Object array initialization without default constructor

Question

include  lt iostream gt  class Car   private    Car        int  no  public    Car int no           no no        void printNo           std  cout lt  lt  no lt  lt std  endl         void printCarNumbers Car  cars  int length        for int i   0  i lt length i             std  cout lt  lt cars i  printNo       int main       int userInput   10    Car  mycars   new Car userInput     for int i  0 i  lt  userInput i             mycars i  new Car i 1     printCarNumbers mycars userInput     return 0          I want to create a car array but I get the following error   cartest cpp  In function    int main       cartest cpp 5  error     Car  Car      is private cartest cpp 21  error  within this context   is there a way to make this initialization without making Car   constructor public

User · Answer

You can use placement-new like this   class Car       int  no  public      Car int no     no no                  int main         void  raw memory   operator new   NUM CARS   sizeof Car        Car  ptr   static cast lt Car   gt  raw memory       for  int i   0  i  lt  NUM CARS    i            new  amp ptr i   Car i                 destruct in inverse order         for  int i   NUM CARS - 1  i  gt   0  --i            ptr i   Car              operator delete   raw memory        return 0      Reference from More Effective C   - Scott Meyers  Item 4 - Avoid gratuitous default constructors

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In C  11 s std  vector you can instantiate elements in-place using emplace back     std  vector lt Car gt  mycars     for  int i   0  i  lt  userInput    i            mycars emplace back i   1      pass in Car   constructor arguments       Voila   Car   default constructor never invoked   Deletion will happen automatically when mycars goes out of scope

User · Answer

Nope   But lo  If you use std  vector lt Car gt   like you should be  never ever use new     then you can specify exactly how elements should be constructed     Well sort of  You can specify the value of which to make copies of     Like this    include  lt iostream gt   include  lt vector gt   class Car   private      Car       if you don t use it  you can just declare it to make it private     int  no  public      Car int no         no no                   use an initialization list to initialize members             not the constructor body to assign them            void printNo                    use whitespace  itmakesthingseasiertoread         std  cout  lt  lt   no  lt  lt  std  endl            int main         int userInput   10          first method  userInput copies of Car 5      std  vector lt Car gt  mycars userInput  Car 5             second method      std  vector lt Car gt  mycars     empty     mycars reserve userInput      optional  reserve the memory upfront      for  int i   0  i  lt  userInput    i          mycars push back Car i       ith element is a copy of this         return 0 is implicit on main s with no return statement         useful for snippets and short code samples      With the additional function   void printCarNumbers Car  cars  int length        for int i   0  i  lt  length  i       whitespace              std  cout  lt  lt  cars i  printNo       int main                     printCarNumbers  amp mycars 0   mycars size          Note printCarNumbers really should be designed differently  to accept two iterators denoting a range

User · Answer

You can create an array of pointers   Car   mycars   new Car  userInput   for  int i 0  i lt userInput  i         mycars i    new Car               for  int i 0  i lt userInput  i         delete mycars i     delete    mycars    or  Car   constructor does not need to be public  Add a static method to your class that builds an array   static Car  makeArray int length       return new Car length

User · Answer

One way to solve is to give a static factory method to allocate the array if for some reason you want to give constructor private    static Car   Car  CreateCarArray int dimensions    But why are you keeping one constructor public and other private   But anyhow one more way is to declare the public constructor with default value    define DEFAULT CAR INIT 0 Car  Car int  no DEFAULT CAR INIT

User · Answer

Good question  I had the same question  and found it here  The real answer is   Dan-Paradox  there is no standard syntactical way of doing it  So  all these answers are a variety of alternatives to get around the problem   I read the answers myself  and didn t particularly find any of them perfect for my personal convention  The method that I ll probably stick with is using a default constructor and a set method   class MyClass     int x y z  public    MyClass    x 0   y 0   z 0       MyClass int  x int  y int  z   x  x   y  y   z  z        for single declarations   void set int  x int  y int  z          x  x      y  y      z  z           The standard initialization constructor is still there  so I can still initialize it normally if I don t need more than one  but if otherwise  I have a set method which sets all the variables that are initialized in the constructor  Thus I could do something like this   int len   25  MyClass list   new MyClass len   for int i   0  i  lt  len  i      list i  set 1  2  3     This works fine and flows naturally  without making code look confusing     Now that s my answer for those wondering how to declare an array of objects that need to be initialized   For you specifically  you re trying to give an array of cars identities  which I d suppose you want to always be unique  You could do it with my method I explained above  and then in the for loop use i 1 as the argument sent to the set method - but from what I ve read in your comments  it seems like you want the ids more internally initiated  so that by default each Car has a unique id  even if someone else uses your class Car   If this is what you want  you can use a static member   class Car     static int current id    int id  public    Car    id current id          int getId     return id       int Car  current id   1           int cars 10  Car  carlist   new Car cars    for int i   0  i  lt  cars  i      cout  lt  lt  carlist i  getId    lt  lt          prints  1 2 3 4 5 6 7 8 9 10    In this way  you don t have to worry at all about initiating the identities since they are managed internally

User · Answer

No  there isn t  New-expression only allows default initialization or no initialization at all   The workaround would be to allocate raw memory buffer using operator new   and then construct objects in that buffer using placement-new with non-default constructor

User · Answer

I don t think there s type-safe method that can do what you want

User · Answer

You can always create an array of pointers   pointing to car objects and then create objects  in a for loop  as you want and save their address in the array   for example      include  lt iostream gt  class Car   private    Car        int  no  public    Car int no           no no        void printNo           std  cout lt  lt  no lt  lt std  endl         void printCarNumbers Car  cars  int length        for int i   0  i lt length i             std  cout lt  lt cars i  printNo       int main       int userInput   10    Car   mycars   new Car  userInput     int i    for i 0 i lt userInput i          mycars i    new Car i 1     note new method         printCarNumbers new mycars userInput       return 0          All you have to change in new method is handling cars as pointers than static objects in parameter and when calling method printNo   for example    void printCarNumbers new Car   cars  int length        for int i   0  i lt length i             std  cout lt  lt cars i - gt printNo        at the end of main is good to delete all dynamicly allocated memory like this  for i 0 i lt userInput i      delete mycars i          deleting one obgject delete   mycars            deleting array of objects   Hope I helped  cheers

User · Answer

My way  Car   cars      else were  extern Car   cars   void main            COLORS    id     cars   new Car 3            Car BLUE               Car RED               Car GREEN

User · Answer

You can use in-place operator new  This would be a bit horrible  and I d recommend keeping in a factory   Car  createCars unsigned number        if  number    0           return 0      Car  cars   reinterpret cast lt Car  gt  new char sizeof Car   number        for unsigned carId   0          carId    number            carId                new cars carId  Car carId             return cars      And define a corresponding destroy so as to match the new used in this

[c++] Object array initialization without default constructor

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