Erasing a specific instance of an object from a vector

Question

I have a class Circle whose instances I keep track of with these:

Circle *f1;
vector<Circle> list;
vector<Circle>::iterator it;

I have managed to create multiple Circles and got them to move around. How can I erase a specific instance of Circle? For example, if a certain circle hits a wall, then it should be erased. I've looked around at other questions and I even tried the code they gave out and no luck. Here's what I've got at the moment:

for (it = list.begin(); it != list.end(); ++it) {

    it->x += 1;

    if (it->x == ofGetWindowWidth()) {
        list.erase(it);
    }
}

I have gotten other statements to work with the if statement such as reversing the direction of their movement. list.erase(it); was a line of code I got from here and I don't understand why it crashes my program.

Answer 1

for (it = list.begin(); it != list.end(); /* nothing here */) {

    it->x += 1;

    if (it->x == ofGetWindowWidth()) {
        it = list.erase(it);
    } else {
        ++it;
    }
}

The problem with your original code is that erasing an element invalidates the iterator to that element - the very same iterator you are trying to increment next. This exhibits undefined behavior.

Answer 2

list.erase invalidates iterators to the erased element. Therefore, after you erase the element pointed to by "it", "it" is invalidated and the ++it, which follows after the for loops body, can crash your program. Rewriting your code to something similiar to the following should prevent your crash:

for(it=list.begin();it!=list.end(); ) {
    //your code
    if(it->x==ofGetWindowWidth())
        it=list.erase(it);
    else
        ++it;
}

Answer 3

The problem with the above code using erase() is that it invalidates the content of it when the element is being erase. You can use, e.g., this instead:

for (it = list.begin(); it != list.end(); ) {
    it->x += 1;

    if (it->x == ofGetWindowWidth()) {
        list.erase(it++);
    }
    else {
        ++it;
    }
}

The branch using erase() moves the kept iterator it off its current location before erase()ing the element. Only the temporary object return from it++ gets invalidated. Of course, for this loop to work, you can't unconditionally increment it, i.e., the non-erase()ing branch needs its own increment.

Answer 4

You could use erase with remove_if. This also works for removal of multiple elements. In your case it's

list.erase(std::remove_if(list.begin(), list.end(),
        [](const Circle& c){return c.x == ofGetWindowWidth();},list.end()), 

Example with integers:

#include <algorithm>
#include <vector>
#include <iostream>

int main()
{
    std::vector<int> str1 = {1,3,5,7};
    str1.erase(std::remove_if(str1.begin(), str1.end(),
                              [](int x){return x<4 && x>2;}), str1.end());
   for(auto i : str1) std::cout << i ;
}

prints 157