Initialization of static variables [C++]

Prev: Commercial programs written in C/C++?
Next: policy base class using static member functions

From: Stephan Brönnimann on 15 Jan 2006 14:40

Given the following sample code:

datetime.h
==========
class Date {
public:
// Constructor with number of days since 01-Jan-1900.
explicit Date(numDays) : numDays_(numDays) {}

// minimum supported date
static const Date& minDate();
private:
// number of days since 01-Jan-1900
long numDays_;
};

datetime.cc
===========
#include "datetime.h"

namespace {
Date minimumDate(-10000);
};

const Date& Date::minDate()
{
return minimumDate;
}

Does the standard guarantee that minimumDate is always initialized if
Date::minDate() is called from the initialization of other static
variables (global or at file scope) in different compilation units?

Note: I'm not looking for a different implementation of
Date::minDate(), e.g., using a static Date inside the function. This
would not be thread-safe because the real (relevant) constructor
computes Date::numDays_ from the day, month and year.

Regards, Stephan
broeni(a)osb-systems.com
Open source rating and billing engine for communication networks.

[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]

From: Mirek Fidler on 16 Jan 2006 09:16

> datetime.h
> ==========
> class Date {
> public:
> // Constructor with number of days since 01-Jan-1900.
> explicit Date(numDays) : numDays_(numDays) {}
>
> // minimum supported date
> static const Date& minDate();
> private:
> // number of days since 01-Jan-1900
> long numDays_;
> };
>
> datetime.cc
> ===========
> #include "datetime.h"
>
> namespace {
> Date minimumDate(-10000);
> };
>
> const Date& Date::minDate()
> {
> return minimumDate;
> }
>
> Does the standard guarantee that minimumDate is always initialized if
> Date::minDate() is called from the initialization of other static
> variables (global or at file scope) in different compilation units?

No.

> Note: I'm not looking for a different implementation of
> Date::minDate(), e.g., using a static Date inside the function. This
> would not be thread-safe because the real (relevant) constructor

Yes, that is corrent, at least for most platforms (I believe that latest
GCC has thread-safe static variables).

Means you have to find a way around for threading safety. Possible
solution would be to use some global POD variable (like pointer) that is
guaranteed to be zeroed at startup and bind thread-locked stuff around it.

Anyway, AFAIK, there is no 100% portable solution here.

Mirek

[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]

From: Thomas Tutone on 16 Jan 2006 09:17

From: Alberto Ganesh Barbati on 16 Jan 2006 09:17

Stephan Br?nnimann wrote:
> Given the following sample code:
>
> datetime.h
> ==========
> class Date {
> public:
> // Constructor with number of days since 01-Jan-1900.
> explicit Date(numDays) : numDays_(numDays) {}
>
> // minimum supported date
> static const Date& minDate();
> private:
> // number of days since 01-Jan-1900
> long numDays_;
> };
>
> datetime.cc
> ===========
> #include "datetime.h"
>
> namespace {
> Date minimumDate(-10000);
> };
>
> const Date& Date::minDate()
> {
> return minimumDate;
> }
>
> Does the standard guarantee that minimumDate is always initialized if
> Date::minDate() is called from the initialization of other static
> variables (global or at file scope) in different compilation units?

Unfortunately, no.

> Note: I'm not looking for a different implementation of
> Date::minDate(), e.g., using a static Date inside the function. This
> would not be thread-safe because the real (relevant) constructor
> computes Date::numDays_ from the day, month and year.

Well... Thread safety may be an issue only if you are creating threads
before entering function main(). If you don't, most probably the
implementation will execute all initializations of static objects in the
main thread, so the most common implementation:

const Date& Date::minDate()
{
static Date minimumDate(-10000);
return minimumDate;
}

won't be a problem if it's called before entering main(). To avoid
multiple initializations *after* entering function main(), just ensure
that's it's called before main() (doh!) o at least before creating any
thread.

HTH,

Ganesh

[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]

From: Maxim Yegorushkin on 16 Jan 2006 11:35

Stephan Br?nnimann wrote:

[]

> datetime.cc
> ===========
> #include "datetime.h"
>
> namespace {
> Date minimumDate(-10000);
> };
>
> const Date& Date::minDate()
> {
> return minimumDate;
> }
>
> Does the standard guarantee that minimumDate is always initialized if
> Date::minDate() is called from the initialization of other static
> variables (global or at file scope) in different compilation units?

It does not. Use something like schwarz counter idiom to guarantee
initialization of minimumDate before its first use.

[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]

| Next | Last
Pages: 1 2 3 4 5 6 7 8
Prev: Commercial programs written in C/C++?
Next: policy base class using static member functions