I've got an Interop wrapper to some unmanaged DLL calls that return details through out paremeters. The functions appear like this:
_myWrapper->My_Method( ... out UInt32 value ... );
So assuming the method is declared like this:
void My_Method(out UInt32 value);
How do I then call this method from within my C++/CLI code? I know how to call reference methods such as this easy enough:
void Another_Method(ref UInt32 value);
UInt32 _value;
_myWrapper->Another_Method(%_value);
I'm doing a little reading and I am reading it can't be done? I don't believe it... Likely this isn't impossible to overcome or workaround, but you've got to be kidding me? Is that really true?
Thank you...
In C++, there's no special call syntax for calling a function with a reference parameter, you just write the call like it was pass-by-value. Of course, you need to supply an lvalue to be overwritten, the rvalue (temporary) result of an arithmetic expression can't be used.
BTW, your code for calling a ref function is wrong too, that might be the source of your troubles.
Example:
C# definition:
void MySharpRef(ref int i) { i = 4; }
void MySharpOut(out int i) { i = 5; }
C++/CLI definition:
void MyPlusRef(System::Int32% i) { i = 14; }
void MyPlusOut([System::Runtime::InteropServices::OutAttribute] System::Int32% i) { i = 15; }
C# call:
int j;
o.MyPlusOut(out j);
o.MyPlusRef(ref j);
C++/CLI call:
System::Int32 k;
p->MySharpOut(k);
p->MySharpRef(k);
Related
Looks like I am doing a basic mistake here. I have a 3 party C++ library(test.dll) in which there is a API defined as follows. And I am invoking this APi by loading the library, getting the API and invoke. I am new to C++ CLI, any pointers to solve the issue will be helpful. Thanks in advance.
3rd part library exported API
FUNCTION_EXPORT void STDCALL GetVersion(UINT16& version);
typedef void (STDCALL *GETVERSION)(UINT16);
I need to call it from C++ Cli
Header file
MyTest.h
namespace MyTest {
public ref class TestClass
{
public:
HMODULE module;
String^ version;
void TestMethod()
};
}
Cpp file
MyTest.cpp
namespace MyTest {
TestClass::TestMethod()
{
this->module = LoadLibrary(engineDllPath);
if (!this->module)
{
return String::Format("LoadLibrary failed");
};
// Get engine version
GETVERSION GetVersionApi = (GETVERSION)GetProcAddress(module, "GetVersion");
if (!GetVersionApi)
{
return;
}
UINT16 major;
GetVersionApi(&uiMajor);
}
}
Getting compilation error
error C2664: 'void (UINT16 &)' : cannot convert parameter 1 from 'UINT16 *' to 'UINT16 &'
Code snippet is to give an idea what I am trying. The main issue is here
UINT16 major;
GetVersionApi(&uiMajor);
what will be the correct way of calling it. Please help.
GetVersion(UINT16& version);
That's not an integer pointer, that's an integer reference. You don't need to type any extra characters to pass a reference.
GetVersionApi(uiMajor);
// ^ no "&"
Is there any way to pass a function pointer to an Objective C method, and then have that method modify the function pointer to point at a C function somewhere else?
Creating a method that accepts a function pointer is simple enough:
- (void)doSomethingWithFunctionPointer:(void(*)(/* args go here */))functionPointer;
I can then call that function inside doSomethingWithFunctionPointer simply by calling:
if (functionPointer)
{
functionPointer();
}
But what if I actually wanted to change the functionPointer to point at something else within doSomethingWithFunctionPointer, so that any code outside that method can then call the changed function pointer to call the function that doSomethingWithFunctionPointer point it to?
I know this is probably a prime example of how not to do things in Objective C (especially considering we've got blocks and what not). It's more just curiosity at this point. It almost sounds like I'd need a function pointer pointer, but I'm not sure how that would work, if it's even possible.
This can be done using pointers to function pointers. Perhaps the most readable way to do it is to typedef your function pointer, like this:
typedef void (*FunPtr)(int a, float b);
Then use a pointer of that typedef-ed type to assign in a function, like this:
void foo(int a, float b) {
printf("FOO : %d %f\n", a, b);
}
void bar(int a, float b) {
printf("BAR : %d %f\n", a, b);
}
// This function receives a pointer to function pointer
void assign(int n, FunPtr *ptr) {
if (n == 0) {
*ptr = foo;
} else {
*ptr = bar;
}
}
Here is how you call assign from your code:
int main(void) {
FunPtr f;
assign(0, &f);
f(10, 20.5);
assign(1, &f);
f(10, 20.5);
return 0;
}
Demo.
Note: You are right about blocks in Objective-C greatly reducing the need for direct use of function pointers. However, you can use a similar typedef trick with pointers to blocks.
I have a method that returns an integer and I now also want to return a small struct or class. If I was using C++ I would pass a reference to the struct in as a parameter. In iOS using ARC, I think the equivalent is to use a pointer to a pointer that has the __autoreleasing attribute which I find a bit cumbersome.
I could return an array containing the two values but then think I would be alloc'ing more than necessary and I could be using this a lot (100,000 calls).
Even with ARC, you can just pass in a struct by reference or an object pointer...
Pass the struct by ref just like you would in C++, e.g. &aStruct
-(int)getStuffOut:(SomeStruct *)aStruct {
if(!aStruct) {
return 0;
}
aStruct->myInt = 12345;
aStruct->myFloat = 12.345f;
return 1;
}
Or:
-(int)getStuffOut:(SomeClass *)anObject {
if(!anObject) {
return 0;
}
anObject.myIntProperty = 12345;
anObject.myFloatProperty = 12.345f;
return 1;
}
If you're using ARC you dont have to worry about memory management if you're using plain Objective-C.
Custom objects are passed by reference, so you can pass your OBJ-C object to your method and fill your stuff in.
Or you can return a Struct that holds the two values
+(struct YourStruct)someMethod:(NSString *)someParam;
+(YourStruct)someMethod:(NSString)someParam {
//some method code
YourStruct st;
//Do something here with the struct
return st;
}
Help!
I'm totally exhausted/frustrated with what seems to be a reasonably easy task. I’m not sure what I'm doing wrong; let alone if I'm doing it correct. I'm "required" to use an existing library (a C static library – over 100,000 lines of straight C code) in developing a WPF application (VS 2010, C# 4.0). Oh, and I can't touch the existing C code - use it as is!
I've read so many postings (advanced topics, how-to, etc), yet I'm so new to C++/CLI that it's just not making sense. From what I've read the best approach is to wrap the C static library as follows:
Unmanaged C static library <---> C++/CLI managed wrapper DLL <--->
managed WPF application
This is the stripped down C header file:
/* Call this function to execute a command. */
int issue_command(int command, long param1, long param2);
/* Completion call back function; you must supply a definition. */
extern int command_completed(int command, long param1, long param2);
struct struct_command_str
{
char command_str[10];
char param1_st[2];
char param2_st[2];
char success;
};
/* You must supply definitions to the following extern items. */
extern int command_status;
extern struct struct_command_str command_str;
The problem(s):
What I can’t seem to do correctly is provide a C++/CLI implementation for the call back functions, and the two extern items (command_status and struct command_str).
Can someone provide a sample C++/CLI implementation for the above missing call back functions and externs?
Thanks in advance for your assistance.
in your C++/CLI managed wrapper project, add 2 files :
a .c file :
extern void doSomething();
int command_status = 0;
struct_command_str command_str = { "command1", "p1", "p2", 't' };
int command_completed(int command, long param1, long param2) {
...
command_status = 1;
...
doSomething();
...
command_status = 2;
...
return 3;
}
a cpp file
void doSomethingManagedWrapper() {
...
call managed code
...
}
void doSomething() {
doSomethingManagedWrapper();
}
when you implement these in your c++/cli module, use the same signature shown in the c header file,but prefixed with extern "C".
also put an extern "C" block around the #include of the C header file.
I am trying to pass an array of ints from C# to C++/CLI. Here's my code:
// SafeArrayTesting_PlusPlus.cpp
#include "stdafx.h"
#include <comdef.h>
using namespace System;
namespace SafeArrayTesting_PlusPlus
{
public ref class MyCppClass
{
public:
MyCppClass();
~MyCppClass();
void SetMyInts(array<int>^ myInts);
};
MyCppClass::MyCppClass(){}
MyCppClass::~MyCppClass(){}
void MyCppClass::SetMyInts(array<int>^ myInts)
{
// Create safearray
SAFEARRAY *safeArrayPointer;
SAFEARRAYBOUND arrayDim[1]; // one dimensional array
arrayDim[0].lLbound= 0;
arrayDim[0].cElements= myInts->Length;
safeArrayPointer = SafeArrayCreate(VT_UNKNOWN,1,arrayDim);
// copy ints to safearray
for (long lo= 0; lo < myInts->Length; lo++)
{
cli::pin_ptr<int> pinnedIntPointer = &(myInts[lo]);
SafeArrayPutElement(
safeArrayPointer,
&lo,
static_cast<void*> (pinnedIntPointer)); // line XX
}
// do something with the safearray here
}
}
// SafeArrayTesting_Main.cs
using SafeArrayTesting_PlusPlus;
namespace SafeArrayTesting_Main
{
class SafeArrayTesting_Main
{
static void Main()
{
var myCppClass = new MyCppClass();
myCppClass.SetMyInts(new[]{42});
}
}
}
When I run this, line XX throws the following exception:
System.AccessViolationException: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.
I have a feeling that I'm doing something basically wrong with the SafeArrayPutElement method. Can you spot the error?
(By the way, I have asked a more complex variation of this question at Passing an array of interfaces from C# to C++/CLI . I think the difference is big enough to warrant two separate questions.)
safeArrayPointer = SafeArrayCreate(VT_UNKNOWN,1,arrayDim);
That creates an array of IUnknown interface pointers. SafeArrayPut() makes sure to increase the reference count of the interface pointer by calling IUnknown::AddRef() since it stores a copy of the pointer in the array.
You can see how that goes kaboom. Create an array of integers instead. Fix:
safeArrayPointer = SafeArrayCreate(VT_I4,1,arrayDim);