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

[Romain Vimont]

The name is probably wrong: a shared pointer manages its own refcount,
while here the refcount is managed by the object itself.

On Tue, Sep 11, 2018 at 03:44:12PM +0200, 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)) {}
> +    ~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
> -- 
> 2.19.0.rc2
> 
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