Example to use shared ptr

Question

Hi I asked a question today about How to insert different types of objects in the same vector array  and my code in that question was    gate  G 1000   G 0    new ANDgate     G 1    new ORgate    gate is a class inherited by ANDgate and ORgate classes class gate                   virtual void Run            A virtual function         class ANDgate  public gate                         void Run              AND version of Run             class ORgate  public gate                         void Run              OR version of Run                    Running the simulator using overloading concept  for                  G i - gt Run        will run perfectly the right Run for the right Gate type       and I wanted to use vectors so someone wrote that I should do that    std  vector lt gate  gt  G  G push back new ANDgate    G push back new ORgate   for unsigned i 0 i lt G size     i      G i - gt Run        but then he and many others suggested that I would better use Boost pointer containers  or shared ptr  I have spent the last 3 hours reading about this topic  but the documentation seems pretty advanced to me       Can anyone give me a small code example of shared ptr usage and why they suggested using shared ptr  Also are there other types like ptr vector  ptr list and ptr deque        Edit1  I have read a code example too that included   typedef boost  shared ptr lt Foo gt  FooPtr          int main       std  vector lt FooPtr gt          foo vector           FooPtr foo ptr  new Foo  2        foo vector push back  foo ptr                    And I don t understand the syntax

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include  lt memory gt   include  lt iostream gt   class SharedMemory       public           SharedMemory int  x   capture x            int  get     return   capture get           protected          std  shared ptr lt int gt   capture      int main int   char         SharedMemory   obj1  new SharedMemory new int 10        SharedMemory   obj2   new SharedMemory   obj1       std  cout  lt  lt     obj1     lt  lt    obj1- gt get    lt  lt     obj2     lt  lt    obj2- gt get        lt  lt  std  endl      delete  obj2       std  cout  lt  lt     obj1     lt  lt    obj1- gt get    lt  lt  std  endl      delete  obj1      std  cout  lt  lt    done    lt  lt  std  endl      This is an example of shared ptr in action   obj2 was deleted but pointer is still valid  output is    test   obj1  10  obj2  10   obj2  10  done

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Learning to use smart pointers is in my opinion one of the most important steps to become a competent C   programmer  As you know whenever you new an object at some point you want to delete it    One issue that arise is that with exceptions it can be very hard to make sure a object is always released just once in all possible execution paths   This is the reason for RAII  http   en wikipedia org wiki RAII  Making a helper class with purpose of making sure that an object always deleted once in all execution paths    Example of a class like this is  std  auto ptr  But sometimes you like to share objects with other  It should only be deleted when none uses it anymore   In order to help with that reference counting strategies have been developed but you still need to remember addref and release ref manually  In essence this is the same problem as new delete   That s why boost has developed boost  shared ptr  it s reference counting smart pointer so you can share objects and not leak memory unintentionally   With the addition of C   tr1 this is now added to the c   standard as well but its named std  tr1  shared ptr lt     I recommend using the standard shared pointer if possible  ptr list  ptr dequeue and so are IIRC specialized containers for pointer types  I ignore them for now   So we can start from your example   std  vector lt gate  gt  G   G push back new ANDgate     G push back new ORgate    for unsigned i 0 i lt G size     i        G i - gt Run          The problem here is now that whenever G goes out scope we leak the 2 objects added to G  Let s rewrite it to use std  tr1  shared ptr     Remember to include  lt memory gt  for shared ptr    First do an alias for std  tr1  shared ptr lt gate gt  so we don t have to     type that in every place  Call it gate ptr  This is what typedef does  typedef std  tr1  shared ptr lt gate gt  gate ptr         gate ptr is now our  smart  pointer  So let s make a vector out of it  std  vector lt gate ptr gt  G      these smart ptrs can t be implicitly created from gate  we have to be explicit about it    gate ptr  new ANDgate   it s a good thing  G push back gate ptr  new ANDgate      G push back gate ptr  new ORgate     for unsigned i 0 i lt G size     i         G i - gt Run          When G goes out of scope the memory is automatically reclaimed   As an exercise which I plagued newcomers in my team with is asking them to write their own smart pointer class  Then after you are done discard the class immedietly and never use it again  Hopefully you acquired crucial knowledge on how a smart pointer works under the hood  There s no magic really

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Using a vector of shared ptr removes the possibility of leaking memory because you forgot to walk the vector and call delete on each element   Let s walk through a slightly modified version of the example line-by-line   typedef boost  shared ptr lt gate gt  gate ptr    Create an alias for the shared pointer type   This avoids the ugliness in the C   language that results from typing std  vector lt boost  shared ptr lt gate gt   gt  and forgetting the space between the closing greater-than signs       std  vector lt gate ptr gt  vec    Creates an empty vector of boost  shared ptr lt gate gt  objects       gate ptr ptr new ANDgate     Allocate a new ANDgate instance and store it into a shared ptr   The reason for doing this separately is to prevent a problem that can occur if an operation throws   This isn t possible in this example   The Boost shared ptr  Best Practices  explain why it is a best practice to allocate into a free-standing object instead of a temporary       vec push back ptr     This creates a new shared pointer in the vector and copies ptr into it   The reference counting in the guts of shared ptr ensures that the allocated object inside of ptr is safely transferred into the vector   What is not explained is that the destructor for shared ptr lt gate gt  ensures that the allocated memory is deleted   This is where the memory leak is avoided   The destructor for std  vector lt T gt  ensures that the destructor for T is called for every element stored in the vector   However  the destructor for a pointer  e g   gate   does not delete the memory that you had allocated   That is what you are trying to avoid by using shared ptr or ptr vector

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The best way to add different objects into same container is to use make shared  vector  and range based loop and you will have a nice  clean and  readable  code   typedef std  shared ptr lt gate gt  Ptr    vector lt Ptr gt  myConatiner   auto andGate   std  make shared lt ANDgate gt     myConatiner push back andGate    auto orGate  std  make shared lt ORgate gt     myConatiner push back orGate    for  auto amp  element   myConatiner      element- gt run

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I will add that one of the important things about shared ptr s is to only ever construct them with the following syntax   shared ptr lt Type gt  new Type          This way  the  real  pointer to Type is anonymous to your scope  and held only by the shared pointer   Thus it will be impossible for you to accidentally use this  real  pointer   In other words  never do this   Type  t ptr   new Type       shared ptr lt Type gt  t sptr ptrT t ptr     t ptr is still hanging around   Don t use it    Although this will work  you now have a Type  pointer  t ptr  in your function which lives outside the shared pointer   It s dangerous to use t ptr anywhere  because you never know when the shared pointer which holds it may destruct it  and you ll segfault   Same goes for pointers returned to you by other classes   If a class you didn t write hands you a pointer  it s generally not safe to just put it in a shared ptr   Not unless you re sure that the class is no longer using that object   Because if you do put it in a shared ptr  and it falls out of scope  the object will get freed when the class may still need it

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The boost documentation provides a pretty good start example  shared ptr example  it s actually about a vector of smart pointers  or  shared ptr doc The following answer by Johannes Schaub explains the boost smart pointers pretty well  smart pointers explained  The idea behind in as few words as possible  ptr vector is that it handles the deallocation of memory behind the stored pointers for you  let s say you have a vector of pointers as in your example  When quitting the application or leaving the scope in which the vector is defined you ll have to clean up after yourself you ve dynamically allocated ANDgate and ORgate  but just clearing the vector won t do it because the vector is storing the pointers and not the actual objects it won t destroy but what it contains        if you just do  G clear      will clear the vector but you ll be left with 2 memory leaks         to properly clean the vector and the objects behind it for  std  vector lt gate  gt   iterator it   G begin    it    G end    it        delete   it       boost  ptr vector lt   will handle the above for you - meaning it will deallocate the memory behind the pointers it stores

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Through Boost you can do it    std  vector lt boost  any gt  vecobj      boost  shared ptr lt string gt  sharedString1 new string  abcdxyz              boost  shared ptr lt int gt  sharedint1 new int 10        vecobj push back sharedString1       vecobj push back sharedint1       for inserting different object type in your vector container  while for accessing you have to use any cast  which works like dynamic cast  hopes it will work for your need

[c++] Example to use shared_ptr?

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