learn_class/modern_cpp_30/smart_ptr/shared_ptr.cpp - master - githubmirror/CPlusPlusThings

learn_class/modern_cpp_30/smart_ptr/shared_ptr.cpp
221 строка8 KB
zhangxing
support bazel complie this project and format code.
30 мар 2024, 17:00
30 мар 2024, 17:003c8a3f2
//
// Created by light on 19-12-12.
//

#include "../RAII/shape.h"
class shared_count {
public:
  shared_count() : count_(1) {}

  // 增加计数
  void add_count() { ++count_; }

  // 减少计数
  long reduce_count() { return --count_; }

  // 获取当前计数
  long get_count() const { return count_; }

private:
  long count_;
};

template <typename T> class shared_ptr {
public:
  explicit shared_ptr(T *ptr = nullptr) noexcept : ptr_(ptr) {
    if (ptr) {
      shared_count_ = new shared_count();
    }
  }

  // 实现强制类型转换需要的构造函数
  template <typename U>
  shared_ptr(const shared_ptr<U> &other, T *ptr) noexcept {
    ptr_ = ptr;
    if (ptr_) {
      other.shared_count_->add_count();
      shared_count_ = other.shared_count_;
    }
  }

  ~shared_ptr() noexcept {
    // 最后一个shared_ptr再去删除对象与共享计数
    // ptr_不为空且此时共享计数减为0的时候,再去删除
    if (ptr_ && !shared_count_->reduce_count()) {
      delete ptr_;
      delete shared_count_;
    }
  }

  T &operator*() const noexcept { return *ptr_; }

  T *operator->() const noexcept { return ptr_; }

  operator bool() const noexcept { return ptr_; }

  T *get() const noexcept { return ptr_; }

  // 带模板的拷贝与移动构造函数 模板的各个实例间并不天然就有 friend
  // 关系，因而不能互访私有成员 ptr_ 和 shared_count_。 需要下面显示声明
  template <typename U> friend class shared_ptr;

  template <typename U> shared_ptr(const shared_ptr<U> &other) noexcept {
    //        cout << "调用了带模板的拷贝构造!" << endl;
    ptr_ = other.ptr_;
    if (ptr_) {
      other.shared_count_->add_count();
      shared_count_ = other.shared_count_;
    }
  }

  template <typename U> shared_ptr(shared_ptr<U> &&other) noexcept {
    //        cout << "调用了带模板的移动构造!" << endl;
    ptr_ = other.ptr_;
    if (ptr_) {
      shared_count_ = other.shared_count_;
      other.ptr_ = nullptr;
      other.shared_count_ = nullptr;
    }
  }

  // copy and swap  始终只有一个对象有管理这块空间的权限
  shared_ptr &operator=(shared_ptr rhs) noexcept {
    rhs.swap(*this);
    return *this;
  }

  void swap(shared_ptr &rhs) noexcept {
    using std::swap;
    swap(ptr_, rhs.ptr_);
    swap(shared_count_, rhs.shared_count_);
  }

  long use_count() const noexcept {
    if (ptr_) {
      return shared_count_->get_count();
    } else {
      return 0;
    }
  }

private:
  T *ptr_;
  shared_count *shared_count_;
};

template <typename T>
void swap(shared_ptr<T> &lhs, shared_ptr<T> &rhs) noexcept {
  lhs.swap(rhs);
}

template <typename T, typename U>
shared_ptr<T> dynamic_pointer_cast(const shared_ptr<U> &other) noexcept {
  T *ptr = dynamic_cast<T *>(other.get());
  return shared_ptr<T>(other, ptr);
}

template <typename T, typename U>
shared_ptr<T> static_pointer_cast(const shared_ptr<U> &other) noexcept {
  T *ptr = static_cast<T *>(other.get());
  return shared_ptr<T>(other, ptr);
}

template <typename T, typename U>
shared_ptr<T> const_pointer_cast(const shared_ptr<U> &other) noexcept {
  T *ptr = const_cast<T *>(other.get());
  return shared_ptr<T>(other, ptr);
}

template <typename T, typename U>
shared_ptr<T> reinterpret_pointer_cast(const shared_ptr<U> &other) noexcept {
  T *ptr = reinterpret_cast<T *>(other.get());
  return shared_ptr<T>(other, ptr);
}

int main() {
  shared_ptr<circle> ptr1(new circle());
  cout << "use count of ptr1 is " << ptr1.use_count() << endl;
  shared_ptr<shape> ptr2, ptr3;
  cout << "use count of ptr2 was " << ptr2.use_count() << endl;
  ptr2 =
      ptr1; // shared_ptr<circle>隐式转换shared_ptr<shape> 调用带模板的拷贝构造
            //    cout<<"======="<<endl;
  //    ptr3 = ptr2;        // 调用的是编译器生成的默认拷贝构造
  //    所以引用计数不会增加 ptr3=ptr2 cout<<"======="<<endl;
  ptr3 = ptr1;
  cout << "此时3个shared_ptr指向同一个资源" << endl;
  cout << "use count of ptr1 is now " << ptr1.use_count() << endl;
  cout << "use count of ptr2 is now " << ptr2.use_count() << endl;
  cout << "use count of ptr3 is now " << ptr3.use_count() << endl;
  if (ptr1)
    cout << "ptr1 is not empty" << endl;
  // 会先调用赋值函数,由编译器决定调用的是拷贝构造还是移动构造,造出一个新的临时对象出来,临时对象会在跳出作用域后被析构掉。
  // 在析构函数中,会先判断该临时对象的是否指向资源,如果没有,析构结束。否则,对引用计数减1,判断引用计数是否为0,如果为0,删除共享引用计数指针,否则不操作。
  cout << "此时2个shared_ptr指向同一个资源" << endl;
  ptr2 = std::move(ptr1);
  if (!ptr1 && ptr2) { // 调用的是bool重载操作符
    cout << "ptr1 move to ptr2" << endl;
    cout << "use count of ptr1 is now " << ptr1.use_count() << endl;
    cout << "use count of ptr2 is now " << ptr2.use_count() << endl;
    cout << "use count of ptr3 is now " << ptr3.use_count() << endl;
  }
  // shape* -> circle*
  // 使用dynamic_cast转换后,指针为空.此时资源还是被dptr2拥有,dptr1为0
  shared_ptr<shape> dptr2(new shape);
  shared_ptr<circle> dptr1 = dynamic_pointer_cast<circle>(dptr2); // 基类转子类

  cout << "use count of dptr1 is now " << dptr1.use_count() << endl; // 0
  cout << "use count of dptr2 is now " << dptr2.use_count() << endl; // 1

  // circle* -> circle*
  // 使用dynamic_cast转换后,指针不为空,此时资源被两者共同使用,引用计数为2
  shared_ptr<shape> dptr3(new circle);
  //    shared_ptr<circle> dptr3(new circle);     //
  //    上面或者当前行,后面输出一样！
  shared_ptr<circle> dptr1_1 =
      dynamic_pointer_cast<circle>(dptr3); // 基类转子类

  cout << "use count of dptr1_1 is now " << dptr1_1.use_count() << endl; // 2
  cout << "use count of dptr3 is now " << dptr3.use_count() << endl;     // 2

  // circle* -> circle*
  // 使用dynamic_cast转换后,指针不为空,此时资源被两者共同使用,引用计数为2
  shared_ptr<circle> dptr3_1(new circle);
  shared_ptr<shape> dptr2_1 =
      dynamic_pointer_cast<shape>(dptr3_1); // 子类转基类 上行转换,安全！

  cout << "use count of dptr2_1 is now " << dptr2_1.use_count() << endl; // 2
  cout << "use count of dptr3_1 is now " << dptr3_1.use_count() << endl; // 2

  // shape* -> circle* 使用static_cast转换后,指针为空  与dynamic_cast相比,不安全
  shared_ptr<shape> sptr2(new shape);
  shared_ptr<circle> sptr1 = static_pointer_cast<circle>(sptr2); // 基类转子类

  cout << "use count of sptr1 is now " << dptr1.use_count() << endl; // 0
  cout << "use count of sptr2 is now " << dptr2.use_count() << endl; // 1

  // circle* -> circle*
  // 使用dynamic_cast转换后,指针不为空,此时资源被两者共同使用,引用计数为2
  shared_ptr<shape> sptr3(new circle);
  //    shared_ptr<circle> sptr3(new circle);     //
  //    上面或者当前行,后面输出一样！
  shared_ptr<circle> sptr1_1 = static_pointer_cast<circle>(sptr3); // 基类转子类

  cout << "use count of sptr1_1 is now " << sptr1_1.use_count() << endl; // 2
  cout << "use count of sptr3 is now " << sptr3.use_count() << endl;     // 2

  // circle* -> circle*
  // 使用static_cast转换后,指针不为空,此时资源被两者共同使用,引用计数为2
  // 等价于dynamic_cast
  shared_ptr<circle> sptr3_1(new circle);
  shared_ptr<shape> sptr2_1 =
      static_pointer_cast<shape>(sptr3_1); // 子类转基类 上行转换,安全！

  cout << "use count of sptr2_1 is now " << sptr2_1.use_count() << endl; // 2
  cout << "use count of sptr3_1 is now " << sptr3_1.use_count() << endl; // 2
  shared_ptr<const int> constV(new int);
  shared_ptr<int> s = const_pointer_cast<int>(constV);
  *s = 10;
  int a = reinterpret_pointer_cast<int>(s);
  cout << a << endl;
}