Ошибка с++

код

#include <iostream>

class temp{
protected:
double value;

public:
virtual double cels() = 0;
virtual double far() = 0;
virtual double kalv() = 0;

virtual void operator=(temp& t) = 0;
temp(double val);
};

temp::temp(double val) : value(val) {}

class Cels : public temp{
public:
double cels();
double far();
double kalv();

void operator=(temp& t);
Cels(double value);
};

Cels::Cels(double val) : temp(val) {}
void Cels::operator=(temp& t){ value = t.cels(); }
double Cels::cels(){ return value; }
double Cels::far(){ return value * 9.0 / 5.0 + 32.0; }
double Cels::kalv(){ return value + 273.15; }

class Far : public temp{
public:
double cels();
double far();
double kalv();

void operator=(temp& t);
Far(double value);
};

Far::Far(double val) : temp(val) {}
void Far::operator=(temp& t){ value = t.far(); }
double Far::cels(){ return (value - 32.0) / 5.0 * 9.0; }
double Far::far(){ return value; }
double Far::kalv(){ return (value - 32.0) * 5.0 / 9.0 + 273.15; }

class Kalv : public temp{
public:
double cels();
double far();
double kalv();

void operator=(temp& t);
Kalv(double value);
};

Kalv::Kalv(double val) : temp(val) {}
void Kalv::operator=(temp& t){ value = t.kalv(); }
double Kalv::cels(){ return value - 273.15; }
double Kalv::far(){ return (value - 273.15) / 5.0 * 9.0 + 32.0; }
double Kalv::kalv(){ return value; }

(.text+0x24): undefined reference to `main'
collect2: error: ld returned 1 exit status

что это?