Define a reference in a range-based for loop

Question

I am totally new to C++, so the question may be a bit naive :D! Recently I have been confused by a question relating to declaration in the range-based for. We are allowed to write something like:

for(int &i : vec)

Then a confusion occurred. It seems to me, the variable i will be defined once and be assigned to different value in each loop. But in the case above, i is a reference which is just an alias and should be bound to only one object. I think one way to come around this is to think that a new i is defined each time. I searched for further explanation on this and found a page: range for!:

Syntax attr(optional)

for ( range_declaration : range_expression ) loop_statement

Explanation

The above syntax produces code similar to the following (__range, __begin and __end are for exposition only):

auto && __range = range_expression ;
for (auto __begin = begin_expr,

            __end = end_expr;

        __begin != __end; ++__begin) {

    range_declaration = *__begin;
    loop_statement
}

The 'range_declaration' is defined within for loop. But isn't a variable defined inside a loop reused which means the reference i defined in the first example is reused? Still I am confused and could any one please give me some hints. Thanks!

Angew is no longer proud of SO · Accepted Answer · 2013-12-04 15:03:35Z

4

A variable defined inside the loop body is local to the loop body and thus destroyed and re-defined each time the loop iterates. So the int &i is freshly initialised in each iteration and there's no problem.

It might be clearer if we perfrom the substitutions (with some simplifications) into the example you've posted:

for (auto b = begin(vec), e = end(vec); b != e; ++b) {
  int &i = *b;
  //loop statement
}

answered Dec 4, 2013 at 15:03

Angew is no longer proud of SO

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2 Comments

dylanG35 Over a year ago

Hi Angew, thank you very much. I get it now. I think my misunderstanding lies in not to be clear about variables defined in a loop. I thought variables is defined once and will be reused later. To be certain, assume the for loop you wrote above has 10 iterations. In the first round, reference i is defined and after the closing curly brace is met, i is destroyed. Now in the second round, a new reference i is defined and will be discarded once the second round is finished. This pattern continues till the end. Have I got it right? Thanks!

Angew is no longer proud of SO Over a year ago

@dylanG35 Yes, you got it right. You can easily verify this yourself by creating a class which does something observable (e.g. write to std::cout) in its constructor and destructor, and declaring a local variable of this class type inside the loop. You'll see the ctor and dtor are invoked in each iteration.

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