Re: Want struct parameter by default set to NULL - C / C++ / MFC Discussion Boards

Re: Want struct parameter by default set to NULL

Wischkony15-Dec-08 1:11

Wischkony

15-Dec-08 1:11

You are right, classes would work, but it is not an option for me Frown | :(

Anyway, thank you

Re: Want struct parameter by default set to NULL

Maximilien15-Dec-08 0:49

Maximilien

15-Dec-08 0:49

yes, that's called "default arguments"

This signature was proudly tested on animals.

Re: Want struct parameter by default set to NULL

Wischkony15-Dec-08 1:09

Wischkony

15-Dec-08 1:09

okay, i did not knew that they were called default arguments.
(I thought default parameter was correct...)

But how can i handle structs???
I want to set an struct argument to NULL by default...

Hm...

Re: Want struct parameter by default set to NULL

CPallini15-Dec-08 0:54

CPallini

15-Dec-08 0:54

C++ allows default arguments, see, for instance, [^].

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.

This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong.
-- Iain Clarke

[My articles]

Re: Want struct parameter by default set to NULL

Wischkony15-Dec-08 1:07

Wischkony

15-Dec-08 1:07

Oh ha,...sry

Maybe my question was not clearly.
I know how to handle default arguments(int,bool,float,double...)...but i dont know how to handle it for Structs!

Re: Want struct parameter by default set to NULL

CPallini15-Dec-08 1:43

CPallini

15-Dec-08 1:43

It makes difference?
For instance:

void foo(MyStruct mystruct=MyStruct());

doesn't fit your needs?
Smile | :)

Re: Want struct parameter by default set to NULL

Wischkony15-Dec-08 2:06

Wischkony

15-Dec-08 2:06

Very good point...
but,

i use structs which also contains structs and many other items, so it would look like this:

void func(
          MyStruct mystruct=MyStruct(MyStruct2(),MyStruct3(),2,3,4,"anystring" )
          );

Am i right?? I think so.

AND another problem is, that if i want to check it in the function, if it is a default, so i have to provide an additional variable:

void func(
          MyStruct mystruct=MyStruct(MyStruct2(),MyStruct3(),2,3,4,"anystring",1 )
          );
...
void func(
MyStruct mystruct=MyStruct(MyStruct2(),MyStruct3(),2,3,4,"anystring",1 )
         )
{
if(mystruct.isDefault==1)//isDefault->last parameter
  {...}
else
  {...}
}

I hoped(and still hoping) there is an easier solution, but you are right, this would work.

Re: Want struct parameter by default set to NULL

CPallini15-Dec-08 2:23

CPallini

15-Dec-08 2:23

Wischkony wrote:
use structs which also contains structs and many other items, so it would look like this:

void func( MyStruct mystruct=MyStruct(MyStruct2(),MyStruct3(),2,3,4,"anystring" ) );

Am i right?? I think so.

Yes, you may also override the MyStruct's default constructor to take care about.

Wischkony wrote:
AND another problem is, that if i want to check it in the function, if it is a default, so i have to provide an additional variable:

void func( MyStruct mystruct=MyStruct(MyStruct2(),MyStruct3(),2,3,4,"anystring",1 ) );...void func(MyStruct mystruct=MyStruct(MyStruct2(),MyStruct3(),2,3,4,"anystring",1 ) ){if(mystruct.isDefault==1)//isDefault->last parameter {...}else {...}}

Provided you've overridden the default MyStruct's constructor the way I told, you may also use the following test (you've also to override the == operator, of course):

if ( mistruct == MyStruct() )

Re: Want struct parameter by default set to NULL

Wischkony15-Dec-08 2:34

Wischkony

15-Dec-08 2:34

->Yes, you may also override the MyStruct's default constructor to take care about.
Yes, i know Smile | :)

By reading and writing answers, i am thinking more and more that this is a good solution and not a bad solution.
I was dreaming of an easy way like this:
void func(mystruct *ms=NULL);
But maybe this will stay a dream Wink | ;)

Thanks a lot for your time.

Re: Want struct parameter by default set to NULL [modified]

CPallini15-Dec-08 2:55

CPallini

15-Dec-08 2:55

Wischkony wrote:
void func(mystruct *ms=NULL);

Well, you have also that option.
Smile | :)

modified on Monday, December 15, 2008 11:53 AM

Re: Want struct parameter by default set to NULL

BonshatS15-Dec-08 1:56

BonshatS

15-Dec-08 1:56

It should work the same way. What error are you getting that it doesn't?

Re: Want struct parameter by default set to NULL

Wischkony15-Dec-08 2:14

Wischkony

15-Dec-08 2:14

Oh, okay,...

void func(myStruct *ms);

works fine.

But with:

void func(myStruct *ms=NULL);
i got the error C2143: Syntax error: missing ')' before '='

Maybe i am totaly wrong, but i think, i got the error because setting a default value for a struct can not be done so easy.

Re: Want struct parameter by default set to NULL

David Crow15-Dec-08 3:12

David Crow

15-Dec-08 3:12

Wischkony wrote:
Maybe i am totaly wrong, but i think, i got the error because setting a default value for a struct can not be done so easy.

It has nothing to do with it being a struct. It has to do with the file having a .c extension.

"Love people and use things, not love things and use people." - Unknown

"The brick walls are there for a reason...to stop the people who don't want it badly enough." - Randy Pausch

Apologize

Wischkony15-Dec-08 3:40

Wischkony

15-Dec-08 3:40

i have to apologize to all here trying help me.

I was running for hours against a wall.

i tried the following:
void func(mystruct *ms=NULL);

and got an error, so thought this is not possible!!

But it is possible, with:
void func(mystruct *ms);
void func(mystruct *ms=NULL)
{
...
}

I am sry,...aeh this is embarrasing...

Re: Apologize

Mark Salsbery15-Dec-08 11:05

Mark Salsbery

15-Dec-08 11:05

Wischkony wrote:
I am sry,...aeh this is embarrasing...

Heh. No problem.

No better way to find silly mistakes and easy solutions than putting it out there for
the world to see Smile | :)

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

DeviceIoControl

john563215-Dec-08 0:36

john5632

15-Dec-08 0:36

Hi,

How can I used DeviceIoControl with FSCTL_GET_VOLUME_BITMAP to get list of free cluster of file system.

Please give any link related to this.

Re: DeviceIoControl

Steve Thresher15-Dec-08 4:29

Steve Thresher

15-Dec-08 4:29

http://msdn.microsoft.com/en-us/library/aa364573(VS.85).aspx[^]

AxisFirst For Business

Unresolved external symbol in a DLL

George_George14-Dec-08 23:32

George_George

14-Dec-08 23:32

Hello everyone,

I am developing a DLL and making another console application to link with the DLL. The DLL expose type CFoo, create an instance and the application consumes it. There is error like this. The source code of DLL and main are listed below, any ideas?

1>main.obj : error LNK2001: unresolved external symbol "class CFoo foo" (?foo@@3VCFoo@@A)

foo.h (defines exposed type in the DLL)
class __declspec (dllexport) CFoo
{
public:

	CFoo();

	int foo1();

	int foo2();
};

dllmain.cpp (defines type implementation inside dll)

#include "foo.h"

CFoo::CFoo()
{
	return;
}

int CFoo::foo1()
{
	return 100;
}

int CFoo::foo2()
{
	return 200;
}

CFoo foo;

main.cpp (the console application which dynamically links with the DLL using import library)

#include "foo.h"

extern CFoo foo;

int main()
{
	int i = foo.foo1();
	i = foo.foo2();

	return 0;
}

thanks in advance,
George

Re: Unresolved external symbol in a DLL

nbugalia15-Dec-08 0:45

nbugalia

15-Dec-08 0:45

George_George wrote:
extern CFoo foo;

Replace the above by -

CFoo foo2;

Re: Unresolved external symbol in a DLL

George_George15-Dec-08 0:55

George_George

15-Dec-08 0:55

I am confused, nbugalia! In my design, I want to create object instance in DLL and access the object instance from the hosting executable from extern reference. Your code change impacts my design... Smile | :)

Any comments?

regards,
George

Re: Unresolved external symbol in a DLL

nbugalia15-Dec-08 1:18

nbugalia

15-Dec-08 1:18

Here is the Dll file code -
foo.h

class __declspec (dllexport) CFoo 
{
public:	
    CFoo();	
    int foo1();	
    int foo2();
};

__declspec (dllexport) CFoo foo;re>

Foo.cpp
<pre>#include "Foo.h"

CFoo::CFoo()
{	
    return;
}
int CFoo::foo1()
{	
    return 100;
}
int CFoo::foo2()
{	
    return 200;
}

and here is the console based application code -
Foo.h

class __declspec (dllimport) CFoo { public: CFoo(); int foo1(); int foo2(); }; __declspec (dllimport) CFoo foo;

test.cpp #include "foo.h" int main() { int i = foo.foo1(); i = foo.foo2(); return 0; }

Don't forget to set the dependency to Foo.lib file in the test project settings.
Hope this will help.

Re: Unresolved external symbol in a DLL

George_George15-Dec-08 1:54

George_George

15-Dec-08 1:54

Cool, nbugalia!

I want to confirm with you that we could expose, function/class/global variable from a DLL, correct?

regards,
George

Re: Unresolved external symbol in a DLL

Naveen15-Dec-08 0:51

Naveen

15-Dec-08 0:51

George_George wrote:
CFoo foo;

This is not enough, you have to export the foo variable, like

__declspec (dllexport) CFoo foo;

George_George wrote:
extern CFoo foo;

Also in the exe, you have to import the variable

__declspec (dllimport) CFoo foo;

nave

[OpenedFileFinder] [My Blog]

Re: Unresolved external symbol in a DLL

George_George15-Dec-08 1:16

George_George

15-Dec-08 1:16

Thanks nave,

I have tested your solution works. My question is, if I create a global variable inside a DLL, then in the hosting executable, could I use extern to refer to the variable and use it?

regards,
George

Re: Unresolved external symbol in a DLL

Naveen15-Dec-08 1:28

Naveen

15-Dec-08 1:28

George_George wrote:
could I use extern to refer to the variable and use it?

No. you should export it to use the variable in the hosting executable..

nave

[OpenedFileFinder] [My Blog]

Use Ctrl+Left/Right to switch messages, Ctrl+Up/Down to switch threads, Ctrl+Shift+Left/Right to switch pages.