The base-class constructor is already automatically called by your derived-class constructor. In C++, if the base class has a default constructor (takes no arguments, can be auto-generated by the compiler!), and the derived-class constructor does not invoke another base-class constructor in its initialisation list, the default constructor will be called. I.e. your code is equivalent to:
class PetStore: public Farm
{
public :
PetStore()
: Farm() // <---- Call base-class constructor in initialision list
{
idF=0;
};
private:
int idF;
string nameF;
}