[LLVMdev] New type of smart pointer for LLVM

[Justin Bogner]

Anton Yartsev <anton.yartsev at gmail.com> writes:
> Hello everyone,
>
> I bring to discussion the necessity/design of a new type of smart pointer.
> r215176 and r217791 rise the problem, D5443 is devoted to the solution.
> r215176 applies several temporary ugly fixes of memory leaks in TGParser.cpp 
> which would be great to be refactored using smart pointers. D5443 demonstrates
> how the solution with a certain type of smart pointer would look like (see
> changes in TGParser::ParseDef(), TGParser::InstantiateMulticlassDef() and
> TGParser::ParseSimpleValue()).
>
> Briefly:
> consider a leaky example:
> {
>   T* p = new T;
>   if (condition1) {
>     f(p); // takes ownership of p
>   }
>   p->SomeMethod();
>
>   if (condition2) {
>     return nullptr; // Leak!
>   }
>
>   g(p); // don't take ownership of p
>   return p;
> }
>
> The preferred solution would look like:
> {
>   smart_ptr<T> p(new T);
>   if (condition1) {
>     f(p.StopOwn()); // takes ownership of p

So this takes ownership, but promises not to destroy the pointee in some
way?

>   }
>   p->SomeMethod();
>
>   if (condition2) {
>     return nullptr; //

I guess p is sometimes destroyed here, depending on condition1?

>   }
>
>   g(p.Get());  // don't take ownership of p
>   return p.StopOwn();
> }

What does it mean to stop owning the pointer twice? Doesn't this leak p
in the case where condition1 was false?

> Neither unique_ptr nor shared_ptr can be used in the place of smart_ptr as
> unique_ptr sets the raw pointer to nullptr after release() (StopOwn() in the
> example above) whereas shared_ptr is unable to release.

I don't understand why shared_ptr wouldn't suffice for the example
above. There are two cases in your example where you try to release the
pointer - in one of them it seems you don't actually want to release it,
because you continue using it after, and in the other the scope ends
immediately after the release. The shared_ptr releases when it goes out
of scope, so it seems to be exactly what you want here.

Maybe I'm just misunderstanding the use case here, but I fear that a
type like this one would just serve to paper over problems where the
ownership is very unclear, without really helping much.

> Attached is a scratch that illustrates how the minimal
> API/implementation of a desired smart pointer sufficient for
> refactoring would look like. Any ideas and suggestions are
> appreciated.
>
> //===-- CondOwnershipPtr.h - Smart ptr with conditional release -*- C++ -*-===//
> //
> //                     The LLVM Compiler Infrastructure
> //
> // This file is distributed under the University of Illinois Open Source
> // License. See LICENSE.TXT for details.
> //===----------------------------------------------------------------------===//
> 
> #ifndef LLVM_ADT_CONDOWNERSHIPPTR_H
> #define LLVM_ADT_CONDOWNERSHIPPTR_H
> 
> namespace llvm {
> 
> template<class T> 
> class CondOwnershipPtr {
>   T* Ptr;
>   bool Own;
> 
>   void Delete() {
>     if (Ptr && !Own)
>       delete Ptr;
>   }

This seems to delete the pointer iff we *don't* own it. I think you have
that backwards...

> public:
>   CondOwnershipPtr() : Ptr(nullptr), Own(true) {}
>   explicit CondOwnershipPtr(T* p) : Ptr(p), Own(true) {}
> 
>   ~CondOwnershipPtr() {
>     Delete();
>   }
> 
>   T* Get() const {
>     return Ptr;
>   }
> 
>   T* StopOwn() const {
>     Own = false;
>     return Ptr;
>   }
> 
>   void Reset(T* P = nullptr) {
>     if (P != Ptr) {
>       Delete();
>       Ptr = P;
>       Own = true;
>     }
>   }
> 
>   bool Owns() const {
>     return Own;
>   }
> 
>   operator bool() const {
>     return Ptr != nullptr;
>   }
> 
>   T& operator*() const {
>     return *Ptr;
>   }
> 
>   T* operator->() const {
>     return Ptr;
>   }
> };
> 
> } // end namespace llvm
> 
> #endif