{"id":241,"date":"2016-08-10T22:19:29","date_gmt":"2016-08-11T06:19:29","guid":{"rendered":"http:\/\/wonghoi.humgar.com\/blog\/?p=241"},"modified":"2025-03-06T14:04:53","modified_gmt":"2025-03-06T22:04:53","slug":"matlab-techniques-oncleanup-destructor","status":"publish","type":"post","link":"https:\/\/wonghoi.humgar.com\/blog\/2016\/08\/10\/matlab-techniques-oncleanup-destructor\/","title":{"rendered":"MATLAB Techniques: onCleanup() ‘destructor’"},"content":{"rendered":"

If your program opens a file, creates a timer(), or whatever resources that needs to be closed when they are no longer needed, before R2008a, you have to put your close resource\u00a0calls at two places: one at the end of successful execution, the other at the exception handling in try…catch block:<\/p>\n

FID = fopen('a.txt')\ntry\n   \/\/ ... do something here\n   fclose(FID);\ncatch\n   fclose(FID);\nend<\/pre>\n
Not only it’s messy that you have to duplicate your code, it’s also error prone when you add code in between. If\u00a0your true intention is to close the resource whenever you exit the current program scope, there’s a better way for you: onCleanup()<\/a> object. The code above can be simplified as:<\/p>\n
FID = fopen('a.txt')\nobj = onCleanup(@() fclose(FID));\n\/\/ ... do something with FID here<\/pre>\n
The way onCleanup()<\/code> works is that it creates a dummy object which you register a function handle that does the cleanup through the constructor argument, and the destructor will blindly call what you’ve registered (the cleanup functor) when its lifecycle ends.<\/p>\n
Due to copyright reasons, I won’t copy the simple code here. Just open onCleanup.m<\/code> in MATLAB editor and you’ll see it that the code (excluding comments) has less words than the description above. Pretty neat!<\/p>\n
Normally we use onCleanup()<\/code> inside a function. The best place to put is is right after you opened a resource because anything in between can go wrong (i.e. might throw exceptions): you want ‘obj’ to be swept (i.e. its destructors called) when that happens.<\/p>\n
Technically, you can make an onCleanup()<\/code> object in the base (root) workspace (aka command window). The destructor will be triggered either when you clear the obj<\/code> explicitly using clear<\/code> or when you exit MATLAB. You can see for yourself with this:<\/p>\n
obj = onCleanup(@() pause);<\/pre>\n
It kind of\u00a0let you do a one-off\u00a0cleanup on exit instead of a recurring cleanup in finish.m.<\/p>\n
So the next time you open a resource that needs to be closed whether the program exits unexpectedly or not, use onCleanup()! It’s one of the elegant,\u00a0smart\u00a0uses of OOP.<\/p>\n\n\n
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Connection to C++ concept: RAII<\/h2>\n\n\n\n
This MATLAB technique is an extension of C++’s RAII where you manage the resource exclusively through acquiring resources at constructors and do cleanup at destructors and let the object’s lifecycle managing the timing.<\/p>\n\n\n\n
The innovation in onCleanup()<\/code> is that it saves simple usages (like opening a file or device) from defining a custom class just to take advantage of the destructor: <\/p>\n\n\n\n