Maps - For Each (Ranged For) or Iterators?

Question

Imagine I had this map:

std::map<int, int> map;

map.insert(std::pair<int, int>(1, 1));
map.insert(std::pair<int, int>(2, 2));
map.insert(std::pair<int, int>(3, 3));
map.insert(std::pair<int, int>(4, 4));
map.insert(std::pair<int, int>(5, 5));

What would be the best to use?

This:

for(std::pair<int, int> pair : map) {} //or
for each(std::pair<int, int> pair in map) {}

Or this:

for(std::map<int, int>::iterator itr = map.begin(); itr != map.end(); itr++) {} //or
for(auto itr = map.begin(); itr != map.end(); itr++) {}

@PiotrSkotnicki It's C++/CLI syntax, IIRC. (C++/CLI is an ECMA standard so while it's not standard C++, it's still a standard nonetheless.) — C. K. Young, Sep 22 '15 at 18:27
P.S. `for each(X in Y)` is nonstandard Microsoft C++; you should use `for(X : Y)` in C++. — nneonneo, Sep 22 '15 at 18:27
Also note that the variable name `map` risks a conflict with `std::map`. — alexgolec, Sep 22 '15 at 18:28
@PiotrSkotnicki Yep, that's why `auto` is so useful, less risk of getting it wrong. (But the OP was copy-constructing `std::pair` from it so that was still valid.) — C. K. Young, Sep 22 '15 at 18:29
@ChrisJester-Young correct, but it's common practice to do `using std::map`. — alexgolec, Sep 22 '15 at 18:30
By for each, I meant the range for. I've always seen it described as for each, which explains the title @PiotrSkotnicki . — jay, Sep 22 '15 at 18:31
@Alex I agree with Piotr, I've always seen/used std::map without using namespace std — jay, Sep 22 '15 at 18:32
I know this is done from personal experience, in my company we have our own map standard map type that lives in the global namespace, so I've grown keenly aware of this practice. — alexgolec, Sep 22 '15 at 18:32
Also @nneonneo I guessed that, just put it there to properly explain what I meant :) — jay, Sep 22 '15 at 18:32
@Alex Putting anything in the global namespace is an antipattern. ;-) — C. K. Young, Sep 22 '15 at 18:32
`for (auto&& p : map)` to get what you almost always want by default — Ryan Haining, Sep 22 '15 at 18:36
I invite you to read the excellent question, answers and comments here: http://stackoverflow.com/questions/13130708/what-is-the-advantage-of-using-universal-references-in-range-based-for-loops — Christian Hackl, Sep 22 '15 at 18:57

C. K. Young · Answer 1 · 2015-09-22T18:57:11.537

2

The best to use is:

for (auto&& entry : map)

edited Sep 22 '15 at 18:57

answered Sep 22 '15 at 18:26

C. K. Young

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The first creates unnecessary copies. The second can sometimes fail (not for maps, but in general). See http://stackoverflow.com/questions/13130708/what-is-the-advantage-of-using-universal-references-in-range-based-for-loops – Christian Hackl Sep 22 '15 at 18:55

score 1 · Accepted Answer · answered Sep 22 '15 at 18:26

1

From a performance perspective they are equivalent. The first two are generally preferred because they are much more readable.

answered Sep 22 '15 at 18:26

alexgolec

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Ah thanks. this is essentially what I wanted to know; I wondered if they had different performance, as I see iterators mentioned everywhere and the method I described above mentioned nowhere. Will accept answer when possible :) – jay Sep 22 '15 at 18:28
@jay But you should still use `auto` (see my answer)! – C. K. Young Sep 22 '15 at 18:33
@ChrisJester-Young Yeah, that's a valid point. I'm accepting this one due to the extra info, but thanks for the info you gave me :) – jay Sep 22 '15 at 18:34
[for each in](https://msdn.microsoft.com/en-us/library/ms177202.aspx) is a non standard Microsoft extension that IMHO is not preferable as it is not portable. – NathanOliver Sep 22 '15 at 18:36
@NathanOliver Yeah I know, I added that there just simply to explain a little bit better. I prefer `for(X : Y)` anyways :) – jay Sep 22 '15 at 18:37
@NathanOliver C++/CLI is actually an ECMA standard so it's not correct to call it nonstandard, but of course it's not standard C++. ;-) – C. K. Young Sep 22 '15 at 18:43

Maps - For Each (Ranged For) or Iterators?

2 Answers2