I trying to brush up on my C++ since as it has been a while since I used it and I'm having a problem with storing pointers. At the end of the method below, the vector "graph" has all of the vertices I inserts, but the edges that were supposed to be added are corrupt (I can see the edges in the debugger, but their data is garbage). I was wondering if someone could help guide?
Thanks!
For reference, the add_edge function is declared as follows:
std::vector<vertice*> m_edges;
...
...
void add_edge(vertice* n);
{
m_edges.push_back(n);
}
The primary problem:
std::vector<vertice> graph;
std::string line;
//Iterate over line
int lineCount = 0;
while(getline(file, line))
{
auto vertices = parse_line(line);
for(size_t i = 0; i < vertices.size(); ++i)
{
auto result = std::find_if(
graph.begin(),
graph.end(),
[&] (vertice cVert) { return cVert.getName() == vertices.at(i); });
std::vector<vertice>::size_type currPos =
std::distance(graph.begin(), result);
if(result == graph.end())
{
graph.emplace_back(vertice{vertices.at(i)});
}
if(i == 0) { continue; }
graph.at(lineCount).add_edge(currPos == graph.size() ?
&graph.back() : &graph.at(currPos));
}
++lineCount;
}
//The vector graph has corrupt nodes here
Pointers to the content of a std::vector may be invalidated when you insert new elements, such as in std::vector::emplace_back:
If the new size() is greater than capacity() then all iterators and references (including the past-the-end iterator) are invalidated. Otherwise only the past-the-end iterator is invalidated.
Consider std::vector::reserve to reserve a capacity before needing to reallocate the elements or use different container, one which does not invalidate references on insert, perhaps std::list.
The problem lies here:
auto v = currPos == graph.size() ? &graph.back() : &graph.at(currPos);
You are taking the address of the graph elements, however if (when) your graph vector resizes during the emplace_back, you end up with dangling pointers. A solution is to fix the size of the vector and instead of push_back/emplace_back, to use directly std::vector::operator[], or to reserve enough memory via std::vector::reserve.
Probably unrelated:
graph.emplace_back(vertice{vertices.at(i)});
should just be
graph.emplace_back(vertices.at(i));
as otherwise you are invoking a copy constructor, see the definition of std::vector::emplace_back.
Related
I have a vector that holds items that are either active or inactive. I want the size of this vector to stay small for performance issues, so I want items that have been marked inactive to be erased from the vector. I tried doing this while iterating but I am getting the error "vector iterators incompatible".
vector<Orb>::iterator i = orbsList.begin();
while(i != orbsList.end()) {
bool isActive = (*i).active;
if(!isActive) {
orbsList.erase(i++);
}
else {
// do something with *i
++i;
}
}
The most readable way I've done this in the past is to use std::vector::erase combined with std::remove_if. In the example below, I use this combination to remove any number less than 10 from a vector.
(For non-c++0x, you can just replace the lambda below with your own predicate:)
// a list of ints
int myInts[] = {1, 7, 8, 4, 5, 10, 15, 22, 50. 29};
std::vector v(myInts, myInts + sizeof(myInts) / sizeof(int));
// get rid of anything < 10
v.erase(std::remove_if(v.begin(), v.end(),
[](int i) { return i < 10; }), v.end());
I agree with wilx's answer. Here is an implementation:
// curFiles is: vector < string > curFiles;
vector< string >::iterator it = curFiles.begin();
while(it != curFiles.end()) {
if(aConditionIsMet) {
it = curFiles.erase(it);
}
else ++it;
}
You can do that but you will have to reshuffle your while() a bit, I think. The erase() function returns an iterator to the element next after the erased one: iterator erase(iterator position);. Quoting from the standard from 23.1.1/7:
The iterator returned from a.erase(q)
points to the element immediately
following q prior to the element being
erased. If no such element exists,
a.end() is returned.
Though maybe you should be using the Erase-remove idiom instead.
erase returns a pointer to the next iterator value (same as Vassilis):
vector <cMyClass>::iterator mit
for(mit = myVec.begin(); mit != myVec.end(); )
{ if(condition)
mit = myVec.erase(mit);
else
mit++;
}
If someone need working on indexes
vector<int> vector;
for(int i=0;i<10;++i)vector.push_back(i);
int size = vector.size();
for (int i = 0; i < size; ++i)
{
assert(i > -1 && i < (int)vector.size());
if(vector[i] % 3 == 0)
{
printf("Removing %d, %d\n",vector[i],i);
vector.erase(vector.begin() + i);
}
if (size != (int)vector.size())
{
--i;
size = vector.size();
printf("Go back %d\n",size);
}
}
As they said, vector's iterators get invalidated on vector::erase() no matter which form of iterator increment you use. Use an integer index instead.
You might want to consider using a std::list instead of a std::vector for your data structure. It is safer (less bug prone) to use when combining erasure with iteration.
Removing items from the middle of a vector will invalidate all iterators to that vector, so you cannot do this (update: without resorting to Wilx's suggestion).
Also, if you're worried about performance, erasing items from the middle of a vector is a bad idea anyway. Perhaps you want to use an std::list?
I have a vector that holds items that are either active or inactive. I want the size of this vector to stay small for performance issues, so I want items that have been marked inactive to be erased from the vector. I tried doing this while iterating but I am getting the error "vector iterators incompatible".
vector<Orb>::iterator i = orbsList.begin();
while(i != orbsList.end()) {
bool isActive = (*i).active;
if(!isActive) {
orbsList.erase(i++);
}
else {
// do something with *i
++i;
}
}
The most readable way I've done this in the past is to use std::vector::erase combined with std::remove_if. In the example below, I use this combination to remove any number less than 10 from a vector.
(For non-c++0x, you can just replace the lambda below with your own predicate:)
// a list of ints
int myInts[] = {1, 7, 8, 4, 5, 10, 15, 22, 50. 29};
std::vector v(myInts, myInts + sizeof(myInts) / sizeof(int));
// get rid of anything < 10
v.erase(std::remove_if(v.begin(), v.end(),
[](int i) { return i < 10; }), v.end());
I agree with wilx's answer. Here is an implementation:
// curFiles is: vector < string > curFiles;
vector< string >::iterator it = curFiles.begin();
while(it != curFiles.end()) {
if(aConditionIsMet) {
it = curFiles.erase(it);
}
else ++it;
}
You can do that but you will have to reshuffle your while() a bit, I think. The erase() function returns an iterator to the element next after the erased one: iterator erase(iterator position);. Quoting from the standard from 23.1.1/7:
The iterator returned from a.erase(q)
points to the element immediately
following q prior to the element being
erased. If no such element exists,
a.end() is returned.
Though maybe you should be using the Erase-remove idiom instead.
erase returns a pointer to the next iterator value (same as Vassilis):
vector <cMyClass>::iterator mit
for(mit = myVec.begin(); mit != myVec.end(); )
{ if(condition)
mit = myVec.erase(mit);
else
mit++;
}
If someone need working on indexes
vector<int> vector;
for(int i=0;i<10;++i)vector.push_back(i);
int size = vector.size();
for (int i = 0; i < size; ++i)
{
assert(i > -1 && i < (int)vector.size());
if(vector[i] % 3 == 0)
{
printf("Removing %d, %d\n",vector[i],i);
vector.erase(vector.begin() + i);
}
if (size != (int)vector.size())
{
--i;
size = vector.size();
printf("Go back %d\n",size);
}
}
As they said, vector's iterators get invalidated on vector::erase() no matter which form of iterator increment you use. Use an integer index instead.
You might want to consider using a std::list instead of a std::vector for your data structure. It is safer (less bug prone) to use when combining erasure with iteration.
Removing items from the middle of a vector will invalidate all iterators to that vector, so you cannot do this (update: without resorting to Wilx's suggestion).
Also, if you're worried about performance, erasing items from the middle of a vector is a bad idea anyway. Perhaps you want to use an std::list?
I have a game where I check collision between bullets and enemies which I store as 2 vector containers. People say if you're gonna erase an element in the for loop you better use iterators and so I did. But I have a problem now with passing the iterator to a function. The thing is I don't necessarily need to erase the element so it has to be a bit more complex.
This is the way I check collision. "CircularCollision" works fine, no mistakes there.
void ResolveColision(Weapon &weap, Map &map)
{
std::vector<Bullet> bullets = weap.GetBullets();
if (!bullets.empty())
{
for (std::vector<Bullet>::iterator i = bullets.begin(); i != bullets.end(); ++i)
{
std::vector<Enemy> enemies = map.GetEnemies();
if (!enemies.empty())
{
for (std::vector<Enemy>::iterator j = enemies.begin(); j != enemies.end(); ++j)
{
if (CircularCollision((*i), (*j)))
{
weap.DeleteByIndex(i);
map.TakeDamageByIndex(j, weap.GetDamage());
std::cout << "HIT!\n";
}
}
}
}
}
}
Here's the method which is supposed to decrease the health of an enemy:
void Map::TakeDamageByIndex(std::vector<Enemy>::iterator &itr, int damage)
{
(*itr).SetHealth((*itr).GetHealth() - damage);
}
Here's the method which deletes the bullet:
void Weapon::DeleteByIndex(std::vector<Bullet>::iterator &itr)
{
destroySprite((*itr).GetSprite());
bullets.erase(itr);
}
I'm sure it looks horrible and it shouldn't work but I have no idea how to do it properly. Please help!
Also, both methods work properly when the for loops operate with indexes (e.g. bullets[i]), in that case the problem is with "Vector subscript out of range" error.
In DeleteByIndex(), change this:
bullets.erase(itr);
To this:
itr = bullets.erase(itr);
std::vector::erase() returns an iterator to the next remaining element after the element that was erased. That next element is where your outer loop needs to continue from on its next iteration.
As such, you need to change your outer loop from a for to a while instead, or else you will skip elements (in fact, your original code suffers from that problem when you were still using indexes):
void ResolveColision(Weapon &weap, Map &map)
{
std::vector<Bullet> bullets = weap.GetBullets();
std::vector<Bullet>::iterator bullerItr = bullets.begin();
while (bullerItr != bullets.end())
{
std::vector<Enemy> enemies = map.GetEnemies();
bool wasAnyHit = false;
for (std::vector<Enemy>::iterator enemyItr = enemies.begin(); enemyItr != enemies.end(); ++enemyItr)
{
if (CircularCollision(*bulletItr, *enemyItr))
{
wasAnyHit = true;
weap.DeleteByIndex(bulletItr);
map.TakeDamageByIndex(enemyItr, weap.GetDamage());
std::cout << "HIT!\n";
break;
}
}
if (!wasAnyHit)
++bulletItr;
}
}
That being said, I would suggest replacing the inner loop with std::find_if() instead. And renaming DeleteByIndex() and TakeDamageByIndex() since they don't take an index anymore. In fact, I would not pass an iterator to TakeDamage...() at all, pass the actual Enemy object instead. Or better, move TakeDamage() into Enemy itself.
Try something more like this:
void ResolveColision(Weapon &weap, Map &map)
{
auto bullets = weap.GetBullets();
auto bulletItr = bullets.begin();
while (bulletItr != bullets.end())
{
auto enemies = map.GetEnemies();
auto &bullet = *bulletItr;
auto enemyHit = std::find_if(enemies.begin(), enemies.end(),
[&](Enemy &enemy){ return CircularCollision(bullet, enemy); }
);
if (enemyHit != enemies.end())
{
weap.DeleteBulletByIterator(bulletItr);
enemyHit->TakeDamage(weap.GetDamage());
std::cout << "HIT!\n";
}
else
++bulletItr;
}
}
void Enemy::TakeDamage(int damage)
{
SetHealth(GetHealth() - damage);
}
void Weapon::DeleteBulletByIterator(std::vector<Bullet>::iterator &itr)
{
destroySprite(itr->GetSprite());
itr = bullets.erase(itr);
}
A few other comments in addition to Remy Lebeau’s answer.
It’s as efficient to pass a STL iterator by value as by reference, so the only reason you would need to pass one by reference is: when you intend to change the index and you want that change to be visible in the caller’s scope. (For example, a UTF-8 parser needs to consume anywhere from one to four bytes.) Since this code doesn’t need to do that, you’re better off just passing the iterator by value.
In general, if you aren’t modifying the variable you pass by reference, you should pass by const reference instead. In the case of Enemy::TakeDamage(), the only thing you do with the iterator is dereference it, so you might as well just pass in an Enemy& and call it with *i as the parameter.
The algorithm is not very efficient: if you delete a lot of items near the start of the list, you would need to move all remaining items of the array multiple times. This runs in O(N²) time. A std::list, although it has a high overhead compared to std::vector, can delete elements in constant time, and might be more efficient if you have a lot of insertions and deletions that are not at the end. You might also consider moving only the objects that survive to a new list and then destroying the old one. At least this way, you only need to copy once, and your pass runs in O(N) time.
If your containers store smart pointers to the objects, you only have to move the pointers to a new location, not the entire object. This will not make up for the overhead of lots of heap allocations if your objects are small, but could save you a lot of bandwidth if they are large. The objects will still be automatically deleted when the last reference to them is cleared.
You could do something like this:
void delByIndex(vector<int>::iterator &i, vector<int>& a)
{
a.erase(i);
}
int main()
{
vector<int> a {1,5,6,2,7,8,3};
vector<int> b {1,2,3,1};
for(auto i=a.begin();i!=a.end();)
{
bool flag = false;
for(auto j=b.begin();j!=b.end();j++)
{
if(*i==*j)
{
flag = true;
}
}
if(flag)
{
delByIndex(i, a);
}
else
i++;
}
for(auto i:a)
cout << i << " ";
return 0;
}
Be careful when using erase as it will change the size of the vector and also invalidates the vector iterator.
I have created a multimap for my road points. The key refers to the road number and the values are vec3 points that make up the road.
I am trying to iterate through the values of each key point and create a road segment at each point on the road (except the last), adjust the values to be on the road points and then store them in a std::vector.
The RoadSegment constructor creates 6 vec3 points and pushes them onto a std::vector.
I have a segmentation fault in the line marked in bold
[for(mapIt = it.first; mapIt != it.second; ++mapIt)]
When i take out the lines creating the new objects and pushing them onto the std::vector it works fine.
Can anyone tell me what the problem is / a solution to the problem??
Many thanks in advance
std::vector<glm::vec3>::iterator SegIt;
for(int i = 0; i < m_genRoads->getKeyValueData().size(); i++)
{
int numberDesired = m_genRoads->getMultimapData().count(i) - 1;
std::multimap<int, glm::vec3>::iterator mapIt;
std::pair<std::multimap<int, glm::vec3>::iterator, std::multimap<int, glm::vec3>::iterator> it;
it = m_genRoads->getMultimapData().equal_range(i);
for(mapIt = it.first; mapIt != it.second; ++mapIt)
{
int distance = std::distance(it.first, mapIt);
if(distance != numberDesired)
{
RoadSegement* roadSegmentPointer = new RoadSegement();
// FUNCTIONS TO ADJUST COORD VALUES TO MATCH THE ROAD POINTS
m_segmentArray.push_back(roadSegmentPointer);
}
else
{
continue;
}
///SOME BUFFER BINDING STUFF
The issue seems to be that you're using iterators that do not exist, all due to returning a temporary object.
it = m_genRoads->getMultimapData().equal_range(i);
Since getMultiMapData() returns a copy of the multimap, that multimap is gone after the line is executed, thus rendering any iterators invalid.
One solution is to return a reference to the multimap, not a new copy of the multimap.
std::multimap<int, glm::vec3>& GenerateRoads::getMultimapData() { return m_roadsMultimap; }
I wrote this method to find the minor of a sparse matrix:
SpMatrixVec SparseMatrix::minor(SpMatrixVec matrix, int col) const{
SpMatrixVec::iterator it = matrix.begin();
int currRow = it->getRow();
int currCol = col;
while(it != matrix.end()) {
if(it->getRow() == currRow || it->getCol() == currCol){
matrix.erase(it);
}
// if we have deleted an element in the array, it doesn't advance the
// iterator and size() will be decreased by one.
else{
it++;
}
}
// now, we alter the cells of the minor matrix to be of proper coordinates.
// this is necessary for sign computation (+/-) in the determinant recursive
// formula of detHelper(), since the minor matrix non-zero elements are now
// in different coordinates. The row is always decreased by one, since we
// work witht he first line, and the col is decreased by one if the element
// was located after 'col' (which this function receives as a parameter).
//change the cells of the minor to their proper coordinates.
for(it = matrix.begin(); it != matrix.end(); it++){
it->setRow(it->getRow()-1);
int newY;
newY = (it->getCol() > col) ? it->getCol() + 1 : it->getCol();
it->setCol(newY);
}
return matrix;
}
Now, i'm probably doing something wrong, because when reaching the second interation of the while loop, the program crashes.
The basic idea was to go over the vector, and see if it is the relevant coordinate, and if so - to delete it. I increment the iterator only if there was no deletion (and in this case, the vector should update the iterator to be pointing the next element..unless i got these things wrong).
Where is the problem?
Thanks a lot.
erase() invalidates your iterator.
You must update it using the return value of erase() for the loop to work:
while(it != matrix.end()) {
if(it->getRow() == currRow || it->getCol() == currCol){
//matrix.erase(it);
it = matrix.erase(it); // Here is the change
}
// if we have deleted an element in the array, it doesn't advance the
// iterator and size() will be decreased by one.
else{
//it++;
++it; // ++i is usually faster than i++. It's a good habit to use it.
}
}
erase invalidates your iterator. Do it = matrix.erase(it) instead.
You can't change a collection while you're iterating between its elements.
Use another temp collection to store the partial results.
edit: Even better, use a functor to delete elements: remove_if
You write the condition.