Problem Calling C++ Lib function from VB.Net

  • Thread starter Thread starter JackRazz
  • Start date Start date
J

JackRazz

I'm trying to call a Managed C++ library function from VB and cannot get it to return
the exponent paramater in Frexp(double floatValue, int* exponent). But I'm getting
the following compile error in the vb code:

'Frexp' has a return type that is not supported or parameter types that are not
supported, which is the passed e variable in frexp(d2, e).

Below is the relevant code from the C++ library. I'm very new to C++ and know its
something very simple. Whats wrong with this C++ code?


Thanks - JackRazz


------------------------------------------------
'This is the test VB code:
Dim d1 As Double
Dim e As Integer
Dim d2 As Double = 16.4
d1 = CLib.MyMath.Frexp(d2, e)

------------------------------------------------
// CLib.h

public:
static double Frexp(double floatValue, int* exponent);


------------------------------------------------
// This is the main DLL file.

double CLib::MyMath::Frexp(double floatValue, int* exponent)
{
return frexp( floatValue, exponent );
};
 
I'm trying to call a Managed C++ library function from VB and cannot get
it to return
the exponent paramater in Frexp(double floatValue, int* exponent). But I'm getting
the following compile error in the vb code:

'Frexp' has a return type that is not supported or parameter types that are not
supported, which is the passed e variable in frexp(d2, e).

Below is the relevant code from the C++ library. I'm very new to C++ and know its
something very simple. Whats wrong with this C++ code?


Thanks - JackRazz


------------------------------------------------
'This is the test VB code:
Dim d1 As Double
Dim e As Integer
Dim d2 As Double = 16.4
d1 = CLib.MyMath.Frexp(d2, e)

------------------------------------------------
// CLib.h

public:
static double Frexp(double floatValue, int* exponent);
Click to expand...

The exponent variable seems to be the problem. You'll need to declare it as
a __gc pointer, and, since it is probably an out only parameter, you'll
likely want something like this:

using namespace System::Runtime::InteropServices;

static double Frexp(double floatValue, [Out] int __gc* exponent)
{
int __pin* e = exponent;

return frexp( floatValue, e );
}
 
Tomas,
Thanks for help. I got it working. I had a couple of problems.

First, I couldn't get the function to compile static on the C++ side:

Frexp: 'identifier' : 'static' should not be used on member functions defined at
file scope

From CLib.h
-------------
namespace CLib
{
public __gc class MyMath
{
public:
static double Frexp(double floatValue, int __gc* exponent);
};
}

From CLib.cpp
----------------
static double CLib::MyMath::Frexp(double floatValue, int __gc* exponent)


I'm confused in that the function is under 'public:' I don't mind non static, just
trying to understand. What am I doing wrong?


Second, I got the following for the out attribute. 'CLib.h(19): error C3108: 'out' :
this attribute may not be used in a managed context' I think this one is cut and
dry - don't use [out]. Just want to make sure.

Thanks for all the help. I would never have gotten this in my own.

JackRazz







|
| The exponent variable seems to be the problem. You'll need to declare it as
| a __gc pointer, and, since it is probably an out only parameter, you'll
| likely want something like this:
|
| using namespace System::Runtime::InteropServices;
|
| static double Frexp(double floatValue, [Out] int __gc* exponent)
| {
| int __pin* e = exponent;
|
| return frexp( floatValue, e );
| }
|
| --
| Tomas Restrepo
| (e-mail address removed)
|
|
 
Hi Jack,
Tomas,
Thanks for help. I got it working. I had a couple of problems.

First, I couldn't get the function to compile static on the C++ side:

Frexp: 'identifier' : 'static' should not be used on member functions defined at
file scope
Click to expand...
From CLib.h
-------------
namespace CLib
{
public __gc class MyMath
{
public:
static double Frexp(double floatValue, int __gc* exponent);
};
}

From CLib.cpp
Click to expand...

You should only put static on the method declaration, not the definition
(just remove it from CLib.cpp).
I'm confused in that the function is under 'public:' I don't mind non static, just
trying to understand. What am I doing wrong?


Second, I got the following for the out attribute. 'CLib.h(19): error C3108: 'out' :
this attribute may not be used in a managed context' I think this one is cut and
dry - don't use [out]. Just want to make sure.
Click to expand...

Case problem: It's [Out] not [out].
Also, make sure you add a using namespace System::Runtime::InteropServices;
 
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