How to erase an vector element in this situation? - c++

Question

I made a game, rambo shoots bullets and bullets hit zombies and I want to erase the zombie who got hit, from the zombie vector.
This nested loop checks collision between every zombie and bullet one by one. It works good for a while but when I start killing more, at some point, it crashes because it wants to use a function of erased zombie.
for ( it = zombies.begin(); it != zombies.end(); ++it ) {
it->attack();
for (rambo.it = rambo.bullets.begin(); rambo.it != rambo.bullets.end(); ++rambo.it) {
if(checkBasicCollision(it,rambo.it) && it != zombies.end()){
zombies.erase(it);
}
}
}
I've added it--; after zombies.erase(it); works better now but it still crashes sometimes.
I think its happening like, for example there are 5 zombies and 20 bullets, zombie iterator is at second zombie, and second zombie starts the bullet loop to check if it got hit. Loop starts, lets say third bullet hit the zombie, but loop is still going, even if zombie is erased, it still continues the loop.
I've added break; after zombies.erase(it); now it hasn't got any problem. But the code looks so dirty. Is there another way to erase the current element easily

Answer 1

While the solution for manually erasing was presented, note that it is not the most idiomatic one. In idiomatic C++ you would make use of the std::remove_if algorithm in the erase-remove idiom like so:
// 1. A predicate that check whether a zombie was it by any bullet:
auto is_zombie_hit = [&rambo](Zombie const& zombie) {
auto is_bullet_hitting_zombie = [&zombie](Bullet const& bullet) {
return checkBasicCollision(zombie, bullet);
};
return std::any_of(
rambo.bullets.begin(),
rambo.bullets.end(),
is_bullet_hitting_zombie
);
};
// 2. Use the erase-remove idiom:
zombies.erase(
std::remove_if(zombies.begin(), zombies.end(), is_zombie_hit),
zombies.end()
);
Note: yes, you can use lambda in-place, however I prefer naming them to indicate their role.
Note: this uses C++11, however replacing lambda with predicates is trivial and an implementation of any_of is easy enough to produce, much like all_of and none_of.

Answer 2

To use erase you need to use the returned value and assign it back to the iterator so it is valid for the next iteration.
for ( it = zombies.begin(); it != zombies.end(); ) {
it->attack();
for (rambo.it = rambo.bullets.begin(); rambo.it != rambo.bullets.end(); ++rambo.it) {
if(checkBasicCollision(it,rambo.it) && it != zombies.end()){
it = zombies.erase(it); // erase will increment the iterator
}
else{
++it; // no erase, increment the iterator manually
}
}
}
From the documetion for vector::erase the return value is:
An iterator pointing to the new location of the element that followed the last element erased by the function call. This is the container end if the operation erased the last element in the sequence.

Answer 3

When erasing vector elements, iterators and indices become invalidated. Also your code is incorrect for the case 2 or more bullets hit the same zombie (if it is possible). Because with the second bullet the inner loop will try to erase the zombie that was already hit. Instead, you should do it like this:
for ( uint i = 0; i < zombies.size(); ++i)
{
for( auto it = rambo.bullets.begin(); it != rambo.bullets.end(); ++it)
{
if(checkBasicCollision(zombies[i], it)
{
zombies.erase( zombies.begin() + i );
--i;
break; // zombie is dead (well, technically it was already dead)
// so no further checks are needed (i.e. exit inner loop)
}
}
}

Answer 4

Perhaps something like this:
auto zombie_tail = std::remove_if(zombies.begin(), zombies.end(), [&](Zombie const & zombie) {
zombie.attack();
return std::any_of(rambo.bullets.begin(), rambo.bullets.end(), [&](Bullet const & bullet) {
return checkBasicCollision(zombie, bullet);
});
});
zombies.erase(zombie_tail, zombies.end());
Alternatively, if you wanted to stay away from c++ algorithms:
for (it = zombies.begin(); it != zombies.end(); ) {
it->attack();
// Attempt to find a bullet that hit.
for(rambo.it = rambo.bullets.begin(); rambo.it != rambo.bullets.end(); ++rambo.it)
if (checkBasicCollision(it, rambo.it))
break;
// Possibly remove the zombie, and advance the iterator
if (rambo.it != rambo.bullets.end())
it = zombies.erase(it);
else
++it;
}

Answer 5

direct, easy to read and grasp, but maybe not very fancy;
for ( auto& z : zombies )
z.attack();
for( auto& b : rambo.bullets )
{
auto itr = zombies.begin();
while( itr != zombies.end() )
{
if( checkBasicCollision(b,*itr) )
itr = zombies.erase(itr);
else
++itr;
}
}
checkBasicCollision now takes references, not iterators