What is the std::vector::iterator's index in the vector?

Question

I have an std::vector<Bullet> bullets and in the for-loop below I want to remove a bullet from the vector if it's not alive anymore.

My plan is to remove the element with pop_back(). If there are more than one element in the vector I want to first swap the element that is to be removed with the last element in the vector, and then call pop_back().

for (std::vector<Bullet>::iterator b = bullets.begin(); b != bullets.end(); ++b) { if(!b->isAlive()) { if (bullets.size() > 1) { std::iter_swap(bullets + ..., bullets.end()); } bullets.pop_back(); } }

The problem is the first parameter in iter_swap. I looked up http://www.cplusplus.com/reference/algorithm/iter_swap/ and the syntax for the first parameter is the vector + the position of the element.

How do I find out b's index in the vector?

Either I'm missing something, or you need an iterator for the first element and already have the right iterator: b. — user395760
– user395760, Commented Jul 10, 2014 at 17:48
@Cyber Because that takes linear time while swap-and-pop takes constant time. — user395760
– user395760, Commented Jul 10, 2014 at 17:48
@delnan Your for loop takes linear time. Remove-erase is swap-and-pop. — Oktalist
– Oktalist, Commented Jul 10, 2014 at 17:52
Don't you want iter_swap(b, bullets.end() - 1) (emphasis on "- 1")? — dlf
– dlf, Commented Jul 10, 2014 at 18:02

Nikos Athanasiou · Accepted Answer · 2014-07-10 17:49:39Z

If the condition governing whether an element is to be removed or not is :

object->isAlive()

Then you should use an STL way to do the removal, namely the erase-remove idiom :

bullets.erase(std::remove_if(bullets.begin(), bullets.end(), [](Bullet const& b) { return !b.isAlive(); }), bullets.end());

Now, to answer your particular question an iterator's it index in a vector v can be obtained like so :

auto indx = std::distance(v.begin(), it);

Dietrich Epp · Accepted Answer · 2014-07-10 17:48:32Z

There's an easier way to filter a std::vector.

#include <algorithm> auto part = std::remove_if( bullets_.begin(), bullets_.end(), [](const Bullet &bullet) { return !bullet.isAlive(); }); bullets_.erase(part, bullets_.end());

This will partition the vector into the alive and dead bullets, and then you delete the segment with the dead bullets.

The std::remove_if() function is like partition() but only the order of the first partition is preserved.

Thanks. What type is that auto there? My doesn't support it.
@A.D. Time to upgrade your compiler... or at least turn on modern C++ support. That's C++11.
Yes, I know. I was wondering what the type is so I can exchange the auto for it and then use your solution.
@A.D. If you don't support auto you'll probably need to replace the lambda with a functor object as well. You can replace auto with the iterator type for your vector.

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