edit: The below code can be run on https://wandbox.org/permlink/1Qry83quzoDveYDi
I implemented various suggestions from the comments, but unfortunately, it is still not clear to my why erasing the item at this particular std::list::iterator (line 86) crashes at runtime. All the explanations given below seem to affirm that the iterator should still be valid at this point.
I was under the impression that iterators to items in a std::list do not get invalidated when an item is inserted into a list (refer to this excellent post).
However, in the below code, the line
items.at(noOfitems-2)->erase(iter++); (line 86)
crashes the program with
malloc: *** error for object 0x100778b28: pointer being freed was not allocated.
Could you please help me understand why (where) this iterator std::list<std::string>::iterator becomes invalid, and how I can make it work without iteratively finding it again?
Am I perhaps misunderstanding the error?
#include <iostream>
#include <iostream>
#include <vector>
#include <random>
#include <unordered_set>
#include <set>
#include <unordered_map>
#include <map>
#include <list>
#include <utility>
#include <chrono>
#include <sstream>
#include <tr1/memory>
class Increment;
struct Item;
struct Pairhash {
public:
template <typename T>
std::size_t operator()(const T &x) const
{
return std::hash<T>()(x) ^ std::hash<T>()(x);
}
};
struct Item {
Item() = default;
std::string name;
int counter;
Item(std::string name) : counter(0)
{
this->name = name;
}
bool operator==(const Item& p) const
{
return (this->name == p.name);
}
bool operator<(const Item& p) const
{
return (this->counter < p.counter);
}
};
class Increment {
private:
std::unordered_map<std::string, std::pair<std::list<std::string>::iterator , std::shared_ptr<Item> >, Pairhash > itemMap;
std::vector<std::shared_ptr<std::list<std::string>>> items;
public:
Increment() = default;
std::list<std::string>::iterator insertItem(std::string & name , int noOfitems)
{
if (noOfitems > items.size())
{
items.emplace_back(std::make_shared<std::list<std::string>>(std::initializer_list<std::string>{ name }));
return items.back()->begin();
}
else
{
items.at(noOfitems-1)->emplace_back(name);
return items.at(noOfitems-1)->rbegin().base(); //Last position in list
}
}
std::list<std::string>::iterator adjustItemPosition(std::string & name, int noOfitems, std::list<std::string>::iterator & iter)
{
if (noOfitems > items.size())
{
std::list<std::string> temp{name};
items.push_back(std::make_shared<std::list<std::string>>(temp));
}
else
{
items.at(noOfitems-1)->emplace_back(name);
}
/* // Works as expected
auto itr = std::find(items.at(noOfitems-2)->begin(), items.at(noOfitems-2)->end(), name);
if (itr != items.at(noOfitems-2)->end())
{
items.at(noOfitems-2)->erase(itr);
}
*/
items.at(noOfitems-2)->erase(iter++); //TODO Crashes
return items.at(noOfitems-1)->rbegin().base(); //Last position in list
}
void incrementByOne(std::string name)
{
auto it = itemMap.find(name);
if (it != itemMap.end()) //Item already in map
{
it->second.second->counter++;
it->second.first = adjustItemPosition(name, it->second.second->counter,
it->second.first);
}
else //New item to be inserted
{
std::shared_ptr<Item> temp = std::make_shared<Item>(Item(name));
temp->counter = 1;
std::list<std::string>::iterator listIter = insertItem(name, 1);
itemMap.emplace(name, std::make_pair( listIter, temp));
}
}
std::string printTop10() const
{
std::stringstream ss;
auto count(0);
for (auto it = items.rbegin(); it != items.rend(); ++it)
{
for (auto item : **it)
{
if (count == 10)
{
break;
}
ss << "We have " << itemMap.at(item).second->counter << " " << item << std::endl;
++count;
}
}
return ss.str();
}
};
int main(int argc, const char * argv[]) {
Increment incrementer;
std::vector<std::string> names{ "Bananas", "Apples", "Peaches", "Durians", "Hazelnuts", "Avocados", "Pineapples", "Cherries", "Almonds", "Olives", "Eggs", "Yoghurts", "Peas", "Blueberries" };
for (int i = 0; i < 100; ++i) {
incrementer.incrementByOne(names.at(i%10));
}
std::cout << incrementer.printTop10() << std::endl;
return 0;
}
Replacing
return items.at(noOfitems-1)->rbegin().base();
with
return std::next(items.at(noOfitems-1)->end(), -1);
in lines 63 and 86 fixes the crash.
Working solution: https://wandbox.org/permlink/I68Szb0XMRKsPZqp
#include <iostream>
#include <iostream>
#include <vector>
#include <random>
#include <unordered_set>
#include <set>
#include <unordered_map>
#include <map>
#include <list>
#include <utility>
#include <chrono>
#include <sstream>
#include <memory>
class Increment;
struct Item;
struct Pairhash {
public:
template <typename T>
std::size_t operator()(const T &x) const
{
return std::hash<T>()(x) ^ std::hash<T>()(x);
}
};
struct Item {
Item() = default;
std::string name;
int counter;
Item(std::string name) : counter(0)
{
this->name = name;
}
bool operator==(const Item& p) const
{
return (this->name == p.name);
}
bool operator<(const Item& p) const
{
return (this->counter < p.counter);
}
};
class Increment {
private:
std::unordered_map<std::string, std::pair<std::list<std::string>::iterator , std::shared_ptr<Item> >, Pairhash > itemMap;
std::vector<std::shared_ptr<std::list<std::string>>> items;
public:
Increment() = default;
std::list<std::string>::iterator insertItem(std::string & name , int noOfitems)
{
if (noOfitems > items.size())
{
items.emplace_back(std::make_shared<std::list<std::string>>(std::initializer_list<std::string>{ name }));
return items.back()->begin();
}
else
{
items.at(noOfitems-1)->emplace_back(name);
return std::next(items.at(noOfitems-1)->end(), -1); // versus return items.at(noOfitems-1)->rbegin().base(); //Crashes
}
}
std::list<std::string>::iterator adjustItemPosition(std::string & name, int noOfitems, std::list<std::string>::iterator & iter)
{
if (noOfitems > items.size())
{
std::list<std::string> temp{name};
items.push_back(std::make_shared<std::list<std::string>>(temp));
}
else
{
items.at(noOfitems-1)->emplace_back(name);
}
/* // Works as expected
auto itr = std::find(items.at(noOfitems-2)->begin(), items.at(noOfitems-2)->end(), name);
if (itr != items.at(noOfitems-2)->end())
{
items.at(noOfitems-2)->erase(itr);
}
*/
items.at(noOfitems-2)->erase(iter++); //TODO Crashes
return std::next(items.at(noOfitems-1)->end(), -1); //versus return items.at(noOfitems-1)->rbegin().base(); //Crashes
}
void incrementByOne(std::string name)
{
auto it = itemMap.find(name);
if (it != itemMap.end()) //Item already in map
{
it->second.second->counter++;
it->second.first = adjustItemPosition(name, it->second.second->counter,
it->second.first);
}
else //New item to be inserted
{
std::shared_ptr<Item> temp = std::make_shared<Item>(Item(name));
temp->counter = 1;
std::list<std::string>::iterator listIter = insertItem(name, 1);
itemMap.emplace(name, std::make_pair( listIter, temp));
}
}
std::string printTop10() const
{
std::stringstream ss;
auto count(0);
for (auto it = items.rbegin(); it != items.rend(); ++it)
{
for (auto item : **it)
{
if (count == 10)
{
break;
}
ss << "We have " << itemMap.at(item).second->counter << " " << item << std::endl;
++count;
}
}
return ss.str();
}
};
int main(int argc, const char * argv[]) {
Increment incrementer;
std::vector<std::string> names{ "Bananas", "Apples", "Peaches", "Durians", "Hazelnuts", "Avocados", "Pineapples", "Cherries", "Almonds", "Olives", "Eggs", "Yoghurts", "Peas", "Blueberries" };
for (int i = 0; i < 100; ++i) {
incrementer.incrementByOne(names.at(i%10));
}
std::cout << incrementer.printTop10() << std::endl;
return 0;
}
I believe the answer as to why this is so was given in https://stackoverflow.com/a/33851951, namely that the pointer to the reverse iterator is in fact a copy of the original reference to it, which is deleted when it goes out of scope. http://cplusplus.github.io/LWG/lwg-defects.html#2360