我在 C++ FAQ 上遇到了一个有趣的基类实现根据我天真的理解，它可以作为某些智能指针实现(例如 shared_ptr)的替代品。这是逐字的示例代码，但请点击上面的链接以获得解释:

class Fred {
public:

  static Fred create1(std::string const& s, int i);
  static Fred create2(float x, float y);

  Fred(Fred const& f);
  Fred& operator= (Fred const& f);
 ~Fred();

  void sampleInspectorMethod() const;   // No changes to this object
  void sampleMutatorMethod();           // Change this object

  ...

private:

  class Data {
  public:
    Data() : count_(1) { }
    Data(Data const& d) : count_(1) { }              // Do NOT copy the 'count_' member!
    Data& operator= (Data const&) { return *this; }  // Do NOT copy the 'count_' member!
    virtual ~Data() { assert(count_ == 0); }         // A virtual destructor
    virtual Data* clone() const = 0;                 // A virtual constructor
    virtual void sampleInspectorMethod() const = 0;  // A pure virtual function
    virtual void sampleMutatorMethod() = 0;
  private:
    unsigned count_;   // count_ doesn't need to be protected
    friend class Fred; // Allow Fred to access count_
  };

  class Der1 : public Data {
  public:
    Der1(std::string const& s, int i);
    virtual void sampleInspectorMethod() const;
    virtual void sampleMutatorMethod();
    virtual Data* clone() const;
    ...
  };

  class Der2 : public Data {
  public:
    Der2(float x, float y);
    virtual void sampleInspectorMethod() const;
    virtual void sampleMutatorMethod();
    virtual Data* clone() const;
    ...
  };

  Fred(Data* data);
  // Creates a Fred smart-reference that owns *data
  // It is private to force users to use a createXXX() method
  // Requirement: data must not be NULL

  Data* data_;   // Invariant: data_ is never NULL
};

Fred::Fred(Data* data) : data_(data)  { assert(data != NULL); }

Fred Fred::create1(std::string const& s, int i) { return Fred(new Der1(s, i)); }
Fred Fred::create2(float x, float y)            { return Fred(new Der2(x, y)); }

Fred::Data* Fred::Der1::clone() const { return new Der1(*this); }
Fred::Data* Fred::Der2::clone() const { return new Der2(*this); }

Fred::Fred(Fred const& f)
  : data_(f.data_)
{
  ++data_->count_;
}

Fred& Fred::operator= (Fred const& f)
{
  // DO NOT CHANGE THE ORDER OF THESE STATEMENTS!
  // (This order properly handles self-assignment)
  // (This order also properly handles recursion, e.g., if a Fred::Data contains Freds)
  Data* const old = data_;
  data_ = f.data_;
  ++data_->count_;
  if (--old->count_ == 0) delete old;
  return *this;
}

Fred::~Fred()
{
  if (--data_->count_ == 0) delete data_;
}

void Fred::sampleInspectorMethod() const
{
  // This method promises ("const") not to change anything in *data_
  // Therefore we simply "pass the method through" to *data_:
  data_->sampleInspectorMethod();
}

void Fred::sampleMutatorMethod()
{
  // This method might need to change things in *data_
  // Thus it first checks if this is the only pointer to *data_
  if (data_->count_ > 1) {
    Data* d = data_->clone();   // The Virtual Constructor Idiom
    --data_->count_;
    data_ = d;
  }
  assert(data_->count_ == 1);

  // Now we "pass the method through" to *data_:
  data_->sampleMutatorMethod();
}

我没有看到任何 C++ 库中使用这种方法；虽然它看起来很优雅。假设单线程环境，为简单起见，请回答以下问题:

1. 这是管理对象生命周期的智能指针方法的合适替代方法，还是自找麻烦？
2. 如果合适，您认为为什么不经常使用它？

最佳答案

Is this a suitable alternative to the smart pointer approach for managing the lifetime of objects, or is it just asking for trouble?

不，我不认为重新发明引用计数是个好主意，尤其是因为我们现在在 C++11 中有了 std::shared_ptr。您可以根据 std::shared_ptr 轻松实现您可能的多态引用计数 Pimpl 习语类。请注意我们如何不必再实现复制构造器、赋值、dtor，并且变异变得更简单 w.r.t.引用计数器和克隆:

// to be placed into a header file ...

#include <memory>
#include <utility>
#include <string>

class Fred
{
public:
    static Fred create1(std::string const& s, int i);
    static Fred create2(float x, float y);

    void sampleInspectorMethod() const;   // No changes to this object
    void sampleMutatorMethod();           // Change this object

private:
    class Data;
    std::shared_ptr<Data> data_;

    explicit Fred(std::shared_ptr<Data> d) : data_(std::move(d)) {}
};

...和实现...

// to be placed in the corresponding CPP file ...

#include <cassert>
#include "Fred.hpp"

using std::shared_ptr;

class Fred::Data
{
public:
    virtual ~Data() {}                               // A virtual destructor
    virtual shared_ptr<Data> clone() const = 0;      // A virtual constructor
    virtual void sampleInspectorMethod() const = 0;  // A pure virtual function
    virtual void sampleMutatorMethod() = 0;
};

namespace {

class Der1 : public Fred::Data
{
public:
    Der1(std::string const& s, int i);
    virtual void sampleInspectorMethod() const;
    virtual void sampleMutatorMethod();
    virtual shared_ptr<Data> clone() const;
    ...
};

// insert Der1 function definitions here

class Der2 : public Data
{
public:
    Der2(float x, float y);
    virtual void sampleInspectorMethod() const;
    virtual void sampleMutatorMethod();
    virtual shared_ptr<Data> clone() const;
    ...
};

// insert Der2 function definitions here

} // unnamed namespace

Fred Fred::create1(std::string const& s, int i)
{
    return Fred(std::make_shared<Der1>(s,i));
}

Fred Fred::create2(float x, float y)
{
    return Fred(std::make_shared<Der2>(x,y));
}

void Fred::sampleInspectorMethod() const
{
    // This method promises ("const") not to change anything in *data_
    // Therefore we simply "pass the method through" to *data_:
    data_->sampleInspectorMethod();
}

void Fred::sampleMutatorMethod()
{
    // This method might need to change things in *data_
    // Thus it first checks if this is the only pointer to *data_
    if (!data_.unique()) data_ = data_->clone();
    assert(data_.unique());

    // Now we "pass the method through" to *data_:
    data_->sampleMutatorMethod();
}

(未经测试)

If it is suitable, why do you suppose it's not used more often?

我认为引用计数，如果你自己实现的话，更容易出错。它也有在多线程环境中运行缓慢的名声，因为引用计数器必须以原子方式递增和递减。但我想由于 C++11 提供了 shared_ptr 和移动语义，这种写时复制模式可能会再次流行起来。如果为 Fred 类启用移动语义，则可以避免一些原子递增引用计数器的成本。因此，将 Fred 对象从一个位置移动到另一个位置应该比复制它更快。

关于c++ - C++ OO 风格的引用计数，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/11788187/