Show HN: A dbg(...) macro for C++

This seems very much inspired by the `dbg` macro recently introduced in Rust. [1]<p>Which in turn was inspired by Haskell s `Debug.Trace` functions [2].<p>It&#x27;s definitely a convenient tool if you can&#x27;t use a debugger or want to follow more complex interactions.<p>[1]<a href="https:&#x2F;&#x2F;doc.rust-lang.org&#x2F;std&#x2F;macro.dbg.html" rel="nofollow">https:&#x2F;&#x2F;doc.rust-lang.org&#x2F;std&#x2F;macro.dbg.html</a><p>[2] <a href="https:&#x2F;&#x2F;hackage.haskell.org&#x2F;package&#x2F;base-4.12.0.0&#x2F;docs&#x2F;Debug-Trace.html" rel="nofollow">https:&#x2F;&#x2F;hackage.haskell.org&#x2F;package&#x2F;base-4.12.0.0&#x2F;docs&#x2F;Debug...</a>

Would be nice to offer runtime enable&#x2F;disable. Out of curiosity, I wrote a little benchmark and submitted as a PR: <a href="https:&#x2F;&#x2F;github.com&#x2F;sharkdp&#x2F;dbg-macro&#x2F;pull&#x2F;57" rel="nofollow">https:&#x2F;&#x2F;github.com&#x2F;sharkdp&#x2F;dbg-macro&#x2F;pull&#x2F;57</a><p>The key idea is that modern CPUs dynamically branch-predict-away if-statements that are rarely&#x2F;never invoked, such as a dynamically disabled dbg() call.<p>With the performance difference magnified 10x by loop unrolling, this is what I&#x27;m seeing on my Mac laptop:<p>830000000 iterations in 3 secs = 2.76667e+08 iters&#x2F;sec (compiled-out). 840000000 iterations in 3 secs = 2.8e+08 iters&#x2F;sec (dynamically disabled).<p>And this is the worst case, where the whole program does nothing but call dbg() - real world programs contain lots of other real work, drowning the minute difference in performance, i.e. in a real program I doubt you&#x27;d see even 0.1% total performance difference, littering it with dynamic dbg() statements.<p>p.s. my C&#x2F;C++ is pretty rusty - feedback welcome, but pls be kind.

I wrote and use regularly similar macros in C++ context.<p>They are less advanced on some areas, but more advanced on some.<p>Particularly nice features:<p>* a structured log indented by stack trace depth (fully portable, put a simple macro at function start, will log start and any exit)<p>* log any expression: `somemacro(foo.bar())` will log `foo.bar() = 42`<p>* log scopes like functions<p>* with possibility to jump to source code line on one click or keypress<p>* browse the log with structured jumps (like step into function vs advance one line, back and forth)<p>...just by generating a text format that is parsed by common pre-existing tools (namely emacs compilation-mode and a few other short settings).<p>I&#x27;ve been planning to share that for a moment. It will eventually appear on <a href="https:&#x2F;&#x2F;github.com&#x2F;fidergo-stephane-gourichon?tab=repositories" rel="nofollow">https:&#x2F;&#x2F;github.com&#x2F;fidergo-stephane-gourichon?tab=repositori...</a><p>I wrote a similar set of macros for C, see my comment <a href="https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=21071497" rel="nofollow">https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=21071497</a> on eerimoq&#x27;s &quot;Show HN&quot; <a href="https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=21040649" rel="nofollow">https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=21040649</a>

Mozilla has something similar in Gecko&#x27;s codebase, inspired by the Rust macro: <a href="https:&#x2F;&#x2F;searchfox.org&#x2F;mozilla-central&#x2F;source&#x2F;mfbt&#x2F;DbgMacro.h" rel="nofollow">https:&#x2F;&#x2F;searchfox.org&#x2F;mozilla-central&#x2F;source&#x2F;mfbt&#x2F;DbgMacro.h</a>

I wrote something similar a couple years ago, except my macro automatically compiled to a no op if compiled in release mode, which is very convenient for switching back-and-forth to the testing.

This is neat. The obvious comparison is glog&#x27;s DLOG feature.

Oh ... this is really neat. I like how you can wrap an subexpression or prettyprint vectors.<p>When the debugger is too cumbersome to use good old prints is always a nice fallback.

The name reminds me of <a href="https:&#x2F;&#x2F;metacpan.org&#x2F;pod&#x2F;DBG" rel="nofollow">https:&#x2F;&#x2F;metacpan.org&#x2F;pod&#x2F;DBG</a>, which, full disclosure, I wrote. Not that it&#x27;s nearly as cool or likely to be used by anyone, but ... it has a superficial resemblance.

I did one of these for C(or at least gcc&#x2F;llvm c) but it&#x27;s a lot harder to abstract over types in c so it&#x27;s a bit more limited<p><a href="https:&#x2F;&#x2F;github.com&#x2F;rtaycher&#x2F;debug_print" rel="nofollow">https:&#x2F;&#x2F;github.com&#x2F;rtaycher&#x2F;debug_print</a>

It looks pretty cool. I agree that installing the header in &#x2F;usr&#x2F;include is a good idea.

Nice idea! I would appreciate a warning from the author specifying whether dbg evaluates its argument twice, as if so one must be careful with side effects when using it.

I was thinking about writing a lib like this... really useful

It would be great if we can see similar things in Java or golang

would be nice if we had something similar for C...

in lisp this would be<p><pre><code> (defmacro dbg [expr] (println &#x27;~expr ~expr))</code></pre>

This feels like too much of a &quot;ricer&quot; feature, at least the way it&#x27;s implemented.<p>Instead, you want:<p>* A pretty-printer library (possibly single-header) * A function for obtaining the typename; see:<p><pre><code> https:&#x2F;&#x2F;stackoverflow.com&#x2F;q&#x2F;35941045&#x2F;1593077 https:&#x2F;&#x2F;stackoverflow.com&#x2F;a&#x2F;56766138&#x2F;1593077 for a constexpr approach. </code></pre> and it won&#x27;t hurt to have:<p>* A logging library with log levels (so that you don&#x27;t necessarily need to recompile to enable these outputs) * A stack trace printing library<p>When you have that, such a macro becomes nearly trivial. Oh, yeah, and - drop the silly ANSI coloring.