BYREF in API Problem from VB to VC! 1

[MagiusCHE]

So.. i need to send from VB6 a STRUCTURED ARRAY into a VC++ DLL!!

i just prepared this 2 file:

VB6 EXE FILE
------------
public type secondarr
value1 as long
value2 as long
end type

publit type strarr
superarr() as secondarr
value3 as long
end type

Private Declare Function tryout Lib &quot;<myc++lib>&quot; (Byref PArr as strarr) as long

public sub PassArray()
dim myarr(0) as starr
redim myarr(0).superarr(0)
myarr(0).value3=3
myarr(0).superarr(0).value1=1
myarr(0).superarr(0).value2=2
msgbox tryout(myarr(0))
end sub

VC
----
[this is a Dynamic-DLL]
[....]
//only function and struct here

[in h]
struct secondarr
{
int value1;
int value2;
};

struct strarr
{
secondarr* superarr;
int value3;
};

[in cpp]
int tryout(strarr * myarr)
{
return strarr[0].superarr[0].value1; // <-- don't work
return stratt[0].value3; // <-- work correctly...
};


...................

so where is the problem????

i need to read a substructired array but... in this case i only operate on first level of array....

any one has other solution???????

thx....

 

[MikeCox]

This may be unintuitive, but do you think it could be because the strarr in VB consists of a UDT array and a long, while the cooresponding strarr in C consists of a pointer to a struct (not a reference to struct) and an int? I think the value secondarr* superarr in the struct strarr will contain a number instead of an actual structure reference like the tryout function is expecting to handle. In other words, you may try making struct strarr like this:

struct strarr {
secondarr superarr;
strarrptr* superarr;
int value3;
};

The first element is more like the VB UDT you've created, and there's still a pointer to the array so u can pass reference to it to the function.

Hope that helps!

-Mike Difference between a madman and a genius:
A madman uses his genius destructively,
A genius uses his madness constructively.

 

[rsathish]

hi mike

i face the same problem as magius. i dont understand your solution to that.

struct strarr {
secondarr superarr;
strarrptr* superarr;
int value3;
};

what is that &quot;strarrptr* superarr&quot;??

i've just started using vc++

reply asap

thanks

sathish

 

[MikeCox]

I have to admit my first post is quite inaccurate, since pointers and arrays are the same thing. In MagiusCHE's code above I noticed something else in the VB function:

public sub PassArray()
dim myarr(0) as starr
redim myarr(0).superarr(0)
myarr(0).value3=3
myarr(0).superarr(0).value1=1
myarr(0).superarr(0).value2=2
msgbox tryout(myarr(0))
end sub

The statement &quot;redim myarr(0).superarr(0)&quot; may declare superarr(0) as a variant in VB, not a structure, since there's no data type specified. It may be worth a try to change it to read &quot;redim myarr(0).superarr(0) As secondarr&quot; and see if C++ will recognize it then. I would like to recall the suggestion I made above, as the rest of MagiusCHE's code seems to be fine.

rsathish, if you're asking why I recommended that he declare 2 variables with the same name (secondarr superarr, and strarrptr* superarr), it was a mistake on my part, sorry. If you're asking what the astrisk * is, it only means the variable is a pointer of that data type, not a variable of that data type. A pointer holds a memory address and allows parts of your code to change the value in that address, rather than a copy of that value. A perfect example is passing values to a function. By default this makes a copy of the variable to pass to the function and whatever changes the function makes to the variable won't be seen after the function returns:
----------------------------------
int Example(){
int MyVar = 4;
Test(MyVar);
return MyVar;
}

void Test(int Value){
Value = 5;
return;
}
-------------------------------
The function Example() will return 4, not 5, because the function didn't actually change MyVar. It changed a copy of it. Here's an example of how to fix this with a pointer:
----------------------------------
int Example(){
int MyVar = 4;
Test(&MyVar);
return MyVar;
}

void Test(int* Value){
*Value = 5;
return;
}
-------------------------------
Now when I call Test() I pass it the &quot;address of&quot; MyVar, and the function now expects to handle a pointer to an integer. Now whenever Test() changes that value, it's changing the variable that the pointer points to.

Hopefully that makes sense, and if not there's a lot of tutorials on the i-net that are better than what I can come up with in just a few minutes.

Hope that helps!

~Mike
Any man willing to sacrifice liberty for security deserves neither liberty nor security.

-Ben Franklin

 

[rsathish]

hi mike thanks for the reply.

actually i used safearray technique to solvethat problem and it worked fine.

anyway will try out ur solution too and let u know the comments.