{"id":32325,"date":"2023-06-26T16:19:51","date_gmt":"2023-06-26T16:19:51","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/cppblog\/?p=32325"},"modified":"2023-06-26T16:19:51","modified_gmt":"2023-06-26T16:19:51","slug":"using-key-events-with-warning-c26800","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/cppblog\/using-key-events-with-warning-c26800\/","title":{"rendered":"Using Key Events with warning C26800"},"content":{"rendered":"

MSVC’s Code Analysis Key Events<\/a> are a useful tool for quickly getting to the root-cause of your static analysis warnings. This blog post highlights how Key Events may be used with warning C26800<\/code><\/a> Use of a moved from object<\/code> to quickly narrow down the problematic code.<\/p>\n

C26800 warning recap<\/h2>\n

Warning C26800 catches code that attempts to use an object that is in a moved-from state:<\/p>\n

#include <iostream>\r\nint main()\r\n{\r\n    std::string string_owner = \"this string will be moved!\";\r\n    std::string new_string_owner = std::move(string_owner);\r\n    std::cout << string_owner << std::endl; \/\/ Warning C26800 Use of a moved from object: ''string_owner'' (lifetime.1).\r\n}<\/code><\/pre>\n
Instead of this string will be moved!<\/code>, the print statement in the above example will produce an empty output line when run.<\/p>\n
The move from string_owner<\/code> into new_string_owner<\/code> happens on the line immediately<\/em> preceding its use in the print statement, so it’s instantly clear why the output is incorrect. But this is a trivial example. Real world code is rarely this straightforward.<\/p>\n
A more complex example, sans Key Events<\/h2>\n
Consider a more convoluted example. The following code defines a function that is not only significantly longer than in the first code snippet, but is also littered with all sorts of intervening business logic (represented by \/\/ ...<\/code>).<\/p>\n
#include <string>\r\nvoid take_ownership(std::string&&) { \/* ... *\/ }\r\nvoid use_string(std::string) { \/* ... *\/ }\r\nvoid my_buggy_function(bool some_condition, bool some_other_condition)\r\n{\r\n    std::string my_string{ \"this is a string!\" };\r\n    bool my_flag{ false };\r\n\r\n    \/\/ ...\r\n\r\n    if (some_condition)\r\n    {\r\n        take_ownership(std::move(my_string)); \/\/ (a)\r\n    }\r\n\r\n    \/\/ ...\r\n\r\n    if (some_other_condition) \/\/ (b)\r\n    {\r\n        my_flag = true;\r\n\r\n        \/\/ ...\r\n\r\n        my_string = \"the string is reset\"; \/\/ (c)\r\n\r\n        \/\/ ...\r\n    }\r\n\r\n    \/\/ ...\r\n\r\n    if (my_flag && !some_other_condition) \/\/ (d)\r\n    {\r\n        take_ownership(std::move(my_string)); \/\/ (e)\r\n    }\r\n\r\n    \/\/ ...\r\n\r\n    use_string(my_string); \/\/ Use of a moved from object: ''my_string'' (lifetime.1). (f)\r\n}<\/code><\/pre>\n
As in the simple example, a developer tasked with solving this bug will be looking to answer the question: where is my_string<\/code> moved? A natural approach would likely be to start on line (f)<\/code>, and to scan backwards through the code. This process would look something like:<\/p>\n