C Macro Tips and Tricks | Hacker News Reader

C Macro Tips and Tricks

mikeash.com

62 pointsyan12 comments

I particularly liked the troll he linked to at the end: "The C Language Is Purely Functional" http://conal.net/blog/posts/the-c-language-is-purely-functio...

That was linked to in a comment, not in the blog post itself.

Hmm.. Bad coding in the site engine:

Traceback (most recent call last): File "index.cgi", line 509, in <module> main() File "index.cgi", line 499, in main if 'iPhone' in os.environ['HTTP_USER_AGENT']: File "/home/mikeash/lib/python2.6/UserDict.py", line 22, in __getitem__ raise KeyError(key) KeyError: 'HTTP_USER_AGENT'

From the article:

""" It's a little-known feature of C that if you put two string literals next to each other in the source code, they get concatenated:

    char *helloworld = "hello, " "world!";

"""

That's a good tip that actually works in other languages too. It helps in Python, for example, when creating long strings. Wrapping parentheses around the string and using the quote-continuation rule makes it nice and easy to read.

For example:

    foo = ('hello world this is '
           'a long string')

is better than:

    foo = 'hello world this is ' + \
          'a long string'

...for two reasons. It avoids string concatenation (the "+" operator) and it avoids the backslash.

For a contained example of many of these used appropriately, look at the source for libev (http://software.schmorp.de/pkg/libev.html). In particular, multiline macros and token pasting are used to generate generic interface functions for the different watcher structs.

The C preprocessor is powerful, but brittle. If Scheme's macros are hygienic, C's are pathogenic, so be judicious.

Great article, although I'd prefer if there was a bit less of a focus on Objective-C and Cocoa, since all the rest applies pretty much the same to plain C.

(However, that technique with blocks looks like a good solution to the "many statement expression in macro expression" problem, would that work with C++0x lambdas?)

Yes, it'd work with 0x lambdas; however, the situations where this occurs in C are much less common in C++. Consider the "max" example given, as written in C++:

template <typename T> T max(T a, T b) { return (a > b) ? a : b; }

int max_ints(int a, int b) { return max(a, b); }

Of course this only works if "a" and "b" have the same type. 0x provides a solution for the situation where they don't:

template <typename Ta, typename Tb> auto max(Ta a, Tb b) -> decltype((a > b) ? a : b) { return (a > b) ? a : b; }

int max_int_char(int a, char b) { return max(a, b); }

My question was more of if you have multiple statements, instead of a single expression, you wanted to stick inside a macro and preserve the ability to call it within another expression.

For example:

  #define SOMEMACRO(x) do { \
     int y=0;               \
     for(int i=0; i<x; i++) \
       y=dosomething(y, i); \
  } while (0)

Which, of course, works fine if you call it as a statement. But it doesn't work if you want to somehow "return" the value of "y" from that: there's no way to embed that kind of loop into an expression.

He showed an example of using Apple's Blocks to solve that, can you do the same with C++0x lambdas?

As I said, 0x lambdas can do that...

... but you probably shouldn't: don't use a macro where a function will suffice, and if you need to use a macro, write a macro that syntactically wraps around a function if possible. C++, especially 0x, provides a lot of metaprogramming functionality you can use to avoid writing macros in many cases. Using this functionality will give the compiler more information about your program for analysis purposes, which means the compiler can help ensure correctness and better efficiency.

Gcc can do some interesting things with macros. I have used multi-statement macros that "return" a value. Another option is using the comma operator, which I think is legitimate C.

To give some context, most of the content of the blog deals with some of the edges of Obj-C and Cocoa, such as defining classes at runtime and other generally weird things.

What a wonderful advertisement for Lisp macros.

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