What is the difference between regular "for" statement and range-based "for" statement in C++

Question

So, what is the difference between these two statement:

for(auto i : VectorName){} for(auto i = VectorName.begin(); i != VectorName.end(); i++){}

For example, I have this program:

#include <iostream> #include <string> #include <vector> using namespace std; int main() { vector<char> vec = {'H','e','l','l','o','W','o','r','l','d','!'}; for(auto i = vec.begin(); i != vec.end(); i++) // This loop has error { cout << i << endl; } for(auto k : vec) //This loop has no problem { cout << k << endl; } return 0; }

I am confused because in this example in this Microsoft docs:

// cl /EHsc /nologo /W4 #include <deque> using namespace std; int main() { deque<double> dqDoubleData(10, 0.1); for (auto iter = dqDoubleData.begin(); iter != dqDoubleData.end(); ++iter) { /* ... */ } // prefer range-for loops with the following information in mind // (this applies to any range-for with auto, not just deque) for (auto elem : dqDoubleData) // COPIES elements, not much better than the previous examples { /* ... */ } for (auto& elem : dqDoubleData) // observes and/or modifies elements IN-PLACE { /* ... */ } for (const auto& elem : dqDoubleData) // observes elements IN-PLACE { /* ... */ } }

They noted that the range for statement is not better than the regular one.

is your question which is better or why doesn't your code work? please stick to one question per post — Alan Birtles
– Alan Birtles, Commented May 19, 2021 at 7:29
"not much better than the previous examples" I.e. it is some improvement, but still not as good as it can be — Caleth
– Caleth, Commented May 19, 2021 at 7:58

Stefan Riedel · Accepted Answer · 2021-05-19 07:40:59Z

The difference in your case is, that the first version with iterators, well, uses iterators (that's why cout << i << endl; is not working), and the second version (the range-based for loop) gives you either a copy, a reference, or const reference.

So this:

for(auto i = vec.begin(); i != vec.end(); i++) { cout << i << endl; // should be *i }

uses iterators (vec.begin() gives you an iterator to the first element).

Whereas this:

for(auto i : vec) { cout << i << endl; }

uses copies of elements in your vector.

While this:

for(auto& i : vec) { cout << i << endl; }

uses references to your vector elements.

Jarod42 · Accepted Answer · 2021-05-19 07:31:38Z

In

for (auto i = vec.begin(); i != vec.end(); i++) { //cout << i << endl; // should be cout << *i << endl; auto& k = *i; cout << k << endl; }

i is an iterator, you have to deference it to retrieve the value to be nearly equivalent to second snippet.

Moberg · Accepted Answer · 2021-05-19 07:32:14Z

Regular for loop uses an explicit loop variable, in your examples an iterator i or iter (many times it is just an integer). To access elements in a vector for example, you need to use the loop variable to extract the elements from the vector.

Range-based for loops automatically gives you the elements inside the container you loop over, so there is no need to extract the elements inside the loop, you can immediately use k/elem.

About the comment you mention: that specific range-based style with no references is usually bad for performance (if the copy is unnecessary).

Andreas DM · Accepted Answer · 2021-05-20 05:24:40Z

So, what is the difference between these two statement:
for(auto i : VectorName){} for(auto i = VectorName.begin(); i != VectorName.end(); i++){} 

The first one will iterate through every item in the collection; i is a copy of the item.

The second one gives you more control over the range; i is an iterator, dereference to access item.

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