string.length() 返回值是unsigned

int main(int argc, const char * argv[]) {
    // insert code here...
    string a = "";
    cout<<a.size()<<endl;
    cout<<a.size()-1<<endl; 
    cout<<a.length()<<endl;
    cout<<a.length()-1<<endl;
    
    return 0;
}

输出：

0

18446744073709551615

0

18446744073709551615