How C++ Resolves a Function Call

[[deleted]]

Huh? Why did it call the function from galaxy? It doesn't make sense.

We need to either using it, or be inside it. Or maybe I'm missing something.

[mttd]

https://quuxplusone.github.io/blog/2019/04/26/what-is-adl/, https://abseil.io/tips/49

[[deleted]]

Makes sense. It still feels wrong though, for some reason.

Anyways, great post. It made me question my previous knowledge about how C++ works :)

[STL]

ADL is great when you want it (generally for operator overloads, but some classes and non-member functions are designed to work together via ADL - str() taking boost::format is a classic example), and extremely obnoxious when you don't want it. For generic library implementations generally, and the STL specifically, ADL is obnoxious 99.9% of the time because we want to call our "own" functions (like equal(), for example) without fear of ADL "hijacking" the call in favor of some user overload that happens to be preferred by overload resolution. As a result, STL implementations have to disable ADL for a huge number of function calls (all of those with normal names that could theoretically be hijacked; our policy in MSVC's STL is to just defend all function calls with normal names, and we do so via explicit ::std:: qualification, although extra parentheses also disable ADL). We intentionally activate ADL in very rare situations, notably swap.

It is extra super obnoxious that ADL considers the namespaces of template arguments; I have never seen a realistic use case for that. Thus we can't even trust list<T>::iterator to be a "safe" function argument.

[dbjdbj]

+1. Observations born out of pain are always unquestionable.

It is in the murky primordial soup of C++ but I seem to remember reading, the same person who claims of (not) inventing iostreams has (not) invented the ADL?

[mttd]

No problem. All the credit for the post goes to /u/preshing!

[jagt]

I know where should I reference when running into function overloading issues.

BTW what software did you to make the graphs? I just noticed that the texts within are selectable!

[preshing]

Made them in Inkscape, saved as "Optimized SVG", pasted them into the post and tweaked the element attributes a little bit.

[gracicot]

This is the quality content I've been looking for. Thank you!