Question

There is some linking error here. I looked for that online, yet still I couldn’t find the problem. How can I fix it?

g++ test.cpp -o test
/tmp/ccDfCj4N.o: In function `Interval::Interval()':
test.cpp:(.text._ZN8IntervalC2Ev[Interval::Interval()]+0x9): undefined reference to     `vtable for Interval'
/tmp/ccDfCj4N.o: In function `IntInterval::~IntInterval()':
test.cpp:(.text._ZN11IntIntervalD0Ev[IntInterval::~IntInterval()]+0x1d): undefined     reference to `Interval::~Interval()'
/tmp/ccDfCj4N.o: In function `IntInterval::~IntInterval()':
test.cpp:(.text._ZN11IntIntervalD1Ev[IntInterval::~IntInterval()]+0x1d): undefined     reference to `Interval::~Interval()'
/tmp/ccDfCj4N.o:(.rodata._ZTI11IntInterval[typeinfo for IntInterval]+0x10): undefined     reference to `typeinfo for Interval'
collect2: ld returned 1 exit status

Here is the code!
All classes are in a same file for a pilot program.

#include <iostream>
#include <vector>
#include <utility>
using namespace std;

#define MAX_IP_RANGE    4294967295

class Interval {

    public:
        virtual Interval * interval_copy() = 0;
        virtual unsigned long get_begin() = 0;
        virtual unsigned long get_end() = 0;
        virtual unsigned long get_length() = 0;
        virtual Interval*     get_intersect(Interval *interval) = 0;   // Examine whether two intervals have intersection
        virtual Interval*     copy() = 0;
        virtual ~Interval();
        virtual bool is_equal(Interval *interval) {
            unsigned long b1 = this->get_begin();
            unsigned long e1 = this->get_end();
            unsigned long b2 = interval->get_begin();
            unsigned long e2 = interval->get_end();
            if (b1 == b2 && e1 == e2)
                    return true;
            return false;
        }

        virtual bool is_within(Interval *interval) {
            unsigned long b1 = this->get_begin();
            unsigned long e1 = this->get_end();
            unsigned long b2 = interval->get_begin();
            unsigned long e2 = interval->get_end();
            if (b1 >= b2 && e1 <= e2)
                return true;
            return false;
        }

        virtual bool contains(Interval *interval) {    // Examine whether this interval contains another interval
            unsigned long b1 = this->get_begin();
            unsigned long e1 = this->get_end();
            unsigned long b2 = interval->get_begin();
            unsigned long e2 = interval->get_end();
            if (b1 <= b2 && e1 >= e2)
                return true;
            return false;
        }

        virtual bool is_empty() {
            return (get_end()<get_begin())?true:false;
        }

        virtual bool is_intersect(Interval *interval) {
            unsigned long b1 = this->get_begin();
            unsigned long e1 = this->get_end();
            unsigned long b2 = interval->get_begin();
            unsigned long e2 = interval->get_end();

            if (b1>e2)
                return false;
            if (b2>e1)
                return false;
            return true;
        }

        virtual void print()
        {
            cout << '('<<get_begin() << ',' << get_end() << ")\n";
        }
};


class IntInterval : public Interval {
    private:
        unsigned long begin;
        unsigned long end;
        IntInterval();

    public:
        virtual Interval * interval_copy() {
            return new IntInterval(begin, end);
        }

        IntInterval(unsigned long a, unsigned long b): begin (a), end  (b)
        {}

        void set_value(unsigned long a, unsigned long b) {
            begin = a;
            end = b;
        }

        void set_begin(unsigned long a) {
            begin = a;
        }

        void set_end(unsigned long b) {
            end = b;
        }

        virtual Interval* copy()
        {
            Interval *new_interval = new IntInterval(begin, end);
            return new_interval;
        }

        virtual unsigned long get_begin() {
            return begin;
        }

        virtual unsigned long get_length() {
            return end-begin+1;
        }

        virtual unsigned long  get_end() {
            return end;
        }

        virtual Interval* get_intersect(Interval *interval);   // Get the intersect part of two intervals
        virtual ~IntInterval() {};
    };

    Interval* IntInterval::get_intersect(Interval *interval) {
        unsigned long begin2 = interval->get_begin();
        unsigned long end2 = interval->get_end();
        if (end < begin2 || begin > end2) {
            return new IntInterval(1, 0);
        }
        return new IntInterval((begin>begin2)?begin:begin2, (end<end2)?end:end2);
    }


    IntInterval * parse_ip(const char * _str) {
        unsigned long  _begin=0;
        unsigned long  _end=0;
        string input(_str);
        if (input.find('-') != string::npos){
            string begin = input.substr(0, input.find('-'));
            string end = input.substr(input.find('-')+1);

            unsigned  int ip1 = 0, ip2 = 0;
            unsigned  int ip3 = 0, ip4 = 0;
            sscanf(begin.c_str(), "%u.%u.%u.%u", &ip1, &ip2, &ip3, &ip4);
            _begin = (ip1 << 24) + (ip2 << 16) + (ip3 << 8) + ip4;

            ip1 = 0; ip2 = 0; ip3 = 0; ip4 = 0;
            sscanf(end.c_str(), "%u.%u.%u.%u", &ip1, &ip2, &ip3, &ip4);
            _end = (ip1 << 24) + (ip2 << 16) + (ip3 << 8) + ip4;

            if ((_begin > _end) ||  (_end > MAX_IP_RANGE)){
                cout<<"ERROR: The IP INTERVAL IS WRONG The range is "<<begin<<"-"<<end<<endl;
                exit(0);
            }
        }
        return new IntInterval(_begin, _end);
    }

    bool compFunc (Interval * i, Interval * j) {
        return (i->get_begin() < j->get_begin());
    }

    int main () {

        vector <vector<pair<string, string> > > nets;

        vector<pair<string, string> > x;
        vector<pair<string, string> > y;

        x.push_back(make_pair("1.1.1.1", "3.0.0.0"));
        x.push_back(make_pair("10.2.5.3", "30.2.5.0"));
        x.push_back(make_pair("100.2.25.2", "130.2.25.2"));

        y.push_back(make_pair("41.0.2.2", "43.2.2.5"));
        y.push_back(make_pair("131.2.2.2", "135.5.5.2"));

        nets.push_back(x);
        nets.push_back(y);

        vector <IntInterval *> _nets;
        for (int i=0; i<(int)nets.size(); i++)
            for(int j=0; j<(int)nets[i].size(); j++) {
                string s = nets[i][j].first + '-' + nets[i][j].second;
                _nets.push_back(parse_ip(s.c_str()));
            }
        sort(_nets.begin(), _nets.end(), compFunc);

        if (_nets.size()>1)
            for (vector<IntInterval *>::iterator it = _nets.begin()+1; it < _nets.end(); ) {
                if ((*it)->get_begin()-1 == (*(it-1))->get_end()) {
                    (*(it-1))->set_end((*it)->get_end());
                    _nets.erase(it);
                }
                else if ((*it)->get_begin()-1 <  (*(it-1))->get_end()) {
                    it++;
                    cout<<"ERROR: Network address overlapping!"<<endl;
                }
                else
                    it++;
            }

        for (int i=0; i<(int)_nets.size(); i++)
            cout << _nets[i]->get_begin() << "  " << _nets[i]->get_end() << endl;

      return 0;
    }

Answer 1

You never provided an implementation for virtual ~Interval(); and several other functions. You must provide an implementation for all non-pure virtual functions you declare. In particular, G++ emits the vtable along with the implementation of the first-declared non-inline function in a class. Omitting its implementation means you won’t have a vtable, and thus won’t be able to construct the class (hence these errors).

In short, define every function you declare, except for pure virtuals. There are some cases where it’s justified to leave out the definition for a declared function, but they are very rare.