Should I use for iterators instead of for range based loop, if so, why?-CodePudding

I just learned about iterators. Should I use this?

for (auto& i : just_a_vec )

Or this?

for (std::vector<std::string>::iterator it = just_a_vec.begin(); it < just_a_vec.end(); it  )

And if so, why this is the case?

CodePudding user response：

Iterators predate range-based for loops, so they used to be the only of these two alternatives available. Nowadays, range-based for has mostly replaced iterators when dealing with a simple loop.

However, iterators can be used in various other contexts as well. For example, you can do std::sort(v.begin(), std::next(v.begin(), 5)) to sort the first five elements of a vector while leaving the rest of it alone.

Going back to iterating over a whole container:

If you can accomplish what you want with a range-based for, then it leads to more legible code, so they are preferable by default.
If you need iterators for some reason, such as using an algorithm that requires them, or because you need to jump ahead or back while iterating, then use those instead.

Also: In the later case, you can/should still use auto when declaring the iterator:

for(auto it = just_a_vec.begin(); it < just_a_vec.end(); it  ) {
}

Edit: as asked: here's a simple, if a bit contrived, example where an iterator-based loop can still be useful:

// adds all values in the vector, but skips over twos values when encountering a 0
// e.g.: {1,2,0,4,5,2} => 5
int my_weird_accum(const std::vector<int>& data) {
  int result = 0; 
 
  for(auto it = data.begin(); it != data.end();   it) {
    auto v = *it;
    result  = v;

    if(v == 0) {
      // skip over the next two
      assert(std::distance(it, data.end()) > 2);
      std::advance(it, 2);
    }
  }
  return 0;
}

CodePudding user response：

Quick personal answer: Its somewhat sylistic and based on what version of c you are using. I typically prefer range based, but there are certainly moments iterators shine as well. Add both to your toolchest. For further reading.

Here is a list of other SO answers that get more into the performance and use cases.

For further information. Google

Range vs iterator loop c