C++ :: Function To Find Out Iterator Of Certain Value In Vector

I defined the following function to find out the iterator of a certain value in the vector. I defined it as such so if the value exist in the vector then return a iterator of it, if not then return a pointer pointing to nonsense.:

vector<tIndex_t>::iterator unlabelTit(const tIndex_t index) {
for(vector<tIndex_t>::iterator it=unlabelT.begin(); it<unlabelT.end(); it++) {
if(index==*it) return it;
} return NULL;
}

But looks this one is not accepted by compiler, saying I cannot do this kind of comparison like:

unlabelTit(i)!=NULL;

so I just delete the return NULL; and then the compiler giving me warning about no return statement before }.

a pointer pointing to nonsense? how to do that?