To me, it tends to bias the sample towards the subset of developer population using SO, which would not be a representative sample size for a true population sampling in a statistical problem - it would be oversampled in one group to the exclusion of all other groups.
Despite the name, cppreference is the best online (there's always `man 3`, of course) resource I've found for C, too. MSDN is good for C++ as well (usually the second place I check). And the GCC and Clang documentation is generally pretty comprehensive about compiler-specific stuff.
The C and C++ sections of StackOverflow are good for finding language lawyers who can give you an exact answer, with citations, to "is this snippet well-defined?" type questions (the TL;DR is usually "if you have to ask, then it isn't"). But they'll scold you for tagging those sorts of questions with both C and C++, even though it's perfectly reasonable to do.•
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• If you're asking about C, it's helpful to know whether your program will still have the same behavior when compiled as C++; if you're asking about C++ but your snippet doesn't have anything C++-specific, it's helpful to future viewers if the answers for both languages are in the same place, since more often than not they're the same (which also means it's helpful to make it clear when they aren't).
I have found a lot less need to look up things like "are there optional parameters in .len()?" because the IDE helps with that.
I need to look up strange errors that I don't understand. (Like when you install a new library and your code breaks). Those are almost never in the official documentation because they didn't plan for it.
I was speaking partially from my experience and, as another poster put it, IDEs help considerably in areas of syntax usage.
Besides, with more stable languages (C++ has a 3 yr standardization cadence and new features are not covered well on SO) you can find in-depth answers on SO. Otherwise, many mailing lists (yes, they still exist) are a better source of more current and more detailed treatment of the topic than, "hey, how do I do X so that I can move on to my next thing at work."
Don't get me wrong, I love SO, but I feel it's more about, "how do I solve this problem now?" vs "I want to learn more about this topic." It's real value is in the comments that others contribute "around" the accepted answers.
C++ devs tend to look at http://en.cppreference.com (using an English locale in this example).
To me, it tends to bias the sample towards the subset of developer population using SO, which would not be a representative sample size for a true population sampling in a statistical problem - it would be oversampled in one group to the exclusion of all other groups.