alt Hacker News> But if vec contains eg long strings, you've now murdered your perf because you're copying them out of the array instead of grabbing refs.
I've seen much more perf-murdering things being caused by
std::map<std::string, int> my_map;
for(const std::pair<std::string, int>& v: my_map) {
...
}
Replies
dubi_steinkek • yesterday at 1:23 PM
Why is this a perf footgun? As someone who doesn't write a lot of c++, I don't see anything intuitively wrong.
Is it that iterating over map yields something other than `std::pair`, but which can be converted to `std::pair` (with nontrivial cost) and that result is bound by reference?
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Wow I really thought this would be a compile error. The implicit cast here really is a footgun. Looks like '-Wrange-loop-construct' (included in -Wall) does catch it:
> warning: loop variable 'v' of type 'const std::pair<std::__cxx11::basic_string<char>, int>&' binds to a temporary constructed from type 'std::pair<const std::__cxx11::basic_string<char>, int>' [-Wrange-loop-construct] 11 | for (const std::pair<std::string, int>& v: m) {
As they say, the power of names...