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Memory management of C libraries in C++ -- Kevin Dungs

By Blog Staff | Dec 4, 2014 09:20 PM | Tags: None

In the "better C than C" department:

Memory management of C libraries in C++

by Kevin Dungs

From the article:

Today I had some time to play around with libgit2, an excellent C-library for Git. But since I was thinking about using it together with the freshly released Proxygen, the fast HTTP-framework from Facebook, I wanted to use the library functions from C++ from the start.

This was a perfect opportunity to play around with C++ memory management objects in the context of a C-library...

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Cap'n Rotbart said on Dec 5, 2014 11:51 AM: 

    using uniqueRepositoryPtr = std::unique_ptr<

      git_repository, decltype(&git_repository_free)>;

    git_repository *rawRepo = nullptr;

    git_repository_open(&rawRepo, "./testrepo");

    uniqueRepositoryPtr repo{std::move(rawRepo), git_repository_free};

A small helper may reduce this by a line or two. Something like


template <typename T, typename DeleterT>

struct uptr_mover

{

 using unique_ptr = std::unique_ptr<T, DeleterT>;

 

 uptr_mover(unique_ptr& u) : u_ptr{&u} {}



 operator T**()

 {

  return &c_ptr;

 }



 ~uptr_mover()

 {

  if (u_ptr->get() != c_ptr) // Prevents double assign / self-reset

   u_ptr->reset(c_ptr);

 }



 T* c_ptr;

 std::unique_ptr<T>* u_ptr;

};



template <typename T, typename DeleterT>

uptr_mover<T, DeleterT> move_into_unique_ptr(std::unique_ptr<T, DeleterT>& u) // Perhaps questionable naming

{

 return uptr_mover<T, DeleterT>{ u };

}

would make this


std::unique_ptr<git_repository, decltype(&git_repository_free)> repo{0, git_repository_free};

git_repository_open(move_into_unique_ptr(repo), "./testrepo");

Also, wrapping the *_free functions in functors may help further - no need for both git_repository_free and decltype(&git_repository_free).