I currently have a problem with vector.erase().
vector<gameObject> gameObjects;
for (auto it = gameObjects.end() - 1; it != gameObjects.begin();)
{
if ((it)->getDestroyed()) {
it = gameObjects.erase(it);
}
else {
--it;
}
}
So gameObject is the base class for everything inside the game and it has a bool flag that basically tells us if the object was destroyed. If the flag is set it should be removed from the vector.
class gameObject
{
protected:
bool toBeDestroyed;
public:
bool getDestroyed();
void markToDestroy();
};
Now the first destroyed object gets removed from the vector successfully and then I get get an error that iterator is not dereferencable, pointing to the vector library at line 73(?).
I then check with the msvc debugger. In the data preview it shows that iterator points to the last/newest element of gameObjects. It is then removed (erase(it)) and AFTERWARDS the data preview doesn't change and calling it->getDestroyed() results in the error message.
Debug assertion failed! vector iterator not dereferencible.
PS: I checked cplusplus.com and vector.erase should return a new, valid iterator so I'm not sure where I'm messing it up.
€: After I was told about the erase-remove idiom I went ahead and ended up with the following, which doesn't compile. Due to my function being a member of gameObject I'm not sure how to successfully call remove_if. Thanks
gameObjects.erase(remove_if(gameObjects.begin(), gameObjects.end(), gameObject::getDestroyed), gameObjects.end());
€2: A lot of you pointed out the first object isn't being checked. I propably should've pointed that out but the first element is ALWAYS the player and shouldn't be removed. Thanks for your comments nevertheless. I'll try with a simple forward loop without getting too fancy ^^.
€3: I tried Jonathan Mees suggested code but I get the exact same error message. I'll try and find out where exactly it happens but I can't just put a breakpoint into the erasing part anymore. Will mess around a bit.
€4: Problem was solved by removing the else {} condition and always decrementing the iterator. Thanks again for all your replies.
Let's say you have 2 objects in your vector and the last one is is marked as destroyed. When you call erase, it will return a new, valid iterator pointing at the element after the erased element. There is no element after the erased element, so the returned iterator is gameObjects.end(). You then continue to the top of the loop and dereference this iterator, which is not valid. You need to decrement your iterator after the erase if you want it pointing at a valid element.
One other note: If you ever wanted your first element removed, it will not be. Your loop exits when the iterator == gameObjects.begin(), so the first element is never checked.
Is there some reason you wanted to do this in reverse? If there is no specific reason, I would recommend you use the method recommended by #Borgleader.
Your loop is a little messed up - you're iterating backwards, ignoring the first element, and testing some elements multiple times. Might I suggest rbegin() as an alternative?
vector::erase returns the:
Iterator following the last removed element. If the iterator pos refers to the last element, the end() iterator is returned.
Meaning that vector::erase will never return vector::begin (unless you removed the only element in the container.) So it will always be dereferenced again after vector::erase is called. It will be dereferenced even if vector::end was returned by the call to vector::erase which is of course illegal.
Instead of this loop, consider using remove_if which is designed for this purpose:
gameObjects.erase(remove_if(begin(gameObjects),
end(gameObjects),
[](const auto& i){ return i.getDestroyed(); }), end(gameObjects));
EDIT:
I noticed you try to use this in your edit. You cannot use a bare function pointer as the predicate. If you want to avoid a lambda, you should consider the use of mem_fn:
gameObjects.erase(remove_if(begin(gameObjects),
end(gameObjects),
mem_fn(&gameObject::getDestroyed)), end(gameObjects));
Live Example
If there's difficulty in reading that line feel free to use as many variable as you like:
auto p = mem_fn(&gameObject::getDestroyed);
auto result = remove_if(begin(gameObjects), end(gameObjects), p);
gameObjects.erase(result, end(gameObjects));
Related
I have to write a function that erases an element out of the list if it's bigger than the previous element.(The previous element is the one which points to the next element before deletion)
I think I've basically finished it but I don't know why it doesn't erase 5 out of my list.
void deleteBigger(list<int> s){
list<int>::iterator test;
for(test = s.begin(); test != s.end(); test++){
int sk1=*test;
cout<<sk1<<endl;
test--;
int sk2=*test;
cout<<sk2<<endl;
if(sk1>sk2){
cout<<"Im here!\n";
s.erase(test);
}
test++;
}
}
It doesn't give an error or anything it just doesn't erase. I tried to test the erase method in the main function of the program, and there it worked fine.
There are three problems with your code:
Your list is passed by value, not reference. So you are changing a copy of your list and it doesn't alter the original container
You try to remove an element from a list while iterating it. Edit: As #Remy Lebeau mentioned in the comments, to be more precise it's a problem because you don't update the iterator properly, but not a problem on its own. Be advised, that when you remove an element from a list, the iterator which pointed to the erased element is considered invalidated.
Upon the first iteration, you decremented the iterator out of the container's bounds
Summing it up, what you might want to have here looks something like this:
void deleteBigger(std::list<int> &s) {
using namespace std;
if (s.empty()) {
return;
}
for(auto test = next(s.cbegin()); test != s.cend(); ++test){
while ((*test > *prev(test)) && (test != s.cend())) {
test = s.erase(test);
}
}
}
I've copied your code and it doesn't work. The problem is your iterator pointer "test". You can't degree a pointer at the begin. The only thing you can do is use a control.
Note: it's wrong decrement a pointer because you are decrementing of (32 bits) the index of memory. In this case there is overriding -- operator that saves your program but be careful next times
Control your program. It's important use a debugger editor where you can stop the program at certain point and control the value of the variables
I have been struggling to put a vector object into a project im doing
I have read what little i could find about doing this and decided to give it a go.
std::vector<BrickFalling> fell;
BrickFalling *f1;
I created the vector. This next piece works fine until i get to the erase
section.
if(brickFall == true){
f1 = new BrickFalling;
f1->getBrickXY(brickfallx,brickfally);
fell.push_back(*f1);
brickFall = false;
}
// Now setup an iterator loop through the vector
vector<BrickFalling>::iterator it;
for( it = fell.begin(); it != fell.end(); ++it ) {
// For each BrickFalling, print out their info
it->printBrickFallingInfo(brick,window,deadBrick);
//This is the part im doing wrong /////
if(deadBrick == true)// if dead brick erase
{
BrickFalling[it].erase;//not sure what im supposed to be doing here
deadBrick = false;
}
}
You can totally avoid the issue by using std::remove_if along with vector::erase.
auto it =
std::remove_if(fell.begin(), fell.end(), [&](BrickFalling& b)
{ bool deadBrick = false;
b.printBrickFallingInfo(brick,window,deadBrick);
return deadBrick; });
fell.erase(it, fell.end());
This avoids the hand-writing of the loop.
In general, you should strive to write erasure loops for sequence containers in this fashion. The reason is that it is very easy to get into the "invalid iterator" scenario when writing the loop yourself, i.e. not remembering to reseat your looping iterator each time an erase is done.
The only issue with your code which I do not know about is the printBrickFallingInfo function. If it throws an exception, you may introduce a bug during the erasure process. In that case, you may want to protect the call with a try/catch block to ensure you don't leave the function block too early.
Edit:
As the comment stated, your print... function could be doing too much work just to determine if a brick is falling. If you really are attempting to print stuff and do even more things that may cause some sort of side-effect, another approach similar in nature would be to use std::stable_partition.
With std::stable_partition you can "put on hold" the erasure and just move the elements to be erased at one position in the container (either at the beginning or at the end) all without invalidating those items. That's the main difference -- with std::stable_partition, all you would be doing is move the items to be processed, but the items after movement are still valid. Not so with std::remove and std::remove_if -- moved items are just invalid and any attempt to use those items as if they are still valid is undefined behavior.
auto it =
std::stable_partition(fell.begin(), fell.end(), [&](BrickFalling& b)
{ bool deadBrick = false;
b.printBrickFallingInfo(brick,window,deadBrick);
return deadBrick; });
// if you need to do something with the moved items besides
// erasing them, you can do so. The moved items start from
// fell.begin() up to the iterator it.
//...
//...
// Now we erase the items since we're done with them
fell.erase(fell.begin(), it);
The difference here is that the items we will eventually erase will lie to the left of the partitioning iterator it, so our erase() call will remove the items starting from the beginning. In addition to that, the items are still perfectly valid entries, so you can work with them in any way you wish before you finally erase them.
The other answer detailing the use of remove_if should be used whenever possible. If, however, your situations does not allow you to write your code using remove_if, which can happen in more complicated situations, you can use the following:
You can use vector::erase with an iterator to remove the element at that spot. The iterator used is then invalidated. erase returns a new iterator that points to the next element, so you can use that iterator to continue.
What you end up with is a loop like:
for( it = fell.begin(); it != fell.end(); /* iterator updated in loop */ )
{
if (shouldDelete)
it = fell.erase(it);
else
++it;
}
I'm interested in removing an element with a specific key out of a map and use this element.
Something that will look like:
itr = MyMap.pop(wantedKey);
//Now MyMap is missing the element which has the key 'wantedKey'.
//Do something with this element through 'itr'.
Is there an stl map method for doing this?
EDIT
Following carleeto's response, I want to clarify: What I need is the element being removed from the map and the program being able to use it afterwards, it could be the element itself as a pair, not necessarily an iterator.
There are two options: use it in-place then remove it, or move it to a local variable, remove the entry, then use it.
// use-remove
auto i = MyMap.find(wantedKey);
if (i != MyMap.end()) {
// use-remove
use(i->second);
MyMap.erase(i);
// or
// move-remove-use
auto x = std::move(i->second);
MyMap.erase(i);
use(x);
} else {
// Not found
}
Not that I know of, but you can use std::map::find to get an iterator and then call std::map::erase with said iterator as an argument when you're done.
From your variable naming, I think you might be confusing concepts here.
itr = MyMap.pop(wantedKey);
//Do something with this element through 'itr'.
Iterators only point to elements in containers. Therefore, if you had received an iterator through a function called pop (even if it existed), the iterator would reference not the element you popped, but probably the one after or before it, like std::vector::erase. This is because the purpose of an iterator is to iterate over the elements in a container. Therefore, if an element is not in the container, you cannot get an iterator to it. However, even if you used the iterator returned by the erase function, it would not reference you would be expecting it to.
So you can erase an element from the map, like so many have pointed out, by searching for it, getting the ierator to it and then calling erase with that iterator. but you cannot get an iterator that points to element you have erased. Hope this clears things up.
UPDATE: If all you want is to access the element and use it, then all you need to do use std::map::find to get an iterator and std::map::erase to remove the item from the map, once you have finished using the iterator. The reason is that even if you have stored a copy of the iterator for future use, once you call erase, it will be invalidated. To be able to access it after you have erased it, depending on scope, you will probably need to copy it.
Finally, what you want to do is a very common task - look up a map based on a key and perform an operation on the associated element. It's quite likely that you have a list of keys to go through. You should also look up functors, std::for_each and std::transform. I realise this is not operating on the element after you have removed it, but I thought I would add it in, seeing as how its a related operation. For example: You could move all elements that match a list of keys into another container (say, a vector, and then use the above to operate on them).
Probably what you want to do is
itr = MyMap.find('thing in a string');
to find the iterator and then use it,
MyMap.erase(itr)
And then erase it.
Pop() belongs to the stack datastructure. To access an element of a map, use the [] operator (http://www.cplusplus.com/reference/map/map/operator%5B%5D/), to remove it from the map use (http://www.cplusplus.com/reference/map/map/erase/).
itr = MyMap.find(wantedKey);
if(itr != MyMap.end()) {
use( itr->second );
MyMap.erase(itr);
}
your.uncle = bob;
Using C++'s std::map<T, U>::find():
map.erase(map.find(key));
The way I did it is below. In my case the map stores std::shared_ptr values, making the copy cheap(ish), and the object ownership transferral clear.
auto it = MyMap.find( wantedkey );
if ( it == MyMap.end() ) throw runtime_error("not found");
auto ret = it->second; // make copy of shared_ptr
MyMap.erase(it);
return ret;
The caller gets a shared_ptr with a reference count of at least one (from the copy). Note the function must return the shared_ptr by value, etc.
I am getting this error when trying to iterate over a map that is pointed to by another object. It works when I am not using a pointer. (Iterating over the member map "pieces") I am therefore wondering what to do, or if it's not possible to iterate through the map like this ? :
Board * Board::ccBoard(){
Board * newBoard = new Board();
map<Vec2, Piece>::iterator it;
for (it = newBoard->pieces.begin(); it != newBoard->pieces.end(); ++it)
newBoard->removePiece(it->first);
return newBoard;
}
Thanks in advance!
The removePiece() function removes the element that it is referring to, invalidating it. An attempt is then made to increment it resulting in the assertion failure. From map::erase():
References and iterators to the erased elements are invalidated.
I am unsure what the intention of the for loop is, it appears that it would effectively empty the map in which case just use map::clear():
newBoard->pieces.clear();
To fix, get rid of the ++it in the for loop and replace it->first by it++->first.
(This will increment the iterator and call erase() using a copy.)
I am trying to do a Z-Index reordering of videoObjects stored in a vector. The plan is to identify the videoObject which is going to be put on the first position of the vector, erase it and then insert it at the first position. Unfortunately the erase() function always causes bad memory access.
Here is my code:
testApp.h:
vector<videoObject> videoObjects;
vector<videoObject>::iterator itVid;
testApp.cpp:
// Get the videoObject which relates to the user event
for(itVid = videoObjects.begin(); itVid != videoObjects.end(); ++itVid) {
if(videoObjects.at(itVid - videoObjects.begin()).isInside(ofPoint(tcur.getX(), tcur.getY()))) {
videoObjects.erase(itVid);
}
}
This should be so simple but I just don't see where I'm taking the wrong turn.
You should do
itVid = videoObjects.erase(itVid);
Quote from cplusplus.com:
[vector::erase] invalidates all iterator and references to elements after position or first.
Return value: A random access iterator pointing to the new location of the element that followed the last element erased by the function call, which is the vector end if the operation erased the last element in the sequence.
Update: the way you access the current element inside your condition looks rather strange. Also one must avoid incrementing the iterator after erase, as this would skip an element and may cause out-of-bounds errors. Try this:
for(itVid = videoObjects.begin(); itVid != videoObjects.end(); ){
if(itVid->isInside(ofPoint(tcur.getX(), tcur.getY()))){
itVid = videoObjects.erase(itVid);
} else {
++itVid;
}
}
Beware, erasing elements one by one from a vector has quadratic complexity. STL to the rescue!
#include <algorithm>
#include <functional>
videoObjects.erase(
std::remove_if(
std::bind2nd(
std::mem_fun_ref(&videoObject::isInside),
ofPoint(tcur.getX(), tcur.getY())
),
),
videoObjects.end()
);
You cannot delete while iterating over the list because the iterator gets invalid. You should use the return iterator of Erase to set it to your current iterator.
erase function returns the next valid iterator.
You would have to make a while loop and do something like
iterator = erase(...)
with corresponding checks.