{"id":5101,"date":"2024-03-14T02:23:16","date_gmt":"2024-03-14T02:23:16","guid":{"rendered":"https:\/\/www.silicloud.com\/blog\/how-to-pass-data-out-of-a-thread-in-qt\/"},"modified":"2025-07-31T18:58:35","modified_gmt":"2025-07-31T18:58:35","slug":"how-to-pass-data-out-of-a-thread-in-qt","status":"publish","type":"post","link":"https:\/\/www.silicloud.com\/blog\/how-to-pass-data-out-of-a-thread-in-qt\/","title":{"rendered":"Qt Thread Data Transfer: Signals &#038; Slots"},"content":{"rendered":"<p>In Qt, you can use the signal and slot mechanism to transfer data between threads. Here are the specific steps:<\/p>\n<ol>\n<li>Define a signal in the thread class to send data.<\/li>\n<li>Connect this signal to a slot function in the main thread to receive data.<\/li>\n<li>Emit this signal in the thread to pass data to the main thread.<\/li>\n<\/ol>\n<p>Here is a simple example:<\/p>\n<pre class=\"post-pre\"><code><span class=\"hljs-meta\">#<span class=\"hljs-keyword\">include<\/span> <span class=\"hljs-string\">&lt;QThread&gt;<\/span><\/span>\r\n<span class=\"hljs-meta\">#<span class=\"hljs-keyword\">include<\/span> <span class=\"hljs-string\">&lt;QObject&gt;<\/span><\/span>\r\n\r\n<span class=\"hljs-keyword\">class<\/span> <span class=\"hljs-title class_\">MyThread<\/span> : <span class=\"hljs-keyword\">public<\/span> QThread\r\n{\r\n    Q_OBJECT\r\nsignals:\r\n    <span class=\"hljs-function\"><span class=\"hljs-type\">void<\/span> <span class=\"hljs-title\">dataReady<\/span><span class=\"hljs-params\">(<span class=\"hljs-type\">int<\/span> value)<\/span><\/span>;\r\n\r\n<span class=\"hljs-keyword\">protected<\/span>:\r\n    <span class=\"hljs-function\"><span class=\"hljs-type\">void<\/span> <span class=\"hljs-title\">run<\/span><span class=\"hljs-params\">()<\/span> <span class=\"hljs-keyword\">override<\/span>\r\n    <\/span>{\r\n        <span class=\"hljs-type\">int<\/span> result = <span class=\"hljs-number\">42<\/span>;\r\n        <span class=\"hljs-function\">emit <span class=\"hljs-title\">dataReady<\/span><span class=\"hljs-params\">(result)<\/span><\/span>;\r\n    }\r\n};\r\n\r\n<span class=\"hljs-keyword\">class<\/span> <span class=\"hljs-title class_\">MyObject<\/span> : <span class=\"hljs-keyword\">public<\/span> QObject\r\n{\r\n    Q_OBJECT\r\n<span class=\"hljs-keyword\">public<\/span> slots:\r\n    <span class=\"hljs-function\"><span class=\"hljs-type\">void<\/span> <span class=\"hljs-title\">onDataReady<\/span><span class=\"hljs-params\">(<span class=\"hljs-type\">int<\/span> value)<\/span>\r\n    <\/span>{\r\n        <span class=\"hljs-built_in\">qDebug<\/span>() &lt;&lt; <span class=\"hljs-string\">\"Data received from thread: \"<\/span> &lt;&lt; value;\r\n    }\r\n};\r\n\r\n<span class=\"hljs-function\"><span class=\"hljs-type\">int<\/span> <span class=\"hljs-title\">main<\/span><span class=\"hljs-params\">(<span class=\"hljs-type\">int<\/span> argc, <span class=\"hljs-type\">char<\/span> *argv[])<\/span>\r\n<\/span>{\r\n    <span class=\"hljs-function\">QApplication <span class=\"hljs-title\">app<\/span><span class=\"hljs-params\">(argc, argv)<\/span><\/span>;\r\n\r\n    MyThread thread;\r\n    MyObject obj;\r\n\r\n    QObject::<span class=\"hljs-built_in\">connect<\/span>(&amp;thread, &amp;MyThread::dataReady, &amp;obj, &amp;MyObject::onDataReady);\r\n\r\n    thread.<span class=\"hljs-built_in\">start<\/span>();\r\n\r\n    <span class=\"hljs-keyword\">return<\/span> app.<span class=\"hljs-built_in\">exec<\/span>();\r\n}\r\n<\/code><\/pre>\n<p>In the example above, the MyThread class inherits from QThread and defines a dataReady signal for sending data. The MyObject class inherits from QObject and defines a slot function onDataReady for receiving data.<\/p>\n<p>In the main function, a thread object and an object are created and the thread&#8217;s dataReady signal is connected to the object&#8217;s onDataReady slot function. When the thread runs, it emits the dataReady signal, passing the data to the slot function onDataReady in the main thread, thereby achieving the functionality of data being passed out from the thread.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>In Qt, you can use the signal and slot mechanism to transfer data between threads. Here are the specific steps: Define a signal in the thread class to send data. Connect this signal to a slot function in the main thread to receive data. Emit this signal in the thread to pass data to the [&hellip;]<\/p>\n","protected":false},"author":5,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_import_markdown_pro_load_document_selector":0,"_import_markdown_pro_submit_text_textarea":"","footnotes":""},"categories":[1],"tags":[5374,5346,5372,5373,5371],"class_list":["post-5101","post","type-post","status-publish","format-standard","hentry","category-uncategorized","tag-qt-multithreading","tag-qt-programming","tag-qt-signals-slots","tag-qt-thread-communication","tag-qt-threading"],"yoast_head":"<!-- This site is optimized with the Yoast SEO Premium plugin v21.5 (Yoast SEO v21.5) - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>Qt Thread Data Transfer: Signals &amp; Slots - Blog - Silicon Cloud<\/title>\n<meta name=\"description\" content=\"Learn how to pass data out of Qt threads using signals and slots. 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