{"id":25328,"date":"2024-03-16T04:45:03","date_gmt":"2024-03-16T04:45:03","guid":{"rendered":"https:\/\/www.silicloud.com\/blog\/what-is-the-usage-of-the-extern-keyword-in-c-2\/"},"modified":"2024-03-22T05:54:29","modified_gmt":"2024-03-22T05:54:29","slug":"what-is-the-usage-of-the-extern-keyword-in-c-2","status":"publish","type":"post","link":"https:\/\/www.silicloud.com\/blog\/what-is-the-usage-of-the-extern-keyword-in-c-2\/","title":{"rendered":"What is the usage of the “extern” keyword in C++?"},"content":{"rendered":"

In C++, the keyword ‘extern’ is used to declare the existence of external variables or functions.<\/p>\n

    \n
  1. When it comes to variables, if we define a global variable in one file and want to use it in another file, we need to declare its existence using the “extern” keyword in the other file. This way, the compiler knows that the variable is defined in another file and can link them together.<\/li>\n<\/ol>\n

    Define global variables in a file.<\/p>\n

    \/\/ File1.cpp<\/span>\r\nint<\/span> globalVariable = 10<\/span>;\r\n<\/code><\/pre>\n
    Use the variable in another file.<\/p>\n
    \/\/ File2.cpp<\/span>\r\nextern<\/span> int<\/span> globalVariable; \/\/ \u4f7f\u7528extern\u5173\u952e\u5b57\u58f0\u660e\u5168\u5c40\u53d8\u91cf\u7684\u5b58\u5728<\/span>\r\n\r\nvoid<\/span> someFunction<\/span>()<\/span> <\/span>{\r\n    int<\/span> localVar = globalVariable; \/\/ \u4f7f\u7528\u5168\u5c40\u53d8\u91cf<\/span>\r\n}\r\n<\/code><\/pre>\n
      \n
    1. When using the same function in multiple files, we need to declare its existence using the extern keyword so that the compiler knows the function is defined in other files and can link them together.<\/li>\n<\/ol>\n
      Define a function in a file.<\/p>\n
      \/\/ File1.cpp<\/span>\r\nvoid<\/span> someFunction<\/span>()<\/span><\/span>; \/\/ \u51fd\u6570\u58f0\u660e<\/span>\r\n\r\nvoid<\/span> someFunction<\/span>()<\/span> <\/span>{\r\n    \/\/ \u51fd\u6570\u5b9a\u4e49<\/span>\r\n}\r\n<\/code><\/pre>\n
      Use the function in another file:<\/p>\n
      \/\/ File2.cpp<\/span>\r\nextern<\/span> void<\/span> someFunction<\/span>()<\/span><\/span>; \/\/ \u4f7f\u7528extern\u5173\u952e\u5b57\u58f0\u660e\u51fd\u6570\u7684\u5b58\u5728<\/span>\r\n\r\nvoid<\/span> anotherFunction<\/span>()<\/span> <\/span>{\r\n    someFunction<\/span>(); \/\/ \u8c03\u7528\u51fd\u6570<\/span>\r\n}\r\n<\/code><\/pre>\n
      Summary: The “extern” keyword is used to declare the existence of global variables or functions in other files. It tells the compiler to link them together during the linking phase.<\/p>\n","protected":false},"excerpt":{"rendered":"
      In C++, the keyword ‘extern’ is used to declare the existence of external variables or functions. When it comes to variables, if we define a global variable in one file and want to use it in another file, we need to declare its existence using the “extern” keyword in the other file. This way, the […]<\/p>\n","protected":false},"author":8,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_import_markdown_pro_load_document_selector":0,"_import_markdown_pro_submit_text_textarea":"","footnotes":""},"categories":[1],"tags":[],"class_list":["post-25328","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"yoast_head":"\nWhat is the usage of the "extern" keyword in C++? - Blog - Silicon Cloud<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.silicloud.com\/blog\/what-is-the-usage-of-the-extern-keyword-in-c-2\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"What is the usage of the "extern" keyword in C++?\" \/>\n<meta property=\"og:description\" content=\"In C++, the keyword ‘extern’ is used to declare the existence of external variables or functions. 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