Class data default initialization

Question

I have following code:

#include <iostream>
using namespace std;

class Base
{
private:
    int i;
    char ch;
public:
    void showdata()
    {
        cout<<"Int:"<<i<<endl;
        cout<<"Char:"<<ch<<endl;
    }
    //int pub_data ;
} ;

int main()
{
    Base ob;
    ob.showdata() ;
    //cout<<"Public Data:"<<ob.pub_data<<endl;
    return 0;
}

This program compiles and runs fine. The output shows that i is initialized with 0 and ch is initialized with '\0'.
If you notice i have commented out 2 statements in this program. First the declaration of public data pub_data and second the line inside main printing this public data.
Now here the problem is, if i uncomment these two lines, the data members of class i.e. i, ch, pub_data do not seem to be initialized and when printed, they display junk values.
So my question is what difference public data makes here?
I'm using g++ 3.4.6

Answer 1

Neither int's nor char's are automatically initialized to 0. The fact that it happened is just luck.

You need to add a constructor that does the initialization:

Base() : i(0), ch(0) {}

Answer 2

None. You're just getting "lucky". Fundamental types remain uninitialized, so your i and ch, as the program stands, could very well not always be 0.

It just so happens adding that public member "messes it up". To correct your class, initialize the members in the initialization list of the constructor:

class Base
{
private:
    int i;
    char ch;
public:
    Base(void) :
    i(0), ch(0) //, pub_data(0)
    {}

    void showdata()
    {
        cout<<"Int:"<<i<<endl;
        cout<<"Char:"<<ch<<endl;
    }
    //int pub_data ;
} ;

Now when a Base gets constructed i, ch, and (when uncommented) pub_data will be properly initialized to meaningful values.