Re: [boost] [smart_ptr] Is there any interest in unique_ptr with type erased deleter?

Subject: Re: [boost] [smart_ptr] Is there any interest in unique_ptr with type erased deleter?
From: Richard Hodges (hodges.r_at_[hidden])
Date: 2017-03-21 09:00:22

It’s an interesting and, on the face of it, obviously useful idea. What is required, more than simply creating a type-erased deleter? Isn’t this simply a partial specialisation of std::unique_ptr?

> On 21 Mar 2017, at 08:04, Andrey Davydov via Boost <boost_at_[hidden]> wrote:
> 
> It is widely known that deleter type is part of std::unique_ptr type. It is
> the most effective decision, but sometimes not very convenient. The most
> important use cases (for me) when std::unique_ptr<T,
> std::default_delete<T>> doesn't work are following:
> 1. unique_ptr to incomplete class
> struct MyClass;
> unique_ptr<MyClass> create();
> auto ptr = create(); // compilation error
> 
> 2. upcast to base without polymorphic destructor
> struct Base
> {
>  // ...
>  // no virtual ~Base() here
> };
> struct Derived : Base
> {
>  // ...
> };
> unique_ptr<Base> ptr = make_unique<Derived>(); // compiles by leaks!
> 
> 3. unique_ptr with non-default deleter
> unique_ptr<FILE, int (FILE *)> open_file(std::string const & path)
> // works but looks ugly, why should the fact that fclose returns `int` be
> visible from the signature of the function `open_path`?
> {
>  return { std::fopen(path, "r"), &std::fclose };
> }
> 
> Of course, all this examples could be fixed if unique_ptr would be replaced
> by shared_ptr, but the semantic of the shared ownership is not desirable
> often.
> I have implemented smart pointer with semantics of unique ownership and
> type erased deleter (https://github.com/AndreyG/unnamed). It has size of 3
> pointers and it doesn't require additional memory allocations for the case
> when deleter is empty class (for example, std::default_delete or
> non-capturing lambda) and for the case when deleter is known at compile
> time function (for instance, fclose). The are some simple examples of usage
> and tests in the repo.
> 
> Why don't just use unique_ptr<T, function<void (void *>>? The main reason
> is that due to the small object optimization std::function has rather big
> size. For instance, sizeof(unique_ptr<T, function<void (void *>>) == 72 for
> MSVC x64! Results for the other platforms can be found in the repo README.
> 
> Does it seem useful?
> 
> -- 
> Andrey Davydov
> 
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