I have a map definition and subsequent manipulation like this.
map<int,string> m;
m.insert(std::pair<int,string>(1,"A");
m.insert(std::pair<int,string>(2,"B");
m.insert(std::pair<int,string>(3,"C");
m.insert(std::pair<int,string>(4,"D");
auto it = m.find(2);
m.erase(m.find(3));
cout<< it->second;
Will "it" be valid after an erase to some other element ?
Will "it" be valid after an erase to some other element ?
Yes, std::map::erase will only invalidate references and iterators to the erased elements.
References and iterators to the erased elements are invalidated. Other references and iterators are not affected.
Note the code m.erase(m.find(3)); has a potential problem, since std::map::find will return end() iterator if nothing is found, but end() iterator cannot be used with std::map::erase.
The iterator pos must be valid and dereferenceable. Thus the end()
iterator (which is valid, but is not dereferencable) cannot be used as
a value for pos.
Yes it will. Only the erased iterator is invalidated when a map erase is performed.
Related
Following lines of C++ code gives runtime error but if erase operation mymap.erase(v) is removed it works:
map<int,int> mymap = {{1,0},{2,1},{9,2},{10,3},{11,4}};
for(auto it=mymap.rbegin();it!=mymap.rend();){
int v=it->first;
++it;
mymap.erase(v);
}
demo
Here iterator it is changed before deleting its value v, so iterator it should remain unaffected I believe.
When you are calling erase(v), you are invalidating the base iterator that the next reverse_iterator (from ++it) is using. So you need to create a new reverse_iterator from the base iterator that precedes the erased value.
Also, rather than erasing the value that the reverse_iterator is referring to, you should erase the base iterator instead, since you already know which element you want to erase. There is no need to make the map go hunting for the value again.
This works for me:
map<int,int> mymap = {{1,0},{2,1},{9,2},{10,3},{11,4}};
for(auto it = mymap.rbegin(); it != mymap.rend(); ){
auto v = --(it.base());
v = mymap.erase(v);
it = map<int,int>::reverse_iterator(v);
}
Demo
On the other hand, this loop is essentially just erase()'ing all elements from mymap, so a better option is to use mymap.clear() instead.
Indeed, std::map::erase:
References and iterators to the erased elements are invalidated. Other references and iterators are not affected.
But std::reverse_iterator
For a reverse iterator r constructed from an iterator i, the relationship &*r == &*(i-1) is always true (as long as r is dereferenceable); thus a reverse iterator constructed from a one-past-the-end iterator dereferences to the last element in a sequence.
Perhaps it gets more clear when you look at the image on the cppreference page.
The crucial part is "Reverse iterator stores an iterator to the next element than the one it actually refers to".
As a consequence (small modification to your code)
auto& element = *it; // fails in the next iteration, because...
int v = element.first;
++it; // now it stores an iterator to element
mymap.erase(v); // iterators to element are invalidated
You are erasing the element that is used by it in the next iteration.
just wondering if I have a map declared like this,
map<int, int> my_map;
and assign end() iterator to a variable, and never change,
auto end_it = my_map.end()
Then later I will change my_map by having many erase and insert of my_map,
will the below always stays true during the life time of my_map?
end_it == my_map.end()
std::map::insert guarantees:
No iterators or references are invalidated.
std::map::erase guarantees:
References and iterators to the erased elements are invalidated. Other references and iterators are not affected.
Therefore the end iterator should stay valid. If you use any other operation, you should check whether it might invalidate the iterators.
Because vectors use an array as their underlying storage, inserting
elements in positions other than the vector end causes the container
to relocate all the elements that were after position to their new
positions.
< http://www.cplusplus.com/reference/vector/vector/insert/ >
I thought that this is the reason that iterator it becomes no longer valid after the last line in code below:
std::vector<int> myvector (3,100);
std::vector<int>::iterator it;
it = myvector.begin();
it = myvector.insert ( it , 200 );
myvector.insert (it,2,300);
But if I change the it's definition into myvector.end();, it's still the same. What is the reason behind this? How exactly does it work and are there situations where iterator insert can be still valid after filling part of vector with some elements? (or single one)
Well yes, that is a reason for iterators to elements after (and at, because insertion is done before the given element) the insertion point are invalidated. If you insert to the end, then there are no elements whose iterators could be invalidated. The end iterator is always invalidated, no matter where you insert. The more relevant description on that page:
Iterator validity
If a reallocation happens, all iterators, pointers and references related to the container are invalidated.
Otherwise, only those pointing to position and beyond are invalidated, with all iterators, pointers and references to elements before position guaranteed to keep referring to the same elements they were referring to before the call.
Here's what it points to if you change the first assignment to end.
it = myvector.end();
it points to end, good.
it = myvector.insert ( it , 200 );
Inserting to end does not invalidate any pointers to elements, but it does invalidate the end iterator which is the old value for it. Luckily, you now assign to the iterator returned by insert. That iterator does not point to the end of the vector but to the newly inserted element.
myvector.insert (it,2,300);
Now it is invalidated again, but you don't reassign it, so it remains so.
Of course, then there is the possibility, after each insert, that the vector was reallocated in which case all previous iterators to any part of the vector would be invalidated. That can be avoided by guaranteeing sufficient space with vector::reserve before initializing the iterators. The new iterator returned by insert will always be valid, even if the vector was reallocated.
Here is a better reference and explanation.
Causes reallocation if the new size() is greater than the old capacity(). If the new size() is greater than capacity(), all iterators and references are invalidated. Otherwise, only the iterators and references before the insertion point remain valid. The past-the-end iterator is also invalidated.
— http://en.cppreference.com/w/cpp/container/vector/insert
In the case where you use end(), size() still goes above capacity(). Try setting the capacity to something larger before insert().
We (all) know, erasing an element, pointer by an iterator invalidates the iterator, for example:
std::map< .. > map_;
std::map< .. >::iterator iter;
// ..
map_.erase( iter ); // this will invalidate `iter`.
But, what about:
map_.erase( map_.begin() );
is this safe? Will map_.begin() be a valid iterator, pointing to the (new) first element of the map?
"test it" is not a solution.
begin() is not an iterator but returns an iterator. After erasing the first element, begin()returns another (valid) iterator.
std::map<int, int> m;
m[1] = 2;
m[2] = 3;
m.erase(m.begin()); // <- begin() points to 1:2
std::cout << m.begin()->second; // <- begin() points to 2:3 now
On cppreference, we see:
All iterators (pos, first, last) must be valid and dereferenceable,
that is, the end() iterator (which is valid, but is not
dereferencable) cannot be used.
That pretty much answers your question. As long as the iterator returned by begin() is valid and dereferencable, it is OK to be used in std::map::erase(). A good way then to check if begin() is OK to be used in std::map::erase is by checking it if it is not equal to end():
if(map.begin() != map.end()) {
map.erase(map.begin());
}
Alternatively, you could also check if the map is empty, and use std::map::erase if it isn't
if(!map.empty()) {
map.erase(map.begin());
}
Of course it is.
map::begin returns a valid iterator referring to the first element in the map container.
http://www.cplusplus.com/reference/map/map/begin/
Pay attention to empty map.
is this safe?
Yes. It invalidates the temporary iterator returned by this call to begin(), and that iterator is destroyed at the end of the statement.
Will map_.begin() be a valid iterator, pointing to the (new) first element of the map?
Yes, unless the map is now empty. Erasing an element does not prevent you from creating new iterators to the remaining elements; that would make the map unusable.
std::vector<int> ints;
// ... fill ints with random values
for(std::vector<int>::iterator it = ints.begin(); it != ints.end(); )
{
if(*it < 10)
{
*it = ints.back();
ints.pop_back();
continue;
}
it++;
}
This code is not working because when pop_back() is called, it is invalidated. But I don't find any doc talking about invalidation of iterators in std::vector::pop_back().
Do you have some links about that?
The call to pop_back() removes the last element in the vector and so the iterator to that element is invalidated. The pop_back() call does not invalidate iterators to items before the last element, only reallocation will do that. From Josuttis' "C++ Standard Library Reference":
Inserting or removing elements
invalidates references, pointers, and
iterators that refer to the following
element. If an insertion causes
reallocation, it invalidates all
references, iterators, and pointers.
Here is your answer, directly from The Holy Standard:
23.2.4.2 A vector satisfies all of the requirements of a container and of a reversible container (given in two tables in 23.1) and of a sequence, including most of the optional sequence requirements (23.1.1).
23.1.1.12 Table 68
expressiona.pop_back()
return typevoid
operational semanticsa.erase(--a.end())
containervector, list, deque
Notice that a.pop_back is equivalent to a.erase(--a.end()). Looking at vector's specifics on erase:
23.2.4.3.3 - iterator erase(iterator position) - effects - Invalidates all the iterators and references after the point of the erase
Therefore, once you call pop_back, any iterators to the previously final element (which now no longer exists) are invalidated.
Looking at your code, the problem is that when you remove the final element and the list becomes empty, you still increment it and walk off the end of the list.
(I use the numbering scheme as used in the C++0x working draft, obtainable here
Table 94 at page 732 says that pop_back (if it exists in a sequence container) has the following effect:
{ iterator tmp = a.end();
--tmp;
a.erase(tmp); }
23.1.1, point 12 states that:
Unless otherwise specified (either explicitly or by defining a function in terms of other functions), invoking a container
member function or passing a container as an argument to a library function shall not invalidate iterators to, or change
the values of, objects within that container.
Both accessing end() as applying prefix-- have no such effect, erase() however:
23.2.6.4 (concerning vector.erase() point 4):
Effects: Invalidates iterators and references at or after the point of the erase.
So in conclusion: pop_back() will only invalidate an iterator to the last element, per the standard.
Here is a quote from SGI's STL documentation (http://www.sgi.com/tech/stl/Vector.html):
[5] A vector's iterators are invalidated when its memory is reallocated. Additionally, inserting or deleting an element in the middle of a vector invalidates all iterators that point to elements following the insertion or deletion point. It follows that you can prevent a vector's iterators from being invalidated if you use reserve() to preallocate as much memory as the vector will ever use, and if all insertions and deletions are at the vector's end.
I think it follows that pop_back only invalidates the iterator pointing at the last element and the end() iterator. We really need to see the data for which the code fails, as well as the manner in which it fails to decide what's going on. As far as I can tell, the code should work - the usual problem in such code is that removal of element and ++ on iterator happen in the same iteration, the way #mikhaild points out. However, in this code it's not the case: it++ does not happen when pop_back is called.
Something bad may still happen when it is pointing to the last element, and the last element is less than 10. We're now comparing an invalidated it and end(). It may still work, but no guarantees can be made.
Iterators are only invalidated on reallocation of storage. Google is your friend: see footnote 5.
Your code is not working for other reasons.
pop_back() invalidates only iterators that point to the last element. From C++ Standard Library Reference:
Inserting or removing elements
invalidates references, pointers, and
iterators that refer to the following
element. If an insertion causes
reallocation, it invalidates all
references, iterators, and pointers.
So to answer your question, no it does not invalidate all iterators.
However, in your code example, it can invalidate it when it is pointing to the last element and the value is below 10. In which case Visual Studio debug STL will mark iterator as invalidated, and further check for it not being equal to end() will show an assert.
If iterators are implemented as pure pointers (as they would in probably all non-debug STL vector cases), your code should just work. If iterators are more than pointers, then your code does not handle this case of removing the last element correctly.
Error is that when "it" points to the last element of vector and if this element is less than 10, this last element is removed. And now "it" points to ints.end(), next "it++" moves pointer to ints.end()+1, so now "it" running away from ints.end(), and you got infinite loop scanning all your memory :).
The "official specification" is the C++ Standard. If you don't have access to a copy of C++03, you can get the latest draft of C++0x from the Committee's website: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2723.pdf
The "Operational Semantics" section of container requirements specifies that pop_back() is equivalent to { iterator i = end(); --i; erase(i); }. the [vector.modifiers] section for erase says "Effects: Invalidates iterators and references at or after the point of the erase."
If you want the intuition argument, pop_back is no-fail (since destruction of value_types in standard containers are not allowed to throw exceptions), so it cannot do any copy or allocation (since they can throw), which means that you can guess that the iterator to the erased element and the end iterator are invalidated, but the remainder are not.
pop_back() will only invalidate it if it was pointing to the last item in the vector. Your code will therefore fail whenever the last int in the vector is less than 10, as follows:
*it = ints.back(); // Set *it to the value it already has
ints.pop_back(); // Invalidate the iterator
continue; // Loop round and access the invalid iterator
You might want to consider using the return value of erase instead of swapping the back element to the deleted position an popping back. For sequences erase returns an iterator pointing the the element one beyond the element being deleted. Note that this method may cause more copying than your original algorithm.
for(std::vector<int>::iterator it = ints.begin(); it != ints.end(); )
{
if(*it < 10)
it = ints.erase( it );
else
++it;
}
std::remove_if could also be an alternative solution.
struct LessThanTen { bool operator()( int n ) { return n < 10; } };
ints.erase( std::remove_if( ints.begin(), ints.end(), LessThanTen() ), ints.end() );
std::remove_if is (like my first algorithm) stable, so it may not be the most efficient way of doing this, but it is succinct.
Check out the information here (cplusplus.com):
Delete last element
Removes the last element in the vector, effectively reducing the vector size by one and invalidating all iterators and references to it.