src/cpp-common/bt2/internal/borrowed-obj.hpp

   1 /*
   2  * Copyright 2019-2020 (c) Philippe Proulx <pproulx@efficios.com>
   3  *
   4  * SPDX-License-Identifier: MIT
   5  */
   6
   7 #ifndef BABELTRACE_CPP_COMMON_BT2_INTERNAL_BORROWED_OBJ_HPP
   8 #define BABELTRACE_CPP_COMMON_BT2_INTERNAL_BORROWED_OBJ_HPP
   9
  10 #include <type_traits>
  11
  12 #include "common/assert.h"
  13
  14 namespace bt2 {
  15 namespace internal {
  16
  17 template <typename ObjT, typename LibObjT, typename RefFuncsT>
  18 class SharedObj;
  19
  20 /*
  21  * An instance of this class wraps a pointer to a libbabeltrace2 object
  22  * of type `LibObjT` without managing any reference counting.
  23  *
  24  * This is an abstract base class for any libbabeltrace2 object wrapper.
  25  *
  26  * `LibObjT` is the direct libbabeltrace2 object type, for example
  27  * `bt_stream_class` or `const bt_value`.
  28  *
  29  * Methods of a derived class can call _libObjPtr() to access the
  30  * libbabeltrace2 object pointer.
  31  */
  32 template <typename LibObjT>
  33 class BorrowedObj
  34 {
  35     static_assert(!std::is_pointer<LibObjT>::value, "`LibObjT` must not be a pointer");
  36
  37     /*
  38      * This makes it possible for a `BorrowedObj<const bt_something>`
  39      * instance to get assigned an instance of
  40      * `BorrowedObj<bt_something>` (copy constructor and assignment
  41      * operator).
  42      *
  43      * C++ forbids the other way around.
  44      */
  45     template <typename AnyLibObjT>
  46     friend class BorrowedObj;
  47
  48     /*
  49      * This is to allow a `SharedObj<_ThisBorrowedObj, LibObjT, ...>`
  50      * instance containing a `BorrowedObj<LibObjT>` instance to access
  51      * _libObjPtr() in order to increment/decrement its libbabeltrace2
  52      * reference count.
  53      */
  54     template <typename ObjT, typename AnyLibObjT, typename RefFuncsT>
  55     friend class SharedObj;
  56
  57 protected:
  58     // libbabeltrace2 object pointer
  59     using _LibObjPtr = LibObjT *;
  60
  61     // This complete borrowed object
  62     using _ThisBorrowedObj = BorrowedObj<LibObjT>;
  63
  64     /*
  65      * Builds a borrowed object to wrap the libbabeltrace2 object
  66      * pointer `libObjPtr`.
  67      *
  68      * `libObjPtr` must not be `nullptr`.
  69      */
  70     explicit BorrowedObj(const _LibObjPtr libObjPtr) noexcept : _mLibObjPtr {libObjPtr}
  71     {
  72         BT_ASSERT(libObjPtr);
  73     }
  74
  75     /*
  76      * Generic copy constructor.
  77      *
  78      * This converting constructor accepts both an instance of
  79      * `_ThisBorrowedObj` and an instance (`other`) of
  80      * `BorrowedObj<ConstLibObjT>`, where `ConstLibObjT` is the `const`
  81      * version of `LibObjT`, if applicable.
  82      *
  83      * This makes it possible for a `BorrowedObj<const bt_something>`
  84      * instance to be built from an instance of
  85      * `BorrowedObj<bt_something>`. C++ forbids the other way around.
  86      */
  87     template <typename OtherLibObjT>
  88     BorrowedObj(const BorrowedObj<OtherLibObjT>& other) noexcept : BorrowedObj {other._mLibObjPtr}
  89     {
  90     }
  91
  92     /*
  93      * Generic assignment operator.
  94      *
  95      * This operator accepts both an instance of
  96      * `_ThisBorrowedObj` and an instance (`other`) of
  97      * `BorrowedObj<ConstLibObjT>`, where `ConstLibObjT` is the `const`
  98      * version of `LibObjT`, if applicable.
  99      *
 100      * This makes it possible for a `BorrowedObj<const bt_something>`
 101      * instance to get assigned an instance of
 102      * `BorrowedObj<bt_something>`. C++ forbids the other way around.
 103      */
 104     template <typename OtherLibObjT>
 105     _ThisBorrowedObj& operator=(const BorrowedObj<OtherLibObjT>& other) noexcept
 106     {
 107         _mLibObjPtr = other._mLibObjPtr;
 108         return *this;
 109     }
 110
 111     // Wrapped libbabeltrace2 object pointer
 112     _LibObjPtr _libObjPtr() const noexcept
 113     {
 114         return _mLibObjPtr;
 115     }
 116
 117 private:
 118     _LibObjPtr _mLibObjPtr;
 119 };
 120
 121 } // namespace internal
 122 } // namespace bt2
 123
 124 #endif // BABELTRACE_CPP_COMMON_BT2_INTERNAL_BORROWED_OBJ_HPP