c++ - 嵌套的 boost::shared_ptr use_count 不更新

Question

我有一个嵌套的 boost::shared_ptr，当它被分配给另一个并超出范围时，它偶尔会被破坏。我发现 use_count 没有更新，除非我将指针复制到临时文件。代码是不言自明的。在第一个 for 循环中，use_count 不更新，而在另一个循环中更新。

#include <vector>
#include <boost/shared_ptr.hpp>
#include <iostream>
using namespace std;



int main(int argc, char const *argv[])
{
  typedef int T;
  typedef std::vector<T> content_1d_t;
  typedef boost::shared_ptr<content_1d_t> storage_1d_t;
  typedef std::vector<storage_1d_t> content_2d_t;
  typedef boost::shared_ptr<content_2d_t> storage_2d_t;

  int dim1 = 10;
  int dim2 = 1;
  content_2d_t* content_1 = new content_2d_t();
  content_1->reserve(dim2);
  storage_2d_t storage_1(content_1);

  for (int i = 0; i < dim2; ++i)
  {
    storage_1->push_back(storage_1d_t(new content_1d_t(dim1)));
  }

  //content_2d_t* content_2 = new content_2d_t(dim2);
  storage_2d_t storage_2 = storage_1;

  for (int i = 0; i < dim2; ++i)
  {
    cout<< "use count before : "<< storage_1->operator[](i).use_count()<<endl;
    storage_2->operator[](i) = storage_1->operator[](i);
    cout<< "use count after: "<< storage_1->operator[](i).use_count()<<endl;
  }

  for (int i = 0; i < dim2; ++i)
  {
    cout<< "use count before : "<< storage_1->operator[](i).use_count()<<endl;
    storage_1d_t ref = storage_1->operator[](i);
    storage_2->operator[](i) = ref;
    cout<< "use count after: "<< storage_1->operator[](i).use_count()<<endl;
  }


  /* code */
  return 0;
}

output

use count before : 1

use count after: 1

use count before : 1

use count after: 2

Answer 1

因为您执行了 storage_2d_t storage_2 = storage_1; 显然，将元素直接返回给它们自己应该不会增加使用次数。

在第二个循环中，您将在持有临时拷贝 (ref) 期间打印使用次数。明确地说，您可以看到确实 - 正如预期的那样 - “之后”计数实际上并没有更高:

for (int i = 0; i < dim2; ++i) {
    cout << "use count before : " << (*storage_1)[i].use_count() << endl;
    {
        storage_1d_t ref = (*storage_1)[i];
        (*storage_2)[i] = ref;
        cout << "use count during: " << (*storage_1)[i].use_count() << endl;
    }
    cout << "use count after: " << (*storage_1)[i].use_count() << endl;
}

现在打印

use count before : 1
use count during: 2
use count after: 1

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---

Brainwave 您是想将 storage_1 深度克隆到 storage_2 中吗？您似乎对您的外部 storage_2d_t 是也是共享指针这一事实感到困惑，因此您通过引用来引用相同的 vector 。

storage_2d_t storage_2 = boost::make_shared<content_2d_t>(*storage_1);
// or
storage_2d_t storage_2 = boost::make_shared<content_2d_t>(storage_1->begin(), storage_1->end());