can not dynamically add element to vector in going over the vector

[John Black]

Hi,
I just found out that in looping the vector, you can not push_back()
to the vector even in some "safe" cases. Here is the code snippet,

vector<pair<UINT32, UINT32> >::iterator itr;
for (itr = vec.begin(); itr< vec.end(); ++itr){

pair<UINT32, UINT32> p1, p2;
... assign value of p1 and p2 ...

if (..in some condition...){
vec.push_back(p1);
vec.push_back(p2);
vec.erase(itr);
return 0;
}
}

I think this is a safe dynamic addition to the vector as you can see
once something is added to the vector, I return from the loop. But from
the debugger I see that after the first push_back(), the iterator, itr,
changes. I just can not image why this happens.

Thanks!

 

[Victor Bazarov]

I just found out that in looping the vector, you can not push_back()
to the vector even in some "safe" cases. Here is the code snippet,

vector<pair<UINT32, UINT32> >::iterator itr;
for (itr = vec.begin(); itr< vec.end(); ++itr){

pair<UINT32, UINT32> p1, p2;
... assign value of p1 and p2 ...

if (..in some condition...){
vec.push_back(p1);
vec.push_back(p2);
vec.erase(itr);

Here 'itr' is invalid, most likely.

return 0;
}
}

I think this is a safe dynamic addition to the vector as you can see
once something is added to the vector, I return from the loop. But from
the debugger I see that after the first push_back(), the iterator, itr,
changes. I just can not image why this happens.

Because it most likely cannot be kept the same.

Victor

 

[John Harrison]

Hi,
I just found out that in looping the vector, you can not push_back()
to the vector even in some "safe" cases. Here is the code snippet,

vector<pair<UINT32, UINT32> >::iterator itr;
for (itr = vec.begin(); itr< vec.end(); ++itr){

pair<UINT32, UINT32> p1, p2;
... assign value of p1 and p2 ...

if (..in some condition...){
vec.push_back(p1);
vec.push_back(p2);
vec.erase(itr);
return 0;
}
}

I think this is a safe dynamic addition to the vector as you can see
once something is added to the vector, I return from the loop. But from
the debugger I see that after the first push_back(), the iterator, itr,
changes. I just can not image why this happens.

Thanks!

There's a simple solution, just use integers instead of iterators.

john

 

[Dave Townsend]

John,

When you push_back an item in the vector, the iterators potentially
will be invalid, think of it, the vector might need to reallocate the
internal storage for the elements, so any pointers/iterators now are
invalid.
Perhaps if you "reserve" up front you might not see the invalidation of
the iterators, but its not something I'd do in production code.

dave

 

[Jeff Flinn]

Hi,
I just found out that in looping the vector, you can not push_back()
to the vector even in some "safe" cases. Here is the code snippet,

Then they are not safe cases.

vector<pair<UINT32, UINT32> >::iterator itr;
for (itr = vec.begin(); itr< vec.end(); ++itr){

'itr < vec.end()' is not portable. Use 'itr != vec.end()' Iterators are not
pointers.

pair<UINT32, UINT32> p1, p2;
... assign value of p1 and p2 ...

if (..in some condition...){

Reverse the order and do the erase first.

vec.erase(itr);

You should be ok, as long as you do in fact exit the loop here. Otherwise
you'd 'itr = vec.erase(itr);' and remove the ++itr from the above for
statement and do the increment explicitly in an else clause.

vec.push_back(p1);
vec.push_back(p2);
return 0;
}
}

I think this is a safe dynamic addition to the vector as you can see
once something is added to the vector, I return from the loop. But from
the debugger I see that after the first push_back(), the iterator, itr,
changes. I just can not image why this happens.

A much better approach is:

typedef vector<pair<UINT32, UINT32> tVec;

tVec::iterator lItr = std::find_if( vec.begin(), vec.end(), condition_fnc );

if( lItr != vec.end() )
{
// some calcs

vec.erase( lItr );

vec.push_back( ... );
}

Jeff F