How do I use a custom deleter with a std unique ptr member

Question

I have a class with a unique ptr member   class Foo   private      std  unique ptr lt Bar gt  bar               The Bar is a third party class that has a create   function and a destroy   function   If I wanted to use a std  unique ptr with it in a stand alone function I could do   void foo         std  unique ptr lt Bar  void    Bar   gt  bar create       Bar  b   destroy b                   Is there a way to do this with std  unique ptr as a member of a class

User · Answer

You can simply use std  bind with a your destroy function   std  unique ptr lt Bar  std  function lt void Bar   gt  gt  bar create    std  bind  amp destroy      std  placeholders   1      But of course you can also use a lambda   std  unique ptr lt Bar  std  function lt void Bar   gt  gt  ptr create       Bar  b   destroy b

User · Answer

It s possible to do this cleanly using a lambda in C  11  tested in G   4 8 2    Given this reusable typedef   template lt typename T gt  using deleted unique ptr   std  unique ptr lt T std  function lt void T   gt  gt     You can write   deleted unique ptr lt Foo gt  foo new Foo       Foo  f    customdeleter f           For example  with a FILE    deleted unique ptr lt FILE gt  file      fopen  file txt    r           FILE  f    fclose f         With this you get the benefits of exception-safe cleanup using RAII  without needing try catch noise

User · Answer

Assuming that create and destroy are free functions  which seems to be the case from the OP s code snippet  with the following signatures   Bar  create    void destroy Bar      You can write your class Foo like this  class Foo        std  unique ptr lt Bar  void    Bar   gt  ptr                public       Foo     ptr  create    destroy                                 Notice that you don t need to write any lambda or custom deleter here because destroy is already a deleter

User · Answer

You just need to create a deleter class   struct BarDeleter     void operator   Bar  b    destroy b          and provide it as the template argument of unique ptr  You ll still have to initialize the unique ptr in your constructors   class Foo     public      Foo     bar create                    private      std  unique ptr lt Bar  BarDeleter gt  bar               As far as I know  all the popular c   libraries implement this correctly  since BarDeleter  doesn t actually have any state  it does not need to occupy any space in the unique ptr

User · Answer

You know  using a custom deleter isn t the best way to go  as you will have to mention it all over your code  Instead  as you are allowed to add specializations to namespace-level classes in   std as long as custom types are involved and you respect the semantics  do that  Specialize std  default delete  template  lt  gt  struct   std  default delete lt Bar gt        default delete     default      template  lt class U gt      constexpr default delete default delete lt U gt   noexcept        void operator   Bar  p  const noexcept   destroy p         And maybe also do std  make unique    template  lt  gt  inline   std  unique ptr lt Bar gt    std  make unique lt Bar gt          auto p   create        if   p          throw std  runtime error  quot Could not  create    a new  Bar   quot        return   p

User · Answer

Unless you need to be able to change the deleter at runtime  I would strongly recommend using a custom deleter type  For example  if use a function pointer for your deleter  sizeof unique ptr lt T  fptr gt      2   sizeof T    In other words  half of the bytes of the unique ptr object are wasted   Writing a custom deleter to wrap every function is a bother  though  Thankfully  we can write a type templated on the function   Since C  17   template  lt auto fn gt  using deleter from fn   std  integral constant lt decltype fn   fn gt    template  lt typename T  auto fn gt  using my unique ptr   std  unique ptr lt T  deleter from fn lt fn gt  gt       usage  my unique ptr lt Bar  destroy gt  p create       Prior to C  17   template  lt typename D  D fn gt  using deleter from fn   std  integral constant lt D  fn gt    template  lt typename T  typename D  D fn gt  using my unique ptr   std  unique ptr lt T  deleter from fn lt D  fn gt  gt       usage  my unique ptr lt Bar  decltype destroy   destroy gt  p create

[c++] How do I use a custom deleter with a std::unique_ptr member?

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