system\core\libutils\include\utils\StrongPointer.h
#ifndef ANDROID_STRONG_POINTER_H
#define ANDROID_STRONG_POINTER_H
#include <functional>
#include <type_traits> // for common_type.
// —————————————————————————
namespace android {
template<typename T> class wp;
// —————————————————————————
template<typename T>
class sp {
public:
inline sp() : m_ptr(nullptr) { }
sp(T* other); // NOLINT(implicit)
sp(const sp<T>& other);
sp(sp<T>&& other) noexcept;
template<typename U> sp(U* other); // NOLINT(implicit) //!– 模板构造函数申明
template<typename U> sp(const sp<U>& other); // NOLINT(implicit)
template<typename U> sp(sp<U>&& other); // NOLINT(implicit)
~sp();
// Assignment
sp& operator = (T* other); //!–重载操作符=函数申明
sp& operator = (const sp<T>& other);
sp& operator=(sp<T>&& other) noexcept;
template<typename U> sp& operator = (const sp<U>& other);//!–重载操作符=模板函数申明
template<typename U> sp& operator = (sp<U>&& other);
template<typename U> sp& operator = (U* other);
//! Special optimization for use by ProcessState (and nobody else).
void force_set(T* other);
// Reset
void clear();
// Accessors
inline T& operator* () const { return *m_ptr; } //!–重载操作符*函数
inline T* operator-> () const { return m_ptr; } //!–重载操作符->函数
inline T* get() const { return m_ptr; }
inline explicit operator bool () const { return m_ptr != nullptr; }
// Punt these to the wp<> implementation.
template<typename U>
inline bool operator == (const wp<U>& o) const {
return o == *this;
}
template<typename U>
inline bool operator != (const wp<U>& o) const {
return o != *this;
}
private:
template<typename Y> friend class sp;
template<typename Y> friend class wp;
void set_pointer(T* ptr);
static inline void check_not_on_stack(const void* ptr);
T* m_ptr; //!–模板成员变量
};
#define COMPARE_STRONG(_op_) \
template <typename T, typename U> \
static inline bool operator _op_(const sp<T>& t, const sp<U>& u) { \
return t.get() _op_ u.get(); \
} \
template <typename T, typename U> \
static inline bool operator _op_(const T* t, const sp<U>& u) { \
return t _op_ u.get(); \
} \
template <typename T, typename U> \
static inline bool operator _op_(const sp<T>& t, const U* u) { \
return t.get() _op_ u; \
} \
template <typename T> \
static inline bool operator _op_(const sp<T>& t, std::nullptr_t) { \
return t.get() _op_ nullptr; \
} \
template <typename T> \
static inline bool operator _op_(std::nullptr_t, const sp<T>& t) { \
return nullptr _op_ t.get(); \
}
template <template <typename C> class comparator, typename T, typename U>
static inline bool _sp_compare_(T* a, U* b) {
return comparator<typename std::common_type<T*, U*>::type>()(a, b);
}
#define COMPARE_STRONG_FUNCTIONAL(_op_, _compare_) \
template <typename T, typename U> \
static inline bool operator _op_(const sp<T>& t, const sp<U>& u) { \
return _sp_compare_<_compare_>(t.get(), u.get()); \
} \
template <typename T, typename U> \
static inline bool operator _op_(const T* t, const sp<U>& u) { \
return _sp_compare_<_compare_>(t, u.get()); \
} \
template <typename T, typename U> \
static inline bool operator _op_(const sp<T>& t, const U* u) { \
return _sp_compare_<_compare_>(t.get(), u); \
} \
template <typename T> \
static inline bool operator _op_(const sp<T>& t, std::nullptr_t) { \
return _sp_compare_<_compare_>(t.get(), nullptr); \
} \
template <typename T> \
static inline bool operator _op_(std::nullptr_t, const sp<T>& t) { \
return _sp_compare_<_compare_>(nullptr, t.get()); \
}
COMPARE_STRONG(==)
COMPARE_STRONG(!=)
COMPARE_STRONG_FUNCTIONAL(>, std::greater)
COMPARE_STRONG_FUNCTIONAL(<, std::less)
COMPARE_STRONG_FUNCTIONAL(<=, std::less_equal)
COMPARE_STRONG_FUNCTIONAL(>=, std::greater_equal)
#undef COMPARE_STRONG
#undef COMPARE_STRONG_FUNCTIONAL
// For code size reasons, we do not want these inlined or templated.
void sp_report_race();
void sp_report_stack_pointer();
// —————————————————————————
// No user serviceable parts below here.
// Check whether address is definitely on the calling stack. We actually check whether it is on
// the same 4K page as the frame pointer.
//
// Assumptions:
// – Pages are never smaller than 4K (MIN_PAGE_SIZE)
// – Malloced memory never shares a page with a stack.
//
// It does not appear safe to broaden this check to include adjacent pages; apparently this code
// is used in environments where there may not be a guard page below (at higher addresses than)
// the bottom of the stack.
//
// TODO: Consider adding make_sp<T>() to allocate an object and wrap the resulting pointer safely
// without checking overhead.
template <typename T>
void sp<T>::check_not_on_stack(const void* ptr) {
static constexpr int MIN_PAGE_SIZE = 0x1000; // 4K. Safer than including sys/user.h.
static constexpr uintptr_t MIN_PAGE_MASK = ~static_cast<uintptr_t>(MIN_PAGE_SIZE – 1);
uintptr_t my_frame_address =
reinterpret_cast<uintptr_t>(__builtin_frame_address(0 /* this frame */));
if (((reinterpret_cast<uintptr_t>(ptr) ^ my_frame_address) & MIN_PAGE_MASK) == 0) {
sp_report_stack_pointer();
}
}
template<typename T>
sp<T>::sp(T* other)
: m_ptr(other) {
if (other) {
check_not_on_stack(other);
other->incStrong(this);
}
}
template<typename T>
sp<T>::sp(const sp<T>& other)
: m_ptr(other.m_ptr) {
if (m_ptr)
m_ptr->incStrong(this);
}
template <typename T>
sp<T>::sp(sp<T>&& other) noexcept : m_ptr(other.m_ptr) {
other.m_ptr = nullptr;
}
template<typename T> template<typename U>
sp<T>::sp(U* other)
: m_ptr(other) {
if (other) {
check_not_on_stack(other);
(static_cast<T*>(other))->incStrong(this);
}
}
template<typename T> template<typename U>
sp<T>::sp(const sp<U>& other)
: m_ptr(other.m_ptr) {
if (m_ptr)
m_ptr->incStrong(this);
}
template<typename T> template<typename U>
sp<T>::sp(sp<U>&& other)
: m_ptr(other.m_ptr) {
other.m_ptr = nullptr;
}
template<typename T>
sp<T>::~sp() {
if (m_ptr)
m_ptr->decStrong(this);
}
template<typename T>
sp<T>& sp<T>::operator =(const sp<T>& other) {
// Force m_ptr to be read twice, to heuristically check for data races.
T* oldPtr(*const_cast<T* volatile*>(&m_ptr));
T* otherPtr(other.m_ptr);
if (otherPtr) otherPtr->incStrong(this);
if (oldPtr) oldPtr->decStrong(this);
if (oldPtr != *const_cast<T* volatile*>(&m_ptr)) sp_report_race();
m_ptr = otherPtr;
return *this;
}
template <typename T>
sp<T>& sp<T>::operator=(sp<T>&& other) noexcept {
T* oldPtr(*const_cast<T* volatile*>(&m_ptr));
if (oldPtr) oldPtr->decStrong(this);
if (oldPtr != *const_cast<T* volatile*>(&m_ptr)) sp_report_race();
m_ptr = other.m_ptr;
other.m_ptr = nullptr;
return *this;
}
template<typename T>
sp<T>& sp<T>::operator =(T* other) {
T* oldPtr(*const_cast<T* volatile*>(&m_ptr));
if (other) {
check_not_on_stack(other);
other->incStrong(this);
}
if (oldPtr) oldPtr->decStrong(this);
if (oldPtr != *const_cast<T* volatile*>(&m_ptr)) sp_report_race();
m_ptr = other;
return *this;
}
template<typename T> template<typename U>
sp<T>& sp<T>::operator =(const sp<U>& other) {
T* oldPtr(*const_cast<T* volatile*>(&m_ptr));
T* otherPtr(other.m_ptr);
if (otherPtr) otherPtr->incStrong(this);
if (oldPtr) oldPtr->decStrong(this);
if (oldPtr != *const_cast<T* volatile*>(&m_ptr)) sp_report_race();
m_ptr = otherPtr;
return *this;
}
template<typename T> template<typename U>
sp<T>& sp<T>::operator =(sp<U>&& other) {
T* oldPtr(*const_cast<T* volatile*>(&m_ptr));
if (m_ptr) m_ptr->decStrong(this);
if (oldPtr != *const_cast<T* volatile*>(&m_ptr)) sp_report_race();
m_ptr = other.m_ptr;
other.m_ptr = nullptr;
return *this;
}
template<typename T> template<typename U>
sp<T>& sp<T>::operator =(U* other) {
T* oldPtr(*const_cast<T* volatile*>(&m_ptr));
if (other) (static_cast<T*>(other))->incStrong(this);
if (oldPtr) oldPtr->decStrong(this);
if (oldPtr != *const_cast<T* volatile*>(&m_ptr)) sp_report_race();
m_ptr = other;
return *this;
}
template<typename T>
void sp<T>::force_set(T* other) {
other->forceIncStrong(this);
m_ptr = other;
}
template<typename T>
void sp<T>::clear() {
T* oldPtr(*const_cast<T* volatile*>(&m_ptr));
if (oldPtr) {
oldPtr->decStrong(this);
if (oldPtr != *const_cast<T* volatile*>(&m_ptr)) sp_report_race();
m_ptr = nullptr;
}
}
template<typename T>
void sp<T>::set_pointer(T* ptr) {
m_ptr = ptr;
}
} // namespace android
// —————————————————————————
#endif // ANDROID_STRONG_POINTER_H
system\core\libutils\include\utils\LightRefBase.h
// This provides primarily wp<> weak pointer types and RefBase, which work
// together with sp<> from <StrongPointer.h>.
这里主要是wp<>和RefBase，其与sp<>一起工作；
// sp<> (and wp<>) are a type of smart pointer that use a well defined protocol
// to operate. As long as the object they are templated with implements that
// protocol, these smart pointers work. In several places the platform
// instantiates sp<> with non-RefBase objects; the two are not tied to each
// other.
// RefBase is such an implementation and it supports strong pointers, weak
// pointers and some magic features for the binder.
// So, when using RefBase objects, you have the ability to use strong and weak
// pointers through sp<> and wp<>.
sp<>和bp<>是一种定义好使用协议的智能指针；在有些场景使用sp<>而没有使用RefBase，它们两不是绑定的。RefBase 是一种支持sp<>、wp<>和bingder 特性而设计的，使用RefBase就能使用sp<>和bp<>来使用强弱指针
// Normally, when the last strong pointer goes away, the object is destroyed,
// i.e. it’s destructor is called. HOWEVER, parts of its associated memory is not
// freed until the last weak pointer is released.
正常情况当对象最后一个强引用解除时，对象就会销毁，即调用其构造函数，但是对象的对应的内存空间没有释放，知道最后的弱引用解除。
// Weak pointers are essentially “safe” pointers. They are always safe to
// access through promote(). They may return nullptr if the object was
// destroyed because it ran out of strong pointers. This makes them good candidates
// for keys in a cache for instance.
// Weak pointers remain valid for comparison purposes even after the underlying
// object has been destroyed. Even if object A is destroyed and its memory reused
// for B, A remaining weak pointer to A will not compare equal to one to B.
// This again makes them attractive for use as keys.
若引用时安全的指针，可以通过promote()将弱引用转化为强引用，如果在cache ，强引用解除了，就会返回nullptr。强引用解除对象被析构时，其内存能被另外使用，但是这个时候两者若引用时不相等的。
// How is this supposed / intended to be used?
// Our recommendation is to use strong references (sp<>) when there is an
// ownership relation. e.g. when an object “owns” another one, use a strong
// ref. And of course use strong refs as arguments of functions (it’s extremely
// rare that a function will take a wp<>).
强引用使用场景：从属（拥有）关系；函数参数
// Typically a newly allocated object will immediately be used to initialize
// a strong pointer, which may then be used to construct or assign to other
// strong and weak pointers.
// Use weak references when there are no ownership relation. e.g. the keys in a
// cache (you cannot use plain pointers because there is no safe way to acquire
// a strong reference from a vanilla pointer).
弱引用使用场景：没有从属关系；cache对象
// This implies that two objects should never (or very rarely) have sp<> on
// each other, because they can’t both own each other.
本文地址：https://blog.csdn.net/lei7143/article/details/109351449
