C++ variable scope error inside for loop

Question

//Constructing set of all Places in Conflict with each other
int l_placeVec, l_placeVec1,p;
for(SP_ListListNode::const_iterator iter = m_postTransitionsSet.begin(),l_placeVec=0; iter != m_postTransitionsSet.end(); iter++,l_placeVec++) {
    for(SP_ListListNode::const_iterator inneriter = m_postTransitionsSet.begin(),l_placeVec1=0; inneriter != m_postTransitionsSet.end(); inneriter++,l_placeVec1++) {
        if((iter != inneriter) && ((**inneriter) == (**iter)) && (((int)((*iter)->size()))>1)) { //when the two lists are same
            SP_ListNode* temper = new SP_ListNode;
            temper->clear();
            for(SP_ListNode::const_iterator iterplaces = m_placeNodes->begin(),p=0; iterplaces != m_placeNodes->end(); iterplaces++,p++) {
                if((p == l_placeVec) || (p == l_placeVec1)) {
                    temper->push_back(*iterplaces);
                }
            }
            m_conflictingPlaces.push_back(temper);
        }
    }
}

The above code is saying: "Unused variable p", though I am using it in the third for loop. In case further information is required, please leave a comment.

But this is something weird I am facing.

You might want to split this up into functions, at the least... — GManNickG
– GManNickG, Commented Jul 23, 2010 at 23:03
If you have a C++0x compatible compiler, consider using the auto keyword to make your loop initializers a little easier to read: en.wikipedia.org/wiki/C++0x#Type_inference — kelm
– kelm, Commented Jul 23, 2010 at 23:17

AnT stands with Russia · Accepted Answer · 2010-07-23 23:07:02Z

4

You declared a completely different variable p in the inner loop. Here

for(SP_ListNode::const_iterator iterplaces = m_placeNodes->begin(),p=0; ...

The above is equivalent to declaring

SP_ListNode::const_iterator p = 0

which, of course, hides the outer p. The outer p remains unused, which is what the compiler is warning you about.

By a coincidence, this inner p is initializable with 0 and is comparable to int, even though its type is SP_ListNode::const_iterator, which is why there is no errors reported when you do this. But it is just a coincidence.

P.S. Just noticed that you did the same thing with all these outer int variables, which explains why the comparisons like p == l_placeVec do not fail.

edited Jul 23, 2010 at 23:07

answered Jul 23, 2010 at 23:01

AnT stands with Russia

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4 Comments

Gaurav Kalra Over a year ago

If i do it this way int p; for(SP_ListNode::const_iterator iterplaces = m_placeNodes->begin(),p=0; ... ... even then p is hidden. and i can't use multiple datatype declarations in for loop too. what is the way out then ?

GManNickG Over a year ago

@Gaurav: You initialize it prior to the for. Of course moving the p doesn't change the result, the first statement in a for-loop is a declaration, regardless of anything outside the loop. That first statement, as is, will always make a new variable named p, now and forever.

AnT stands with Russia Over a year ago

@Gaurav Kalra: Stop redeclaring p. Either do p = 0 before the cycle, or declare SP_ListNode::const_iterator iterplaces before the cycle. In the latter case the cycle will look as follows: for(iterplaces = m_placeNodes->begin(),p=0;....

Johannes Schaub - litb Over a year ago

@Gaurav not true. In C++ you can: stackoverflow.com/questions/75538/hidden-features-of-c/…

Ryan · Accepted Answer · 2010-07-23 23:03:57Z

0

"SP_ListNode::const_iterator iterplaces = m_placeNodes->begin(),p=0;" creates a new p of type SP_ListNode::const_iterator...so the outer p is not used.

answered Jul 23, 2010 at 23:03

Ryan

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