Class data default initialization

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 pr开发者_Python百科inted, they display junk values.

So my question is what difference public data makes here?

I'm using g++ 3.4.6

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) {}

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.

As opposed to Java or C#, where memory allocated for a newly created objects ALWAYS set to zero, this NOT happends in C++. There are several rules that describe when object initialization is guaranteed to take place and when it isn't.

Consider folowing example:

class Base
{
private:
    int i;
    char ch;
    std::string str;
public:
    Base()
      : i(0) //built-in fields remains unitialized. We should initialize it manually
      , ch('\0') //another built-in field
      //, str() //this call is redundant due automatic default constructors calls for all user-defined types
    {}
    void showdata()
    {
        cout<<"Int:"<<i<<endl; //valid only after manual initialization
        cout<<"Char:"<<ch<<endl; //valid only after manual initialization
        cout<<"String:"<<str<<endl; //always valid 
    }
    //int pub_data ;
} ;

You should remember, that ALL buit-in fields you should initialized manually in class constructor.

P.S. The fact, that in the first case your code works - is pure accident.

What has been answered already is correct, but to make sure your values are zero-initialized you can also simply declare your object as

Base ob();  // notice the parentheses here

or

Base ob{};  // this compiles only on c++11

For more details, check out this insightful answer: https://stackoverflow.com/a/620402/3073460