i was coding solution for the problem C. The Smallest String Concatenation . the solution depends mainly on overriding the cmp() function. first version caused RTE

bool cmp(const string &a, const string &b){

    return a+b <= b+a;
}

second version Accepted without = sign.

bool cmp(const string &a, const string &b){

    return a+b < b+a;
}

why the = sign caused RTE ?