How to initialize static class member

[subramanian]

I am a beginner in C++.

Suppose I want to build a class: I have given below the starting code:

class Date {
int day, month, year;

static Date default_date;

};

Someone, kindly, completely tell me how to initialize(ie the definition
of) the static member default_date in the above class and the
constructors needed for this. Also how should I access the members of
default_date ie the syntax to access it. I need the full
implementation. I am asking this is for learning purpose.

 

[Salt_Peter]

I am a beginner in C++.

Suppose I want to build a class: I have given below the starting code:

class Date {
int day, month, year;

static Date default_date;

};

Someone, kindly, completely tell me how to initialize(ie the definition
of) the static member default_date in the above class and the
constructors needed for this. Also how should I access the members of
default_date ie the syntax to access it. I need the full
implementation. I am asking this is for learning purpose.

#include <iostream>
#include <ostream>

class Date {
int day, month, year;
static Date default_date;
public:
Date() : day(default_date.day),
month(default_date.month),
year(default_date.year) { }
explicit Date(int d, int m, int y)
: day(d), month(m), year(y) { }
};

Date Date::default_date(1,1,2000); // static member

int main()
{
Date date;
Date today(31,12,2006);
}

___
But then wouldn't be simpler do the above in the default ctor directly
instead of using a static member?

class Date {
int day, month, year;
public:
Date() : day(1),
month(1),
year(2000) { }
explicit Date(int d, int m, int y)
: day(d), month(m), year(y) { }
};

 

[Rolf Magnus]

explicit Date(int d, int m, int y)
: day(d), month(m), year(y) { }

No point in making that constructor explicit. It isn't a conversion
constructor anyway.

 

[subramanian]

But then wouldn't be simpler do the above in the default ctor directly

instead of using a static member?

class Date {
int day, month, year;
public:
Date() : day(1),
month(1),
year(2000) { }
explicit Date(int d, int m, int y)
: day(d), month(m), year(y) { }

Thanks for the explanation. I am not able to understand this question
and the above code ? Also, the static member is missing. Can you kindly
explain ?

 

[Daniel T.]

I am a beginner in C++.

Suppose I want to build a class: I have given below the starting code:

class Date {
int day, month, year;

static Date default_date;

};

Someone, kindly, completely tell me how to initialize(ie the definition
of) the static member default_date in the above class and the
constructors needed for this.

Date Date::default_date;

Also how should I access the members of
default_date ie the syntax to access it. I need the full
implementation. I am asking this is for learning purpose.

void printDate( const Date& date )
{
cout << date.month << '/' << date.day << '/' << date.year;
}

int main() {

// assignment
Date::default_date.day = 31;
Date::default_date.month = 12;
Date::default_date.year = 2006;

// reading
int d = Date::default_date.day;
int m = Date::default_date.month;
int y = Date::default_date.year;

// pass to function
printDate( Date::default_date );
}

 

[Ron Natalie]

Thanks for the explanation. I am not able to understand this question
and the above code ? Also, the static member is missing. Can you kindly
explain ?

By the way, explicit is only useful on a converting (that is, one
that can be called with only one argument) constructor.

 

[subramanian]

Hello Daniel . T.

You have mentioned

Date Date::default_date;

Is the above statement the definition of the memeber : static Date
default_date.
(I read somewhere each static member should be defined.)

Here is the default constructor called ? But we have not provided it.
So will the compiler supply it ?

Thanks

 

[Daniel T.]

Hello Daniel . T.

You have mentioned

Date Date::default_date;

Is the above statement the definition of the memeber : static Date
default_date.
Yes.

Here is the default constructor called ?
Yes.

But we have not provided it. So will the compiler supply it ?

Yes, but the compiler supplied default constructor will do nothing. It
will not even initialize the values to 0.

 

[John Femiani]

Guys,

There are a couple ways to initialize static members that I know of:
1) declare the static variable in the class, and initilize it exactly
once inside of a .cpp (or cxx or whatever) file. You generally can't do
that in a header because it might get included by more than one cpp
file and the linker won't know what to do.
2) if it is a const integer, you can sometimes do it inline with the
class (depends on the compiler I think)
3) if it is an constant ordinal type, you can use an enumeration.
4) you can wrap it inside of a function;

static Data& date_thing() {static int value = initial_value; return
value;}


John

 

[Salt_Peter]

Guys,

There are a couple ways to initialize static members that I know of:
1) declare the static variable in the class, and initilize it exactly
once inside of a .cpp (or cxx or whatever) file. You generally can't do
that in a header because it might get included by more than one cpp
file and the linker won't know what to do.
2) if it is a const integer, you can sometimes do it inline with the
class (depends on the compiler I think)
3) if it is an constant ordinal type, you can use an enumeration.
4) you can wrap it inside of a function;

static Data& date_thing() {static int value = initial_value; return
value;}


John

The point here is that a constructor must still initialize the members
of the static variable.
Hence, the need for a default ctor, in which case having a static
default_date member is a moot point. Simply initialize the members in a
default ctor.