shared_ptr 与 new 运算符：使用哪个

Question

在下面的函数中，当发出特定请求时，我使用了 cpprestsdk (https://github.com/Microsoft/cpprestsdk) to make http requests to a network camera. The function below is probably a callback called by the lcm library (http://lcm-proj.github.io/) 中的 http_client。

我在第 11 行遇到了问题。我之前使用的是 new 运算符：

auto init_session_response = new init_session_response_t;

创建指针并在退出函数之前手动删除它。但是在第 49 行的 pplx 任务延续中尝试修改 init_session_response 对象时出现访问冲突异常。

init_session_response->status_code =
              ptz_camera::status_codes_t::OK;

当我开始使用 std::shared_ptr 时，这个问题就消失了。有人可以向我解释为什么使用 shared_ptr 可以解决问题吗？ http_client* 是否也应该使用 std::shared_ptr 创建？

1 void lcm_handler::on_init_session_req(const lcm::ReceiveBuffer* rbuff,
2   const std::string& channel,
3   const ptz_camera::init_session_request_t* req)
4 {
5     std::cout << "Received init session req on channel: " << channel << 
6       "; Camera: " << req->ip_address << std::endl;
7 
8   auto ip_address = req->ip_address;
9 
10    // Note use of std::shared_ptr
11  auto init_session_response = make_shared<ptz_camera::init_session_response_t>();
12  
13  auto key_position = this->ip_client_map.find(ip_address);
14  if (key_position == ip_client_map.end())
15  {
16      std::cout << "Creating a new client for the ip: "
17          << req->ip_address << endl;
18
19      wstring username = this->convert_to_wstring(req->username);
20      wstring password = this->convert_to_wstring(req->password);
21
22      wstring main_uri = L"http://" + convert_to_wstring(ip_address);
23      auto config = http_client_config();
24      auto login = credentials(username, password);
25      config.set_credentials(login);
26      config.set_timeout(std::chrono::milliseconds(500));
27
28      http_client* client = new http_client(main_uri, config);
29      std::cout << "Client created...\n";
30
31      uri_builder uri = uri_builder(U("/") + uri_constants::stw_cgi).
32          append_path(uri_constants::attributes_cgi).append_path(uri_constants::attributes);
33
34      auto request = uri.to_string();
35  
36      client->request(methods::GET, request)
37          .then([this, ip_address, client, init_session_response]
38              (pplx::task<http_response> request_task) -> pplx::task<wstring>
39      {
40          try 
41          {
42              auto response = request_task.get();
43              if (response.status_code() == status_codes::OK)
44              {
45                  std::cout << "Saving client...";
46                  this->ip_client_map[ip_address] = client;
47                  std::cout << "success.\n";
48
49                  init_session_response->status_code =
50                  ptz_camera::status_codes_t::OK;
51              }
52
53              else
54              {
55                  cout << "GET request to client failed! HTTP Error: "
56                      << response.status_code() << std::endl;
57
58                  init_session_response->status_code =
59                      ptz_camera::status_codes_t::ERR;
60              }
61
62              return response.extract_string();
63          }
64              
65          catch (const exception& e)
66          {
67              cout << "Caught exception: " << e.what() << endl;
68              return create_task([e, this]() -> wstring
69              {
70                  return convert_to_wstring(e.what());
71              });
72          }               
73
74      })
75          .then([init_session_response, this](wstring response)
76      {
77          string n = this->convert_to_string(response);
78          init_session_response->response_message = n;
79      });
80  }
81
82
83  else
84  {
85      string message = "Client for ip: " + req->ip_address + " already exists\n";
86      cout << message << endl;
87      init_session_response->response_message = message;
88      init_session_response->status_code = ptz_camera::status_codes_t::OK;
89  }   
90
91  this->lcm->publish(ptz_camera_channels::init_session_res_channel,
92      init_session_response.get());   
93}

Answer 1

当您遇到违规访问错误时，您必须删除代码中某些地方的指针（例如，在某些 then lambda 中），但您没有 post 您的代码使用原始指针所以我不能说是哪一行。

通过使用std::shared_ptr，当它传递给lambda时，它被值捕获，所以它增加了use_count并确保init_session_response有效并且在labmda中没有被破坏，解决了这个问题。

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