{"id":358,"date":"2009-08-10T10:19:00","date_gmt":"2009-08-10T09:19:00","guid":{"rendered":"https:\/\/blogs.mentor.com\/colinwalls\/?p=358"},"modified":"2026-03-26T16:30:46","modified_gmt":"2026-03-26T20:30:46","slug":"i-or-i","status":"publish","type":"post","link":"https:\/\/blogs.sw.siemens.com\/embedded-software\/2009\/08\/10\/i-or-i\/","title":{"rendered":"++i or i++?"},"content":{"rendered":"

If you are reading this blog, you are probably knowledgeable about embedded software and, therefore, like me, you consider yourself proficient in C. C is still the dominant language for embedded work. I read an article<\/a> recently which espoused the view that “real men program in C” – I rather liked that. It also suggested that, since relatively few new graduates know C, there is an increasing shortage of expertise. This is great news for us old hands.<\/p>\n

However, we should not be complacent. I think that I know C quite well. After all, it is quite a small language, so getting a grip should not be hard. Even aspects of it that are specifically interesting to embedded developers [and there are quite a few] should not be too difficult to master. But sometimes it is possible to get caught out …<\/p>\n

I will pose a question: what is the difference between ++i<\/strong> and i++<\/strong>?
\nI am sure that you know that both expressions result in the variable i<\/strong> being incremented, but the first expression yields the incremented value if i<\/strong>, whereas the second returns the value prior to incrementing. OK so far. But what if the expression result is discarded in a situation like this, for example:<\/p>\n

for (i=0; i<5; i++)<\/strong><\/p>\n

Does it make any difference which form of the increment operator is used in this context? I would have said no. But then I had a conversation with one of our compiler team and now I know better.<\/p>\n

Functionally, it makes no difference – ++i<\/strong> and i++<\/strong> would both do the right thing. However, embedded developers are interested in more than just functionality – we are always interested in overheads. Think about what code might be compiled for each form of the expression. For ++i<\/strong>, the variable is simply incremented; for i++<\/strong>, the value needs to be stored somewhere and then the variable is incremented. This means that the post increment form needs some additional storage, which is an unwanted overhead.<\/p>\n

Of course, for a simple integer, the overhead is quite small. But, if the variable were actually a complex C++ object, it might be a more serious matter.<\/p>\n

It can be argued that a good compiler would observe that the expression result is not needed and this code would be optimized away. I sincerely hope that this would be the case. However, it is sloppy programming to just rely on compiler optimization to address the ignorance or laziness of the programmer.<\/p>\n

If you have any other examples of subtle C features that are interesting to embedded developers, please email<\/a> me.<\/p>\n","protected":false},"excerpt":{"rendered":"

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