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Archive for May, 2011

STL and erase()

The inconsistency of erase() method in STL containers has always bothered me.

Say if you would want to loop through a std::list, print its items, and erase the item from the list, you could do the following.

std::list<int> l(6); // add in 6 zeros
std::list<int>::iterator itr = l.begin();
while(l.end() != itr)
{
	std::cout << *itr << std::endl;
	itr = l.erase(itr);
}

Pretty straight forward.

The erase() method will remove the item pointed by the current iterator, and move the iterator to the next element.

Now what if you want to do the same to a std::set?

std::set<int> s; // add in 6 zeros
//do some initialization here
std::set<int>::iterator s_itr = s.begin();
while(s.end() != s_itr)
{
	std::cout << *s_itr << std::endl;
	s_itr = s.erase(s_itr); // COMPILER ERROR!
}

Accck! This won’t compile under gcc because erase() method in an associative container returns void. To work around the problem, you need to use the post-increment operator of the iterator within the erase() method.

std::set<int> s; // add in 6 zeros
//do some initialization here
std::set<int>::iterator s_itr = s.begin();
while(s.end() != s_itr)
{
	std::cout << *s_itr << std::endl;
	s.erase(s_itr++); // use post-increment
}

In C++, even erasing an item from a STL container is subtle and error-proning.

The Standard

To make matter worse, Visual Studio’s erase() method implementation violates the standard, and is consistent across all containers. This confuses a lot of people.

As far as I can see, this inconsistency has been deemed as a defect by LGW since 1999. Hell, even Josuttis mentioned a few dissatisfactions in his infamous STL Bible.

In the current C++0x standard, it looks like this issue will finally be put to rest. So… just a few more years of pain before the next compiler upgrade.

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