Question: I am trying to use class inheritance. I have taken a code from a book and tweaked it a bit to create Class Mammal and a Class Dog which inherits Mammal member data. The code is:

```
#include<iostream>

enum BREED{Labrador,Alcetian,Bull,Dalmetian,Pomerarian};

class Mammal{

 public:
    Mammal();
    ~Mammal(){}
    double getAge(){return age;}
    void setAge(int newAge){age=newAge;}
    double getWeight(){return weight;}
    void setWeight(double newWeight){weight=newWeight;}
    void speak(){std::cout<<"Mammal sound!\n";}
 protected:
    int age;
    double weight;

};

class Dog : public Mammal{

 public:
    Dog();
    ~Dog(){}
    void setBreed(BREED newType){type=newType;}
    BREED getBreed(){return type;}
 private:
    BREED type;
};

int main(){
 Dog Figo;

 Figo.setAge(2);
 Figo.setBreed(Alcetian);
 Figo.setWeight(2.5);
 std::cout<<"Figo is a "<<Figo.getBreed()<<" dog\n";
 std::cout<<"Figo is "<<Figo.getAge()<<" years old\n";
 std::cout<<"Figo's weight is "<<Figo.getWeight()<<" kg\n";
 Figo.speak();

 return 0; 
}

```

When I run this code,, it gives me the following error:

C:\cygwin\tmp\cc7m2RsP.o:prog3.cpp:(.text+0x16): undefined reference to `Dog::Dog()'
collect2.exe: error: ld returned 1 exit status

Any help would be appreciated. Thanks.

Answer: 
You didn't define constructors, you only declared them:

```
class Mammal{
public:
    Mammal();  // <-- declaration

...

class Dog : public Mammal{
public:
    Dog();  // <-- declaration

```

When you declare a constructor (or any function) then the C++ compiler will look for the constructor's definition elsewhere. And when it can't find it it complains. For basic definitions try this:

```
class Mammal{
public:
    Mammal() {};  // <-- definition

...

class Dog : public Mammal{
public:
    Dog() {};  // <-- definition

```

You can also remove them entirely since they don't seem to do anything (C++ will insert default constructor for you).
This because you have just declared the non-parameterized constructors `Dog();` and `Mammal();` in respective classes, but not defined it.

To remove the error do following:

* In `class Mammal`, change line `Mammal();` to `Mammal(){}` **AND**
* In `class Dog`, change line `Dog();` to `Dog() {}`.