAs i    A 3           What is happening         1  A 3  Build your A object  fine         2  A  operator  A const amp   is called to assign the value           onto the result of the array access  Because you did           not define this operator the compiler generated one is           used      The compiler generated assignment operator is fine for nearly all situations  but when RAW pointers are in play you need to pay attention  In your case it is causing a problem because of the shallow copy problem  You have ended up with two objects that contain pointers to the same piece of memory  When the A 3  goes out of scope at the end of the loop it calls delete    on its pointer  Thus the other object  in the array  now contains a pointer to memory that has been returned to the system   The compiler generated copy constructor  copies each member variable by using that members copy constructor  For pointers this just means the pointer value is copied from the source object to the destination object  hence shallow copy    The compiler generated assignment operator  copies each member variable by using that members assignment operator  For pointers this just means the pointer value is copied from the source object to the destination object  hence shallow copy    So the minimum for a class that contains a pointer   class A       size t     mSize      int        mArray      public              Simple constructor destructor are obvious           A size t s   0   mSize s mArray   new int mSize             A                delete    mArray                Copy constructor needs more work          A A const amp  copy                          mSize    copy mSize               mArray   new int copy mSize                    Don t need to worry about copying integers                  But if the object has a copy constructor then                 it would also need to worry about throws from the copy constructor               std  copy  amp copy mArray 0   amp copy mArray c mSize  mArray                            Define assignment operator in terms of the copy constructor             Modified  There is a slight twist to the copy swap idiom  that you can                       Remove the manual copy made by passing the rhs by value thus                       providing an implicit copy generated by the compiler           A amp  operator  A rhs     Pass by value  thus generating a copy                          rhs swap  this      Now swap data with the copy                                   The rhs parameter will delete the array when it                                  goes out of scope at the end of the function              return  this                      void swap A amp  s  noexcept                         using std  swap               swap this mArray s mArray                swap this mSize  s mSize                           C  11          A A amp  amp  src  noexcept                mSize 0                 mArray NULL                          src swap  this                       A amp  operator  A amp  amp  src  noexcept                         src swap  this          You are moving the state of the src object                                      into this one  The state of the src object                                      after the move must be valid but indeterminate                                                                            The easiest way to do this is to swap the states                                      of the two objects                                                                            Note  Doing any operation on src after a move                                       is risky  apart from destroy  until you put it                                       into a specific state  Your object should have                                      appropriate methods for this                                                                             Example  Assignment  operator   should work                                                 std  vector   has clear   which sets                                               a specific state without needing to                                               know the current state               return  this

User · Answer

Why not have a setSize method    A  arrayOfAs   new A 5   for  int i   0  i  lt  5    i        arrayOfAs i  SetSize 3       I like the  copy  but in this case the default constructor isn t really doing anything   The SetSize could copy the data out of the original m array  if it exists    You d have to store the size of the array within the class to do that   OR  The SetSize could delete the original m array    void SetSize unsigned int p newSize          I don t care if it s null because delete is smart enough to deal with that      delete myArray      myArray   new int p newSize       ASSERT myArray

[c++] Dynamically allocating an array of objects

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