Question

我有一个嵌套的boost :: shared_ptr，它在被分配到另一个并且超出范围时偶尔会被破坏。我发现use_count没有更新，除非我将指针复制到temp。代码不言自明。在第一个for循环中，use_count不会更新，而另一个则更新。

#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;
}

输出

之前使用计数：1

使用后计数：1

之前使用计数：1

使用后计数：2

Answer 1

由于您显然storage_2d_t storage_2 = storage_1;，将元素直接分配回自身不应增加使用次数。

我是第二个循环，您在持有临时副本（ref）期间打印使用计数。说清楚，你可以看到 - 正如预期的那样 - ＆＃34;＆＃34;＆＃34;＆＃34;＆＃34;计数实际上并不高：

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

查看 Live On Coliru

Brainwave 你的意思是将storage_1深入克隆到storage_2吗？您的外部storage_2d_t 也是共享指针这一事实似乎让您感到困惑，因此您可以通过引用引用相同的向量。

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());

嵌套的boost :: shared_ptr use_count没有更新

1 个答案: