passing structs

[IrishJoker]

Hi,

I am looking for a way to pass structs or a similar type of thing from a C++ DLL to VB.

Has anyone got any ideas.....

Regards

IJ...

[twanvl]

You could do it the way windows API does it:

C++:

struct my_struct
{
    int a,b;
    char c;
    something* d;
};
void _stdcall my_function(my_struct& the_struct)
{
    the_struct.a=1;
    the_struct.b=2;
    the_struct.c="3";
    the_struct.d=&my_something;
}

VB:

Type my_struct
    'The same as the other struct, but VB doesn't have pointers so use a Long instead
    Long a
    Long b
    Byte c
    Long d
End Type
Declare Sub my_function Lib "dll.dll" (ByRef Struct As my_struct)
Sub MySub()
    Dim ReturnValue As my_struct
    my_function ReturnValue
    MsgBox ReturnValue.a
End Sub

[IrishJoker]

Thanks man, your a genius

But how do I get the string value, not too sure about that

Thanks

IJ...

[twanvl]

A VB string passed ByVal can be used in C++ as a normal wchar_t:

#include <string>
#include <algorithm>
void _stdcall takes_a_string(wchar_t* the_string)
{
    std::wstring my_string = the_string;
    the_string=L"hi" + L", bye";
    std::copy(my_string.begin(),my_string.end(),the_string);
    the_string[my_string.size()]=0;
}

You could also use C style string functions instead of the C++ ones. Also, make sure that the string is large enough, and that it is not a variant:

Declare Sub takes_a_string Lib "dll.dll" (ByVal aString as String)
Sub MyOtherSub()
    Dim MyString As String
    MyString=String(100," ") ' Make sure it is large enough
    takes_a_string MyString
    ' Everything until the \0, which markes the end of a C style string
    MyString=Left$(MyString,InStr(MyString,vbNullChar)-1)
    MsgBox MyString
End Sub

Also note that although you are passing the string ByVal, you can still modify it in C++
I don't know if it also works for a string in a struct...

[Zaffir]

Could pointers (in C) be used for something like this?

[Sastraxi]

You mean, to pass strings? Maybe. Actually quite possible...

Why not try? Use VarPtr, or Copymemory!

[sbasak]

The example creates shows how to

1. create a DLL in C/C++ which returns a structure
2. calls that DLL in VB (using VarPtr and CopyMemory)

[ccode]
#include<string.h>
#include<math.h>
#include<malloc.h>
struct quad
{
float r1;
float r2;
char* status;
};
quad* _stdcall solve_quad(int *a);
quad* _stdcall solve_quad(int *a)
{
quad *x;
x=(quad*)malloc(sizeof(quad));
int p,q,r;
float determ;
p=*(a+0);
q=*(a+1);
r=*(a+2);
determ=q*q-4*p*r;
if(determ<0) /* no real root */
{
x->r1=0;
x->r2=0;
x->status=(char*)malloc(13*sizeof(char));
strcpy(x->status,"No real root");
return x;
}
else /* real root exists */
{
x->r1=((-q)+sqrt(determ))/(2*p);
x->r2=((-q)-sqrt(determ))/(2*p);
x->status=(char*)malloc(15*sizeof(char));
strcpy(x->status,"Two real roots");
return x;
}
}
[/ccode]

Create also the .def file as usual.
LIBRARY ArrayStructure
EXPORTS solve_quad

Now use the following code to call the function in VB.

Private Declare Sub CopyMemory Lib "kernel32" Alias
"RtlMoveMemory" _
(Destination As Any, ByVal Source As Any, ByVal Length As
Long)
Private Declare Function solve_quad Lib "arraystructure.dll" _
(ByVal m As Long) As Long
Private Sub Command1_Click()
Dim adr As Long
Dim z(3) As Long
Dim Root1 As Single
Dim Root2 As Single
Dim Solution As String
z(0) = 1
z(1) = -5
z(2) = 6
adr = solve_quad(VarPtr(z(0)))
Call CopyMemory(Root1, adr, 4)
Call CopyMemory(Root2, adr + 4, 4)
Call CopyMemory(Solution, adr + 8, 4)
MsgBox "Root1 = " & Root1 & vbCrLf & "Root2 = " & Root2 _
& vbCrLf & "Solution = " & Solution
End Sub

Execute the VB program and you’ ll see two roots and status of solution.