[vlc-devel] [PATCH] include: add C++ wrapper for C shared pointers

[Romain Vimont]

On Wed, Sep 12, 2018 at 03:08:35PM +0800, Zhao Zhili wrote:
> 
> 
> On 2018年09月11日 21:44, Romain Vimont wrote:
> > Add a helper to create RAII wrappers for C shared pointers, which
> > automatically call the Hold() and Release() functions associated to the
> > raw pointer.
> > 
> > Just declare a new shared pointer wrapper type:
> > 
> >      using InputItemPtr = vlc_shared_ptr_type(input_item_t,
> >                                               input_item_Hold,
> >                                               input_item_Release);
> > 
> > Then use this new type directly:
> > 
> >      input_item_t *ptr = /* ... */;
> >      InputItemPtr item(ptr);
> >      QString name = item->psz_name;
> >      InputItemPtr other = item; /* hold automatically */
> > ---
> >   include/vlc_cxx_helpers.hpp | 63 +++++++++++++++++++++++++++++++++++++
> >   1 file changed, 63 insertions(+)
> > 
> > diff --git a/include/vlc_cxx_helpers.hpp b/include/vlc_cxx_helpers.hpp
> > index 832a964e53..464b71a60a 100644
> > --- a/include/vlc_cxx_helpers.hpp
> > +++ b/include/vlc_cxx_helpers.hpp
> > @@ -122,6 +122,69 @@ inline std::unique_ptr<T[], void (*)(void*)> wrap_carray( T* ptr ) noexcept
> >   } // anonymous namespace
> > +///
> > +/// Wraps a C shared pointer having associated Hold() and Release() functions
> > +//
> > +/// This is a RAII wrapper for C shared pointers (which are manually managed by
> > +/// calling explicitly their Hold() and Release() functions).
> > +///
> > +/// The Hold() and Release() functions must accept exactly one parameter having
> > +/// type T* (the raw pointer type). Their return type is irrelevant.
> > +///
> > +/// To create a new shared pointer wrapper type for my_type_t, simply declare:
> > +///
> > +///     using MyTypePtr =
> > +///             vlc_shared_ptr_type(my_type_t, my_type_Hold, my_type_Release);
> > +///
> > +/// Then use it to wrap a raw C pointer:
> > +///
> > +///     my_type_t *raw_ptr = /* ... */;
> > +///     MyTypePtr ptr(raw_ptr);
> > +
> > +// In C++17, the template declaration could be replaced by:
> > +//     template<typename T, auto HOLD, auto RELEASE>
> > +template <typename T, typename H, typename R, H HOLD, R RELEASE>
> > +class vlc_shared_ptr {
> > +    T *ptr;
> > +public:
> > +    explicit vlc_shared_ptr(T *ptr = nullptr) : ptr(ptr) { if (ptr) HOLD(ptr); }
> > +    vlc_shared_ptr(const vlc_shared_ptr &o) : vlc_shared_ptr(o.ptr) {}
> > +    vlc_shared_ptr(vlc_shared_ptr &&o) noexcept :
> > +        ptr(std::exchange(o.ptr, nullptr)) {}
> 
> std::exchange is introduced in C++14.

std::exchange(c++11, c++14); // ;-)

Thank you, I'll change that.

> 
> > +    ~vlc_shared_ptr() { if (ptr) RELEASE(ptr); }
> > +    vlc_shared_ptr &operator=(const vlc_shared_ptr &o)
> > +    {
> > +        reset(o.ptr);
> > +        return *this;
> > +    }
> > +    vlc_shared_ptr &operator=(vlc_shared_ptr &&o) noexcept
> > +    {
> > +        ptr = std::exchange(o.ptr, nullptr);
> > +        return *this;
> > +    }
> > +    bool operator==(const vlc_shared_ptr &o) const { return ptr == o.ptr; }
> > +    bool operator!=(const vlc_shared_ptr &o) const { return !(*this == o); }
> > +    operator bool() const { return ptr; }
> > +    T &operator*() { return *ptr; }
> > +    const T &operator*() const { return *ptr; }
> > +    T *operator->() { return ptr; }
> > +    const T *operator->() const { return ptr; }
> > +    T *get() { return ptr; }
> > +    const T *get() const { return ptr; }
> > +    void reset(T *newptr) {
> > +        if (newptr)
> > +            /* hold before in case ptr == newptr */
> > +            HOLD(newptr);
> > +        if (ptr)
> > +            RELEASE(ptr);
> > +        ptr = newptr;
> > +    }
> > +};
> > +
> > +// useful due to the unnecessarily complex template declaration before C++17
> > +#define vlc_shared_ptr_type(type, hold, release) \
> > +    vlc_shared_ptr<type, decltype(&hold), decltype(&release), &hold, &release>
> > +
> >   #ifdef VLC_THREADS_H_
> >   namespace threads
> 
> 
> 
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