Question

look at the following simple code:

#include <iostream>
#include <string>
using namespace std;

int main()
{
    string s("1234567890");
    string::iterator i1 = s.begin();
    string::iterator i2 = s.begin();
    string s1, s2;
    s1.append(i1, ++i1);
    s2.append(++i2, s.end());

    cout << s1 << endl;
    cout << s2 << endl;
}

what would you expect the output to be?

would you, like me, expect it to be:

1
234567890

wrong!
it is:

234567890

i.e. the first string is empty.

seams that prefix increment operator is problematic with iterators. or am I missing something?

Answer 1

Not a bug.

The order in which the arguments to

s1.append(i1, ++i1);

are evaluated is not specified by the standard. The compiler is free to use any order it chooses. In this case, it evaluates the second argument (++i1) before the first (i1) and you specify a null range to copy.