[英]shared_ptr vs. new operator: which one to use
在下面的函數中,我利用了http_client
( https://github.com/Microsoft/cpprestsdk )的http_client向網絡攝像機發出http請求。 下面的函數可能是在發出特定請求時由lcm庫( http://lcm-proj.github.io/ )調用的回調。
我在第11行遇到問題。我以前使用過new運算符:
auto init_session_response = new init_session_response_t;
創建指針並在退出函數之前手動將其刪除。 但是在第49行的pplx任務繼續中嘗試修改init_session_response
對象時遇到訪問沖突異常。
init_session_response->status_code =
ptz_camera::status_codes_t::OK;
當我開始使用std :: shared_ptr時,這個問題就消失了。 有人可以向我解釋為什么使用shared_ptr解決了這個問題嗎? 還應該使用std :: shared_ptr創建http_client *嗎?
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}
當您收到違規訪問錯誤時,您必須在代碼中的某個位置刪除了指針(例如,在then
lambda中),但是您沒有使用原始指針發布代碼,因此我無法說出哪一行。
通過使用std::shared_ptr
,當它傳遞給lambda時,它會被值捕獲,因此它增加了use_count
並確保init_session_response
有效且在labmda中不被破壞,從而解決了該問題。
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