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I am working on a microcontroller with tight memory constraints. Hence I watch memory consumption.

I have some library with classes which are only visible in the cpp file. These class do not show up in the header file. The classes were directly implemented. Now I started to separate the declaration from the implementation. The point is that I need to expose some of them in the header file. However I noticed that this separation affects the program size. For some of the classes it increases memory consumption for some it decreases it.

Why is it that separation of definition and implementation affects compiled program size? How might I leverage this to decrease compiled program size?

Udo Klein
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1 Answers1

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When a class is only used inside a single translation unit (file) the compiler is free to perform whatever optimisations it likes. It can completely get rid of the v-table, split the class up and turn it into a more procedural structure if this works better. When you export the class outside, the compiler can't make assumptions about who might be using it and so the optimisations it can perform are more limited.

However, particularly on microcontrollers, there are lots of aggressive post linker optimisations such as procedural abstraction that might be done to reduce code size on the finished program. Sometimes if the compiler has optimised the separate modules less due to the situation described above, bigger gains can be achieved at this stage as there is more unoptimised repeated code blocks.

These days extra memory is so cheap it is rarely worth trying to write your program around saving a few bytes. Having a clearer and easier to maintain code base will quickly pay for any BOM savings at the first instance you have to add new features. If you really want to carefully control memory usage then I'd recommend moving to C (or an extremely limited subset of C++) and getting a really good understanding of how your compiler is optimising.

Jon
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