MESSAGE
| DATE | 2016-11-21 |
| FROM | ruben safir
|
| SUBJECT | Subject: [Learn] Fwd: hidden static
|
From learn-bounces-at-nylxs.com Mon Nov 21 15:52:12 2016
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C++ static variables --
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From: Popping mad
Newsgroups: comp.lang.c++
Subject: hidden static
Date: Sat, 19 Nov 2016 01:49:12 +0000 (UTC)
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Xref: panix comp.lang.c++:1125334
I've a declared static as a public member of an
object but it doesn't seem to show up in the compiler
/home/ruben/src/test_static/test.cpp|22| undefined reference to `blah::A::here'
If I put it on top it errors are a private member. When I move it
to public, it is not being seen
The three test case files look like this
test.cpp
18 #include "test.h"
19 #include
20 namespace blah{
21 void A::read(int in){
22 A::here = in;
23 std::cout << here << std::endl;
24 }
25 }
test.h
1 namespace blah{
2 class A{
3 public:
4 static int here;
5 A(int in=0){
6 here = in;
7 };
8 void read(int);
9 };
10
11 }
~
main.cpp
18 #include "test.h"
19 #include
20 namespace std{
21
22 int main(int argc, char** argv){
23
24 blah::A* a = new blah::A{9};
25 cout << a->here << endl;
26 return 0;
27 }
28
29 }
~
makefile
1 CXX:=g++
2 CXXFLAGS:=-Wall -ggdb -pg -pthread
3
4 LDFLAGS:=-L/usr/local/lib/mysql -lmysqlpp -lmysqlclient
5
6 test : test.o main.o
7 ${CXX} ${CXXFLAGS} -o testme test.o main.o
8
9 main.o : main.cpp
10 ${CXX} ${CXXFLAGS} -o main.o -c main.cpp
11
12 test.o : test.cpp test.h
13 ${CXX} ${CXXFLAGS} -c test.cpp
14
15 clean :
16 rm testme *.o make.deps
17 touch *.cpp *.h
18
19 include make.deps
20 make.deps: *.cpp ; gcc -M *.cpp >$-at-
~
I need an extern?
1 || g++ -Wall -ggdb -pg -pthread -o testme test.o main.o
2 || /usr/lib/gcc/x86_64-pc-linux-gnu/6.2.1/../../../../lib/gcrt1.o: In function `_start':
3 || (.text+0x20): undefined reference to `main'
4 || test.o: In function `blah::A::read(int)':
5 /home/ruben/src/test_static/test.cpp|22| undefined reference to `blah::A::here'
6 /home/ruben/src/test_static/test.cpp|23| undefined reference to `blah::A::here'
7 || main.o: In function `std::main(int, char**)':
8 /home/ruben/src/test_static/main.cpp|25| undefined reference to `blah::A::here'
9 || main.o: In function `blah::A::A(int)':
10 /home/ruben/src/test_static/test.h|6| undefined reference to `blah::A::here'
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From: Ian Collins
Newsgroups: comp.lang.c++
Subject: Re: hidden static
Date: Sat, 19 Nov 2016 16:07:01 +1300
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Xref: panix comp.lang.c++:1125336
On 11/19/16 02:49 PM, Popping mad wrote:
> I've a declared static as a public member of an
> object but it doesn't seem to show up in the compiler
>
> /home/ruben/src/test_static/test.cpp|22| undefined reference to `blah::A::here'
>
>
> If I put it on top it errors are a private member. When I move it
> to public, it is not being seen
>
> The three test case files look like this
>
>
> test.cpp
>
> 18 #include "test.h"
> 19 #include
> 20 namespace blah{
> 21 void A::read(int in){
> 22 A::here = in;
> 23 std::cout << here << std::endl;
> 24 }
> 25 }
>
>
> test.h
>
> 1 namespace blah{
> 2 class A{
> 3 public:
> 4 static int here;
> 5 A(int in=0){
> 6 here = in;
> 7 };
> 8 void read(int);
> 9 };
> 10
> 11 }
Who have declared A::here, but you haven't defined it anywhere.
--
Ian
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Date: Sat, 19 Nov 2016 10:30:14 +0200
From: Paavo Helde
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Xref: panix comp.lang.c++:1125337
On 19.11.2016 3:49, Popping mad wrote:
> I've a declared static as a public member of an
> object but it doesn't seem to show up in the compiler
>
> /home/ruben/src/test_static/test.cpp|22| undefined reference to `blah::A::here'
See https://isocpp.org/wiki/faq/ctors#link-errs-static-data-mems
While there, consider reading the whole https://isocpp.org/faq
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From: Popping mad
Newsgroups: comp.lang.c++
Subject: Re: hidden static
Date: Sat, 19 Nov 2016 17:01:03 +0000 (UTC)
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Xref: panix comp.lang.c++:1125342
On Sat, 19 Nov 2016 10:30:14 +0200, Paavo Helde wrote:
> See https://isocpp.org/wiki/faq/ctors#link-errs-static-data-mems
>
> While there, consider reading the whole https://isocpp.org/faq
This is an excellent explanation, but I'm still not clear on this. I
think it is saying that a static has to be declared and defined within a
compilational unit. I'm not sure why, though. And I've asked before,
and read, but I'm still not clear on what a compilation unit is.
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Xref: panix comp.lang.c++:1125350
On 19.11.2016 19:01, Popping mad wrote:
> On Sat, 19 Nov 2016 10:30:14 +0200, Paavo Helde wrote:
>
>> See https://isocpp.org/wiki/faq/ctors#link-errs-static-data-mems
>>
>> While there, consider reading the whole https://isocpp.org/faq
>
>
> This is an excellent explanation, but I'm still not clear on this. I
> think it is saying that a static has to be declared and defined within a
> compilational unit. I'm not sure why, though.
There are other things in C++ which are declared in a header file and
then defined in some translation unit. The prime example are functions
(member and non-member). That's just the way how C++ does things.
When defining a class static member in some cpp file, one tells the
compiler to reserve memory for it in an the data segment of this
translation unit, plus one also specifies the arguments instructing the
compiler how to construct it.
However, for some simpler scenarios the initialization of the class
static member can be written directly in the declaration, which is
probably the most confusing point here.
> And I've asked before,
> and read, but I'm still not clear on what a compilation unit is.
It is just whatever is compiled into a single object .o/.obj file.
hth
Paavo
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From: Juha Nieminen
Newsgroups: comp.lang.c++
Subject: Re: hidden static
Date: Mon, 21 Nov 2016 07:52:39 +0000 (UTC)
Organization: Netfront http://www.netfront.net/
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Xref: panix comp.lang.c++:1125395
Popping mad wrote:
> This is an excellent explanation, but I'm still not clear on this. I
> think it is saying that a static has to be declared and defined within a
> compilational unit. I'm not sure why, though. And I've asked before,
> and read, but I'm still not clear on what a compilation unit is.
A static member variable needs to exist somewhere in the executable
(because, among other things, you need to be able to eg. have a pointer
pointing to it). You have to tell the compiler which compilation unit
(in practice which object file) the actual instance of that variable is.
The compiler won't guess; you need to tell it explicitly where it should
be put.
(Incidentally, C++17 will introduce the concept of inline variables,
which will allow you to define static member variables in the header
file itself, causing the linker to merge all the instantiations into
one. But that will be available only after compilers start supporting
it.)
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Subject: Re: hidden static
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Xref: panix comp.lang.c++:1125403
Juha Nieminen writes:
>Popping mad wrote:
>> This is an excellent explanation, but I'm still not clear on this. I
>> think it is saying that a static has to be declared and defined within a
>> compilational unit. I'm not sure why, though. And I've asked before,
>> and read, but I'm still not clear on what a compilation unit is.
>
>A static member variable needs to exist somewhere in the executable
>(because, among other things, you need to be able to eg. have a pointer
>pointing to it).
The confusing part for some folks is that gcc/binutils, when using -O3, won't
require the definition unless the address of the static const value
is taken. Remove -O3 and you'll start getting link errors.
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NNTP-Posting-Date: Mon, 21 Nov 2016 08:48:30 -0600
Date: Mon, 21 Nov 2016 14:48:15 +0000
From: Chris Vine
Newsgroups: comp.lang.c++
Subject: Re: hidden static
Message-ID: <20161121144815.63a2899c-at-dell.homenet>
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Xref: panix comp.lang.c++:1125407
On Mon, 21 Nov 2016 13:26:07 GMT
scott-at-slp53.sl.home (Scott Lurndal) wrote:
> Juha Nieminen writes:
> >Popping mad wrote:
> >> This is an excellent explanation, but I'm still not clear on
> >> this. I think it is saying that a static has to be declared and
> >> defined within a compilational unit. I'm not sure why, though.
> >> And I've asked before, and read, but I'm still not clear on what a
> >> compilation unit is.
> >
> >A static member variable needs to exist somewhere in the executable
> >(because, among other things, you need to be able to eg. have a
> >pointer pointing to it).
>
> The confusing part for some folks is that gcc/binutils, when using
> -O3, won't require the definition unless the address of the static
> const value is taken. Remove -O3 and you'll start getting link
> errors.
My recollection in the case of the declaration of a const static
integer member variable (typically with a literal value) is that you
don't need to provide a definition for it if it is not ODR-used,
whatever the optimization level. Taking an address is one way of
ODR-using it. Casting it to lvalue of another type is another. I don't
at the moment have the C++ standard in front of me to give you the
reference.
These days I guess you would probably declare such a static member
variable constexpr rather than const anyway.
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Date: Mon, 21 Nov 2016 17:31:03 +0000
From: Chris Vine
Newsgroups: comp.lang.c++
Subject: Re: hidden static
Message-ID: <20161121173103.17c0ed02-at-dell.homenet>
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Xref: panix comp.lang.c++:1125410
On Mon, 21 Nov 2016 14:48:15 +0000
Chris Vine wrote:
> On Mon, 21 Nov 2016 13:26:07 GMT
> scott-at-slp53.sl.home (Scott Lurndal) wrote:
> > Juha Nieminen writes: =20
> > >Popping mad wrote: =20
> > >> This is an excellent explanation, but I'm still not clear on
> > >> this. I think it is saying that a static has to be declared and
> > >> defined within a compilational unit. I'm not sure why, though.
> > >> And I've asked before, and read, but I'm still not clear on what
> > >> a compilation unit is. =20
> > >
> > >A static member variable needs to exist somewhere in the executable
> > >(because, among other things, you need to be able to eg. have a
> > >pointer pointing to it). =20
> >=20
> > The confusing part for some folks is that gcc/binutils, when using
> > -O3, won't require the definition unless the address of the static
> > const value is taken. Remove -O3 and you'll start getting link
> > errors. =20
>=20
> My recollection in the case of the declaration of a const static
> integer member variable (typically with a literal value) is that you
> don't need to provide a definition for it if it is not ODR-used,
> whatever the optimization level. Taking an address is one way of
> ODR-using it. Casting it to lvalue of another type is another. I
> don't at the moment have the C++ standard in front of me to give you
> the reference.
I do now. It's =C2=A79.4.2/3. "If a non-volatile const static data member
is of integral or enumeration type, its declaration in the class
definition can specify a brace-or-equal-initializer in which every
initializer-clause that is an assignment-expression is a constant
expression (5.20). ... . The member shall still be defined in
a namespace scope if it is odr-used (3.2) in the program and the
namespace scope definition shall not contain an initializer." This
implies that the initialised const static integral member need not be
defined in namespace scope (its declaration is sufficient) if it is not
ODR-used.
Chris
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Date: Mon, 21 Nov 2016 18:18:06 +0000
From: Chris Vine
Newsgroups: comp.lang.c++
Subject: Re: hidden static
Message-ID: <20161121181806.58043cf8-at-dell.homenet>
References:
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<20161121173103.17c0ed02-at-dell.homenet>
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Xref: panix comp.lang.c++:1125411
On Mon, 21 Nov 2016 17:31:03 +0000
Chris Vine wrote:
> On Mon, 21 Nov 2016 14:48:15 +0000
> Chris Vine wrote:
> > On Mon, 21 Nov 2016 13:26:07 GMT
> > scott-at-slp53.sl.home (Scott Lurndal) wrote: =20
[snip]
> > > The confusing part for some folks is that gcc/binutils, when using
> > > -O3, won't require the definition unless the address of the static
> > > const value is taken. Remove -O3 and you'll start getting link
> > > errors. =20
> >=20
> > My recollection in the case of the declaration of a const static
> > integer member variable (typically with a literal value) is that you
> > don't need to provide a definition for it if it is not ODR-used,
> > whatever the optimization level. Taking an address is one way of
> > ODR-using it. Casting it to lvalue of another type is another. I
> > don't at the moment have the C++ standard in front of me to give you
> > the reference. =20
>=20
> I do now. It's =C2=A79.4.2/3. "If a non-volatile const static data memb=
er
> is of integral or enumeration type, its declaration in the class
> definition can specify a brace-or-equal-initializer in which every
> initializer-clause that is an assignment-expression is a constant
> expression (5.20). ... . The member shall still be defined in
> a namespace scope if it is odr-used (3.2) in the program and the
> namespace scope definition shall not contain an initializer." This
> implies that the initialised const static integral member need not be
> defined in namespace scope (its declaration is sufficient) if it is
> not ODR-used.
However I do now recall having experiences similar to the one you
mention. Some forms of ODR-use, such as assigning to a const
reference, may be optimized out with a sufficiently high level of
optimization, and will compile and link even though technically the
code in question is not conformant. That may be what you had in mind.
But where the static member is not ODR-used, you should be safe from
that problem.
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