if문의 condition부분에다가 함수의 return값을 받으면 안되나요?(c++)

 
      tmp3 = tmp;
        while(true){
            tmp2 = tmp;
            tmp = bigSum(tmp, tmp3);
 
            cout << "a :";
            print_bigNum(a);
            cout << ", tmp :";
            print_bigNum(tmp);
         //   cout << ", " << abscmp(a, tmp);
            cout << endl;
 
            if(abscmp(a, tmp)){
                div2 = bigSum(div2, one);
            } else{
                tmp = tmp2;
                break;
            }
        }

...
a :80, tmp :60
a :80, tmp :70
a :80, tmp :80
Program ended with exit code: 0

a :80, tmp :50, 1
a :80, tmp :60, 1
a :80, tmp :70, 1
a :80, tmp :80, 1
a :80, tmp :90, 0
Program ended with exit code: 0

bool abscmp(bigNum a, bigNum b){
    int c = MAX_DIGIT-1;
    while(a.n[geshifilter-c] == b.n[c]) {
        c--;
    }
    if(a.n[c] >= b.n[c]) return true;
    else return false;
}