c++ - Unexpected evaluate result when calling std::set functions in "if" statement -

i found behavior of calling std::set function in "if" statement can't understand, here code.

#include<set> #include<iostream> #include<cstdio> using namespace std; set<int>s;int t; set<int>::iterator i; int main() {     while (cin>>t) {         if ((i=s.insert(t).first)==s.begin()) /*expected:  insert new element,  iterator of new inserted element , save i, , compare begin of set see if smallest. */             puts("the new int smallest");         else puts("the new int not smallest");     }     return 0; } 

if input:

3 2 1 

the output be:

the new int not smallest new int not smallest new int not smallest 

however, if move insert out of "if":

while (cin>>t) {         (i=s.insert(t).first);         if (i==s.begin())             puts("the new int smallest");         else puts("the new int not smallest");     } 

then can expected output:

the new int smallest new int smallest new int smallest 

i tried test using following code:

int a() {     puts("fun encountered");     return 1; } int b() {     puts("fun b encountered");     return 1; } int main() {     int x;     if ((x=a())==b()); } 

and output is:

fun encountered fun b encountered 

seems order expected in first code. confused. reason first code went wrong?

this great reason not write horribly complex code!

from [intro.execution]:

except noted, evaluations of operands of individual operators , of subexpressions of individual expressions unsequenced.

when write:

(i=s.insert(t).first)==s.begin() 

you have no guarantee begin() called before or after insert(). if it's called before insert(), inserted element won't equal begin() since they're pointing different things.

just write assignment , test on separate lines. space isn't premium. not mention that

auto = s.insert(t).first; if (t == s.begin()) { ... } 

is easier read , understand anyway, in addition being well-defined behavior.