Getting an invalid reference in a range-based for loop [closed] - c++

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auto& kphist = this->kphist;
for (auto& it : kphist) {
it.second.aging(); // EXC-BAD-ACCESS
if(it.second.age > LAST_DAY){
kphist.erase(it.first);
continue;
}
}
kphist is a private member
Class A{
private:
unordered_map<int, KeyPointHistory> kphist;
}
The debugger shows all item in kphist is valid, how is it possible to have a bad reference inside the for loop. What possibly can go wrong?

Answer 1

From cppreference.com for std::unordered_map::erase(): References and iterators to the erased elements are invalidated. Other iterators and references are not invalidated. Thus, you cannot use std::unordered_map::erase() from within a range for loop (since this will try to increment an invalid iterator).
To avoid incrementing an invalidated iterator, you can simply increment first
and then erase using the original iterator:
for(auto i=map.begin(),end=map.end(); i!=end; ) { // no increment here
auto it=i++; // but here instead
if(must_remove(it))
map.erase(it);
}
In fact, since erase() returns the iterator to the next element, you can avoid the extra iterator it (thanks to Hurkyl to pointing this out in a comment):
for(auto i=map.begin(),end=map.end(); i!=end; ) { // no increment here
if(must_remove(i))
i = map.erase(i); // but here
else
++i; // or here instead
}
No need to make a list of keys of elements to be erased ...
Btw, why don't you use a std::map (rather than an std::unordered_map) as your key is an int (which is easily orderable)? Also, why do you make a reference kphist of a member variable of the same name?

Answer 2

You cannot erase the contents / iterator while you iterate over it and neither should you.
Save the element index in a different container and when you're done loop through that and erase the elements you got.
What possibly can go wrong?
Everything!

Answer 3

You can erase from an unordered_map by directly passing the iterator to the item to be erased. When you do so, erase() returns the subsequent iterator, so you can do something like this:
for (auto pos = kphist.begin(); pos != kphist.end(); ) {
it.second.aging();
if(it.second.age > LAST_DAY)
pos = kphist.erase(it);
else
++pos;
}
As a bonus, this will probably be a bit faster than passing the key to be erased--since you're providing the iterator, it can get to the item to be erased directly rather than re-hashing the key to find the position you already knew.