error from string insert

I try to insert a character into a string at every position that modulus 3 == 0.

Code:

#include <iostream>
#include <string>

using std::cout;
using std::endl;
using std::string;

int main( )
{
	string s = "01020304050607080910";
	string::iterator it;
	string::iterator begin = s.begin();
	string::const_iterator end = s.end();
	int cnt = 0;
	
	for( ; it != end; ++it ) {
		++cnt;
		cout << cnt << endl;
		if( cnt % 3 == 0 )
			it = s.insert( it, ' ' ); // <-- what's wrong here?

	}
	cout << "done scan" << endl;
	cout << s << endl;

	return 0;
}

I've got "Segmentation fault".
What's wrong with my insert?

I'm not sure what is wrong, maybe because you are modifying the iterator while traversing it ? but I would just do it like this ->

Code:

        int len = s.length();
	for(int i = 0; i < len; ++i)
	{
		if(i % 3 == 0){
			s.insert(i," ");
			len = s.length();
		}
	}

Did you intend the iterator it to iterate over any particular string? If so, you should probably make it refer somewhere, rather than pointing into space.

you never initialize your it

@all, thanks, I've got it.

When I insert a character, the string size (or capacity) is increased; however, my iterator at end and beginning is const, that is, it causes conflict in memory between const iterator and the new inserted character in string.Finally, it crashes, "Segmentation fault"

I fixed like this and it works well.

Code:

#include <iostream>
#include <string>

using std::cout;
using std::endl;
using std::string;

int main( )
{
	string s = "01020304050607080910";
	string::iterator it;

	int cnt = 0;
	
	for( it = s.begin(); it != s.end(); ++it ) {
		++cnt;
		if( cnt % 4 == 0 )
			it = s.insert( it, ' ' ); 

	}
	cout << s << endl;

	return 0;
}