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    <p>Apart from inlining for efficiency and conditional compilation, macros can be used to raise the abstraction level of low-level C code. C doesn't really insulate you from the nitty-gritty details of memory and resource management and exact layout of data, and supports very limited forms of information hiding and other mechanisms for managing large systems. With macros, you are no longer limited to using only the base constructs in the C language: you can define your own data structures and coding constructs (including classes and templates!) while still nominally writing C! </p> <p>Preprocessor macros actually offer a <a href="http://en.wikipedia.org/wiki/Turing_complete" rel="noreferrer">Turing-complete</a> language executed at compile time. One of the impressive (and slightly scary) examples of this is over on the C++ side: the <a href="http://www.boost.org/doc/libs/1_38_0/libs/preprocessor/doc/index.html" rel="noreferrer">Boost Preprocessor</a> library uses the <a href="http://en.wikipedia.org/wiki/C_(programming_language)" rel="noreferrer">C99</a>/<a href="http://en.wikipedia.org/wiki/C_Plus_Plus" rel="noreferrer">C++98</a> preprocessor to build (relatively) safe programming constructs which are then expanded to whatever underlying declarations and code you input, whether C or C++. </p> <p>In practice, I'd recommend regarding preprocessor programming as a last resort, when you don't have the latitude to use high level constructs in safer languages. But sometimes it's good to know what you can do if your back is against the wall and the weasels are closing in...! </p>
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    1. POWhat are C macros useful for?
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    1. USPontus Gagge
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    1. COWhy is it better to use the preprocessor to define data structures than it is to use a struct? And surely the point at which you start defining classes and templates using the preprocessor is the point at which you might consider using C++, or another language with support for these constructs.
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      1. USJack Ryan
    2. CODepends on what degrees of freedom you want to have: macros allow you to produce a whole range of data structures consistently. And you're quite right: it's a last resort, as I wrote. But sometimes, you have to work with the tools you get.
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    3. COYou can actually use structs and other constructs and then add some sintactic sugar with the preprocessor,
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      1. USAndrea
 

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