This is based on the GNU C++11 compiler on Codeforces. Results inconclusive.
I will be omitting discussion of input and output, which is well-known.
- No significant performance difference between ranged fors (26322469, 26322507) and iterating over elements (26316124)
- No significant performance difference between assignment with
tie(26322531) and regular assignment (26316124) - No significant performance difference between
emplace,emplace_back(26316124) andpush,push_back(26322556)
It is well known that auto is compile-time, so there will be no performance difference.
First of all, thanks for research. I really don't know why you have so much downvotes, but I can't do more than one upvote :(
Now about results and my thoughts:
compilers usually convert for(auto smth : container) into iterating with iterator. As far as I know, vector's iterator in GCC is actually a pointer. And when you are using operator [] for vectors, you actually just add some value to pointer. So difference is predictably indistinguishable.
tie was made for using with tuples. And it is just a syntactic sugar for easy converting tuple to set of variables. I think GCC converts it to regular assignment
emplace and emplace_back are good for structures and classes with no copy constructors inside. Or if class has easy creating and hard copying constructors.
Ofc, it's just IMHO.
Yes, I checked the assembly code. Ranged fors are converted to iterators, and tie is converted to assignment, in GCC with -O2 switch at least.
emplaceandemplace_backalso bring some syntactic sugar to the language. Comparevec.push_back(make_pair(a, b))vsvec.push_back({a, b})vsvec.emplace_back(a, b).It is shorter but not always clearly. I mean, it doesn't give you any information about container — what type it stores. Sometimes it isn't required, but sometimes it costs couple of minutes.
Couple of minutes sounds overestimated, but yes, you lose some information by using this version of emplace_back. However this version mostly used with
pairand it's kind of clear in most cases.Could you, please, give an example of such situation? I don't see a problem here.
Something like this:
@ Trying to understand some functions of inherited code
@ After
Nhours of analysis you find method with 50+ lines of code@ Some bit operations, some methods, some attributes
@
for (auto i : q) some_name.emplace_back(i.a, i.b, i.c)@ wtf is some_name and what does it store
@ You are Marlin from "Finding Nemo" and some_name field is Nemo. But you have found it!
@
mutable set<T> some_name;@ this class is template class and you have no idea what does some_name mean