Reset iterator inside loop with erase method - c++

Question

Here's a little piece of code, which is making us a little mad...
for (vector<QSharedPointer<Mine>>::iterator itMine = _mines.begin();
itMine != _mines.end();) {
auto point2 = shipFire[l].getBulletLine().p2();
if (_mines[pos]->checkCollision(point2)) { // _mines is a Vector<QSharedPointer<Mine>>
Explosion explosionMine(_mines[pos]->point());
_explosions.push_back(explosionMine);
itMine = _mines.erase(itMine);
bulletErased = true;
scoreFrame += 100;
}
else {
++itMine;
pos++;
}
}
Here is the problem, the itMine is erasing two of our vector<..<Mine>> and it's making the program to shutdown unexpectedly
We thought about it, and came up with this : All of our iterators are being invalidated after erasing the one of the Mine, right ?
But we are a little confused about how to change our actual code to fit to the new one ?
The main question is : how do we reset this itr ?
ps : if you have any questions, or if you need a little more of code to understand the logic behind, feel free to ask more question !
Best Regards, and thank you in advance.

Answer 1

A not uncommon solution to this problem is to swap the current position with the back position. The last item can then be removed without invalidating any iterators. See live example.
#include <iostream>
#include <vector>
#include <memory>
int main()
{
struct Mine
{
Mine( int id = 0 ): id( id ) { }
bool check_collision_point( int point ) const { return id % point == 0; }
int id;
};
auto mines = std::vector<std::shared_ptr<Mine>>{ 7 };
for( auto i = 0u; i < mines.size(); ++i )
{
mines[i] = std::make_shared<Mine>( i );
}
// save point outside loop
auto point = 2;
for( auto it = mines.begin(); it != mines.end(); )
{
// don't use mines[pos] in loop
if( (*it)->check_collision_point( point ) ){
std::iter_swap(it, mines.rbegin() );
mines.resize( mines.size() -1 );
}
else
++it;
}
for( auto it = mines.begin(); it != mines.end(); ++it)
std::cout << (*it)->id << " ";
}

Answer 2

Update
for (auto itMine = _mines.begin(); itMine != _mines.end();){
auto point2 = shipFire[l].getBulletLine().p2(); // thomas : -> here we need to do it in the loop, to get every bullet position while looping.
if (_mines[pos]->checkCollision(point2)){
Explosion explosionMine(_mines[pos]->point());
_explosions.push_back(explosionMine);
if (_mines[pos]->checkCollision(point2)){
mines_to_be_deleted.push_back(*itMine);
shipFire.erase(it);
bulletErased = true;
scoreFrame += 100;
qDebug() << "Collision bullet mine";
}
}
//else
++itMine;
pos++;
}
for (auto itrDelete = mines_to_be_deleted.begin(); itrDelete != mines_to_be_deleted.end();)
{
_mines.pop_back(); // Here instead of that, we need to erase the current itr (here is our mistake)
//mines_to_be_deleted.erase(itrDelete);
//_mines.resize(_mines.size());
qDebug() << "mine deleted";
++itrDelete;
}
mines_to_be_deleted.clear();
At this point we came up with this.
We tried everything you told us, but none of them worked, we thinked about swapping the current itr to the end of the vector. Thanks for your help.
For now, the program is running perfectly, but when we hit a Mine with a bullet, a ramdom one disapear.
For those who want to know, we have found the solution.
while(!mines_to_be_deleted.empty()) { // _mine_to_be_deleted is a QList<Sharedpointer<Mine>>
std::remove(begin(_mines),end(_mines),mines_to_be_deleted.front());
_mines.pop_back();
mines_to_be_deleted.pop_front();
}
using std::remove allow us to erase the Mine without creating a malloc()