5 ms·

The Ugliest C Feature: tgmath.h

107 points by bcaa7f3a8bbc 7y ago

s_gourichon 7y ago
The ternary operator is indeed special as it (is the only one AFAIK that) introduces in the language some type constraints between the two result branches, that affect the way one or the other is interpreted. A similar trick in C++ is explained on https://www.artima.com/cppsource/foreach2.html https://www.artima.com/cppsource/foreach2.html and allows to return the type of an expression without evaluating it. From the link: > You may be thinking this is needlessly convoluted. After all, couldn't we just use encode_type(container) without the conditional operator and get the same result? No, because that would have caused container to be evaluated. With the conditional operator, only one branch of the condition is ever executed. In this case, since the condition is always true, the first branch will always be taken. The second branch is "dead code"—it will never execute—yet it extends a ghostly finger into the land of the living and exerts its influence on the conditional expression's type. Spooky!
- smitty1e 7y ago
  That's just C++ wishing that it could have a type system closer to Haskell's.
- saagarjha 7y ago
  Qt apparently does something similar to implement Q_FOREACH: https://stackoverflow.com/a/10522384/5230900 https://stackoverflow.com/a/10522384/5230900
  - likeliv 7y ago
    That was pre-c++11. Now using C++11, it is much simpler (using 'auto'). And the C++17 version is even simpler https://code.woboq.org/qt5/qtbase/src/corelib/global/qglobal.h.html#1085-1107 https://code.woboq.org/qt5/qtbase/src/corelib/global/qglobal...
- aardvark179 7y ago
  That sort of operator is often tough, just look at section 15.25 in the Java language specification (https://docs.oracle.com/javase/specs/jls/se13/html/jls-15.html#jls-15.25 https://docs.oracle.com/javase/specs/jls/se13/html/jls-15.ht...) in the type of such expressions.
- MauranKilom 7y ago
  For those wondering (like I did) "But there already is a C++ facility whose sole purpose is determining the type of an expression [or entity] without evaluation it?" decltype() only exists since C++11 (and the above article is from 2006). https://en.cppreference.com/w/cpp/language/decltype https://en.cppreference.com/w/cpp/language/decltype
The_rationalist 7y ago
C11 obscoleted this with the _Generic macro
- EdSchouten 7y ago
  C11 doesn't obsolete it, right? It only permits it to be implemented portably. For the curious, here's an implementation of tgmath.h on top of _Generic: https://github.com/NuxiNL/cloudlibc/blob/master/src/include/tgmath.h https://github.com/NuxiNL/cloudlibc/blob/master/src/include/...
  - kzrdude 7y ago
    Far from obsolete.. more is coming :) 'Add type generic interfaces for string functions to the C library, see N2068' has been mentioned as a possible C2x addition https://gustedt.wordpress.com/2018/11/12/c2x/ https://gustedt.wordpress.com/2018/11/12/c2x/
- fanf2 7y ago
  And _Generic is still somewhat ugly: it has the awkward property that all the branches must pass the type checker even if they are not used. This is mostly OK for numerical functions where you get some help from C’s implicit conversions making the checker more lenient, but if you try to apply _Generic in other contexts you soon encounter problems.
amelius 7y ago
I always thought it was setjmp. https://en.wikipedia.org/wiki/Setjmp.h https://en.wikipedia.org/wiki/Setjmp.h
- saagarjha 7y ago
  setjmp isn’t really that ugly, IMO.
  - ludamad 7y ago
    Yeah, low level and rare isn't the same as "bizarrely ripped from the design of another language"
deleted 7y ago
[deleted]
RcouF1uZ4gsC 7y ago
With apologies to Philip Greenspun: Any sufficiently complicated C program contains an ad-hoc, informally-specified, bug-ridden, slow implementation of half of C++.
- wayoutthere 7y ago
  Sure, but the C++ STL is to code as this [1] is to knives. [1] https://www.amazon.com/Wenger-16999-Swiss-Knife-Giant/dp/B001DZTJRQ https://www.amazon.com/Wenger-16999-Swiss-Knife-Giant/dp/B00....
  - benj111 7y ago
    For the low low price of $9999.99. Is that the maximum you can charge on Amazon? Seems strange that they'd want to put a limit on it.
  - jjoonathan 7y ago
    Now let's see boost in this metaphor :P (Are there enough knives in the entire swiss army to make this work?)
- deleted 7y ago
  [deleted]
- marcosdumay 7y ago
  Yes, but it's most of the good 1/5 of it, with just a little bit of the atrocious 1/3, and the rest taken from the "who cares" 7/15.
likeliv 7y ago
... and this is why I will never touch C++ and stay with simple C. (Ah no..., wrong thread. This is actually about C. /s)
techslave 7y ago
Has C jumped the shark?
jwilk 7y ago
(2005) Discussed in 2011: https://news.ycombinator.com/item?id=3392728 https://news.ycombinator.com/item?id=3392728

107 points by bcaa7f3a8bbc 7y ago

The ternary operator is indeed special as it (is the only one AFAIK that) introduces in the language some type constraints between the two result branches, that affect the way one or the other is interpreted. A similar trick in C++ is explained on https://www.artima.com/cppsource/foreach2.html https://www.artima.com/cppsource/foreach2.html and allows to return the type of an expression without evaluating it. From the link: > You may be thinking this is needlessly convoluted. After all, couldn't we just use encode_type(container) without the conditional operator and get the same result? No, because that would have caused container to be evaluated. With the conditional operator, only one branch of the condition is ever executed. In this case, since the condition is always true, the first branch will always be taken. The second branch is "dead code"—it will never execute—yet it extends a ghostly finger into the land of the living and exerts its influence on the conditional expression's type. Spooky!

smitty1e 7y ago
That's just C++ wishing that it could have a type system closer to Haskell's.
saagarjha 7y ago
Qt apparently does something similar to implement Q_FOREACH: https://stackoverflow.com/a/10522384/5230900 https://stackoverflow.com/a/10522384/5230900
- likeliv 7y ago
  That was pre-c++11. Now using C++11, it is much simpler (using 'auto'). And the C++17 version is even simpler https://code.woboq.org/qt5/qtbase/src/corelib/global/qglobal.h.html#1085-1107 https://code.woboq.org/qt5/qtbase/src/corelib/global/qglobal...
aardvark179 7y ago
That sort of operator is often tough, just look at section 15.25 in the Java language specification (https://docs.oracle.com/javase/specs/jls/se13/html/jls-15.html#jls-15.25 https://docs.oracle.com/javase/specs/jls/se13/html/jls-15.ht...) in the type of such expressions.
MauranKilom 7y ago
For those wondering (like I did) "But there already is a C++ facility whose sole purpose is determining the type of an expression [or entity] without evaluation it?" decltype() only exists since C++11 (and the above article is from 2006). https://en.cppreference.com/w/cpp/language/decltype https://en.cppreference.com/w/cpp/language/decltype

The_rationalist 7y ago

C11 obscoleted this with the _Generic macro

EdSchouten 7y ago
C11 doesn't obsolete it, right? It only permits it to be implemented portably. For the curious, here's an implementation of tgmath.h on top of _Generic: https://github.com/NuxiNL/cloudlibc/blob/master/src/include/tgmath.h https://github.com/NuxiNL/cloudlibc/blob/master/src/include/...
- kzrdude 7y ago
  Far from obsolete.. more is coming :) 'Add type generic interfaces for string functions to the C library, see N2068' has been mentioned as a possible C2x addition https://gustedt.wordpress.com/2018/11/12/c2x/ https://gustedt.wordpress.com/2018/11/12/c2x/
fanf2 7y ago
And _Generic is still somewhat ugly: it has the awkward property that all the branches must pass the type checker even if they are not used. This is mostly OK for numerical functions where you get some help from C’s implicit conversions making the checker more lenient, but if you try to apply _Generic in other contexts you soon encounter problems.

amelius 7y ago

I always thought it was setjmp. https://en.wikipedia.org/wiki/Setjmp.h https://en.wikipedia.org/wiki/Setjmp.h

saagarjha 7y ago
setjmp isn’t really that ugly, IMO.
- ludamad 7y ago
  Yeah, low level and rare isn't the same as "bizarrely ripped from the design of another language"

deleted 7y ago

[deleted]

RcouF1uZ4gsC 7y ago

With apologies to Philip Greenspun: Any sufficiently complicated C program contains an ad-hoc, informally-specified, bug-ridden, slow implementation of half of C++.

wayoutthere 7y ago
Sure, but the C++ STL is to code as this [1] is to knives. [1] https://www.amazon.com/Wenger-16999-Swiss-Knife-Giant/dp/B001DZTJRQ https://www.amazon.com/Wenger-16999-Swiss-Knife-Giant/dp/B00....
- benj111 7y ago
  For the low low price of $9999.99. Is that the maximum you can charge on Amazon? Seems strange that they'd want to put a limit on it.
- jjoonathan 7y ago
  Now let's see boost in this metaphor :P (Are there enough knives in the entire swiss army to make this work?)
deleted 7y ago
[deleted]
marcosdumay 7y ago
Yes, but it's most of the good 1/5 of it, with just a little bit of the atrocious 1/3, and the rest taken from the "who cares" 7/15.

likeliv 7y ago

... and this is why I will never touch C++ and stay with simple C. (Ah no..., wrong thread. This is actually about C. /s)

techslave 7y ago

Has C jumped the shark?

jwilk 7y ago

(2005) Discussed in 2011: https://news.ycombinator.com/item?id=3392728 https://news.ycombinator.com/item?id=3392728