C++ Understanding Classes and Constructors - c++

Question

I am new to C++ (coming from Java). I am having trouble with putting together classes in C++.
My goal in this program is to simply implement a basic Animal class with a few strings and counter.
I want to be able to read in from a text file that I have created and set the lines in the text file to each of the these variables.
Species
Family
Phylum
Descendants
I then want the program to print out the results of all 3 classes.
I don't understand how to implement a default constructor.
Here is my class.
#include <iostream>
#include <string>
using namespace std;
class Animal
{
string species;
string family;
string phylum;
string desc;
static int count;
public:
bool readIn(ifstream&file, const string frame);
void printInfo() const;
void setAnimal(string s, string f, string p, string d);
static int getCount();
Animal(string s, string f, string p, string d);
Animal(ifstream& file, const string fname);
};
These are the function definitions:
#include "animal.h"
#include <iostream>
#include <string>
using namespace std;
Animal::Animal(string s, string f, string p, string d)
{
setAnimal(s,f,p,d);
}
static int Animal::getCount()
{
int i=0;
i++;
return i;
}
bool Animal::readIn(ifstream &myFile, const string fname)
{
myFile.open(fname);
if(myFile)
{
getline(myFile, species);
getline(myFile, family);
getline(myFile, phylum);
getline(myFile, desc);
myFile.close();
return true;
}
else
return false;
}
Animal::Animal(ifstream& file, const string fname)
{
if(!readIn(file, fname) )
species="ape";
family="ape";
phylum="ape";
desc="ape";
count = 1;
}
void Animal::printInfo() const
{
cout << species << endl;
cout << family << endl;
cout << phylum << endl;
cout << desc << endl;
}
void Animal::setAnimal(string s, string f, string p, string d)
{
species = s, family = f, phylum = p, desc = d;
}
int main()
{
ifstream myFile;
Animal a;
Animal b("homo sapien", "primate", "chordata", "erectus");
Animal c(myFile, "horse.txt");
a.printInfo();
b.printInfo();
c.printInfo();
}

Answer 1

A default constructor is one which can be called with no parameters specified. This description might seem a little verbose, so consider a couple possibilities.
Typically, or maybe by default (no pun), a default constructor will simply be a constructor which takes no parameters:
class Animal
{
public:
Animal() {}; // This is a default constructor
};
Other times though you might write a construcutor that does take parameters, but all the parameters have defaults:
class Animal
{
public:
Animal(int age=42) : age_(age) {}; // This is a default constructor
private:
int age_;
};
This is also a default constructor because it can be called with no parameters:
Animal a; // OK
You will not want to have 2 default constructors in a class. That is, don't try to write a class like this:
class Animal
{
public:
Animal() {};
Animal(int age=42) : age_(age) {};
private:
int age_;
};
In C++, if you have a class that has no default constructor, the compiler will generate one for you automatically. However, the compiler doesn't automatically generate a default constructor if you have already declared any other constructor yourself. So in your case, since you have already declared 2 other constructors (both are "convert" constructors), the compiler won't generate a default constructor for you. Since your class, as defined, doesn't have a default constructor, you can't default construct Animal objects. In other words, this won't compile:
Animal a;

Answer 2

A default constructor is simply a constructor that takes no parameters. The compiler generates one for you if you don't define any constructors of your own.
This auto generated one does nothing other than call the no-param constructors of the class' bases and members.
You can define a no-param constructor yourself.

Answer 3

To implement a default constructor, just do what you've already done but provide no parameters:
static int getCount();
Animal(string s, string f, string p, string d);
Animal(ifstream& file, const string fname);
Animal(); //Default Constructor
Then in your implementation:
Animal::Animal(){
species="ape";
family="ape";
phylum="ape";
desc="ape";
count = 1;
}