dmg@ssc-vax.UUCP (David Geary) (07/06/89)
Here's something interesting:
First, the file works.cxx:
----------------------------------------------------------------------
#include "//ernie/user/geary/c++/include/stream.hxx" // c++ i/o
class junk { public: junk() { puts("Constructing Junk"); } };
junk myjunk;
main() { }
----------------------------------------------------------------------
When I compile and run, I get:
$ works
Constructing Junk
$
(In other words, everything is ok). However, watch this:
The file noworks.cxx:
----------------------------------------------------------------------
#include "//ernie/user/geary/c++/include/stream.hxx" // c++ i/o
class junk { public: junk() { cout << "Constructing Link\n"; } };
junk myjunk;
main() { }
----------------------------------------------------------------------
When I compile and run, I get:
$ noworks
Segmentation Fault
$
(In other words, everything is NOT ok ;-). The only difference
between the two is that I use puts() in works.cxx, and use
cout << in noworks.cxx. How, watch this:
The file worksagain.cxx
----------------------------------------------------------------------
#include "//ernie/user/geary/c++/include/stream.hxx" // c++ i/o
class junk { public: junk() { cout << "Constructing Link\n"; } };
main()
{
junk myjunk;
}
----------------------------------------------------------------------
When I compile and run, I get:
$ worksagain
Constructing Junk
$
So now, I can use cout << ... but I cannot have the object myjunk
external, it has to be in main().
Can anybody provide an explanation as to why this is happening?
--
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~ David Geary, Boeing Aerospace, Seattle ~
~ "I wish I lived where it *only* rains 364 days a year" ~
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~beard@ux1.lbl.gov (Patrick C Beard) (07/06/89)
The first case works because you are calling a standard C function to do console i/o, which requires no explicit initialization to work. The third case works because all static constructors (which the iostream classes have) are called before you use "cout". The second case doesn't work because of ordering problems in the calling of static constructors. If the stream's constructor was called first, it would work. Anyway, it's really bad practice to write code that depends on the order of calling of static constructors. Don't do it. In fact, I think static constructors are nothing but trouble (mho, of course). I hope this explains your problem. Patrick Beard beard@lbl.gov
vasta@apollo.COM (John Vasta) (07/07/89)
In article <2774@ssc-vax.UUCP> dmg@ssc-vax.UUCP (David Geary) writes: > > Here's something interesting: > [ ... ] >The file noworks.cxx: >---------------------------------------------------------------------- >#include "//ernie/user/geary/c++/include/stream.hxx" // c++ i/o > >class junk { public: junk() { cout << "Constructing Link\n"; } }; > >junk myjunk; > >main() { } >---------------------------------------------------------------------- > >When I compile and run, I get: > >$ noworks >Segmentation Fault > > Can anybody provide an explanation as to why this is happening? The cout object from the stream library must be initialized when the program starts up. So does your myjunk object. The order of execution of constructors for static objects in different modules is not defined. It looks like your object is getting attention first in this case, and its constructor is trying to use an uninitialized stream object. Although some people have found ways of determining or even controlling the execution order of static constructors/destructors, these techniques are not portable, so I'd advise you not to count on them. Making the object automatic (e.g. local to main) makes sure that all static objects are initialized first. If you need global visibility for an object which depends on other static objects being initialized, you can create the object dynamically (using new) and put a pointer to it in file scope. -- John Vasta Apollo Computer (division of Hewlett Packard) vasta@apollo.com M.S. CHA-01-LT (508) 256-6600 x6362 330 Billerica Road, Chelmsford, MA 01824 UUCP: {decwrl!decvax, mit-eddie, attunix}!apollo!vasta
ark@alice.UUCP (Andrew Koenig) (07/07/89)
In article <4443be62.1ad5a@apollo.COM>, vasta@apollo.COM (John Vasta) writes: > The cout object from the stream library must be initialized when the > program starts up. So does your myjunk object. The order of execution > of constructors for static objects in different modules is not defined. C++ 2.0 does two things to make this less of a bother. First, the 2.0 stream library goes to some trouble to ensure that it works correctly even in the face of static constructors and destructors. Second, if you have a static object that is local to a function, that object will be initialized no later than the first time that the flow of control passes through that declaration, even if that function is called from a static constructor. Example: struct T { T(); }; void f() { static T t; }; In this example, T::T() will be called to initialize t no later than the first time f() is executed. -- --Andrew Koenig ark@europa.att.com
wmm@sdti.com (0006-William M. Miller(0000)) (07/07/89)
In article <2774@ssc-vax.UUCP> dmg@ssc-vax.UUCP (David Geary) writes: >So now, I can use cout << ... but I cannot have the object myjunk >external, it has to be in main(). > > Can anybody provide an explanation as to why this is happening? Yes, you've been bitten by the "undefined static constructor order" bug. "cout" is a static object of class "ostream" which is defined in the library code somewhere. "myjunk" (in the nonworking example) is a static object of class "junk" which is defined in your program. When static objects are defined in different files, the order in which their constructors are invoked is not defined (within a file, it's done in declaration order). In your case, "myjunk" is being constructed before "cout," so saying "cout <<" in the constructor uses an uninitialized object. -- Non-disclaimer: My boss and I always see eye-to-eye (every time I look in the mirror). ...!genrad!mrst!sdti!wmm
rds@goanna.oz.au (Rowan D. Stevens) (01/24/90)
---- HELP! ---- I am looking for information about either a user interface methodology, a name of a research paper, an actual CASE tool or development environment, or anything which might help me to locate the following:- A tool or method which will allow a user interface to be developed visually (similiar to Microsoft's Dialog Box Editor) but will output the operational interface shell in some object-oriented programming type language (ie. c++) The method should allow modelling of window-oriented and text user interfaces. Anything close to this, I would still like to hear about. If someone out there is carrying out research in this or a similar area, then I would be very interested in talking further about this. I can be contacted at: Royal Melbourne Institute of Technology C/-Department of Computer Science GPO BOX 2476V Melbourne, Vic 3001 Australia Phone: +61-3-660-3216 ACSnet: rds@goanna.rmit.oz Much appreciated! Regards --> Rowan Stevens
shiao@cunixf.cc.columbia.edu (Dennis Shiao) (04/11/90)
Does anyone have any information(general->specific) on Object Oriented Programming ? A final project in one of my classes entails writing a processor to take C-- code (C, minus some functionality -- a pseudo language) and produce from it valid C code. Topics of Interest: Symbol Tables Parse Trees Object Oriented Programming Lex Yacc Thanks a lot.... Dennis internet --> shiao@cunixf.cc.columbia.edu bitnet ----> shiao@cunixf.BITNET