c++ - Why below exception safety code using auto

here's code not exception safe

//header file declaration void f(auto_ptr<t1>, auto_ptr<t2>);  //implementation file: f(auto_ptr<t1>(new t1), auto_ptr<t2>(new t2));

exception safe solution above suggested :

//implementation file: {     auto_ptr<t1> t1(new t1);     auto_ptr<t2> t2(new t2);     f(t1, t2); }

my question why it's different when both uses auto_ptr handle resource allocation?

this problem-solution part of "more exceptional c++" herb sutter.

the problem first code snippet order 2 sub-expressions building f() parameters executed undefined c++ standard. is, if happens like:

this work fine. quite possible order like:

and in case if step 2 (alloc t2) throws, allocated memory t1 leak irretrievably.

for prevent this, explicit ordering done in second code snippet you've provided.

Silver