how to assign pointer in a managed array of pointers c++/cli?

Question

I am new to managed code and i need to pass array of pointers to different structures to windows form using C++/CLI , but it didn`t work !

My problem is in the managed array, how can i correctly access its elements .

The code sequence :

array<void*> ^ ptr;//here ptr value is undefined , type array<void*> ^
ptr = gcnew array<void*> (2);// length 0x2 , 0x0 and 0x1 values are undefined of type void

class1::struct1 structObj1;
class2::struct2 structObj2;

ptr[0] = &structObj1;// value is empty of type void!!
ptr[1] = &structObj2;//value is empty of type void!!

When i watched ptr , i found the above comments.

Notice that repeating code but using unmanaged array works probably

void* ptr[2];//here ptr value is undefined , type void*[]

class1::struct1 structObj1;
class2::struct2 structObj2;

ptr[0] = &structObj1;// value is address1 of type void*
ptr[1] = &structObj2;//value is address2 of type void*

Can anyone see where is the problem??

Do I need to use unmanaged array then convert to managed? If yes, how can I do it ??

Neither the CLR nor the debugger have much use for a void*. No type info is available for it. It is very unclear what type "struct1" might be, you cannot do anything with it in a Winforms app if it is an unmanaged struct. If it is managed then a way ahead is to use an array of Object^. Still miserable in the Winforms code but at least it has a shot at it. — Hans Passant
– Hans Passant, Commented May 15, 2013 at 15:29
He might be doing a WinForms app in C++/CLI, and need a way to pass unmanaged data from one managed class to another. — David Yaw
– David Yaw, Commented May 15, 2013 at 15:49

David Yaw · Accepted Answer · 2013-05-16 16:12:59Z

2

Passing unmanaged pointers in a managed array may be valid C++/CLI, but it's definitely not the ideal way to do things. Do consider creating a custom managed class (ref class in C++/CLI) to hold the structures, instead of passing around pointers.

For this, I'm assuming that struct1 and struct2 are unmanged structs. This answer only applies if that is the case.

Your existing code works for me. Here's my version, with some debugging added in.

public struct struct1 { int foo; };
public struct struct2 { float bar; };

int main(array<System::String ^> ^args)
{
    array<void*> ^ ptr;
    ptr = gcnew array<void*> (2);

    for(int i = 0; i < ptr->Length; i++)
        Debug::WriteLine("ptr[{0}] = {1:X8}", i, reinterpret_cast<int>(ptr[i]));

    struct1 structObj1;
    struct2 structObj2;

    ptr[0] = &structObj1;
    ptr[1] = &structObj2;

    for(int i = 0; i < ptr->Length; i++)
        Debug::WriteLine("ptr[{0}] = {1:X8}", i, reinterpret_cast<int>(ptr[i]));

    struct1* pointerToStructObj1 = reinterpret_cast<struct1*>(ptr[0]);

    structObj1.foo = 4;
    Debug::WriteLine("pointerToStructObj1->foo = {0}", pointerToStructObj1->foo);
}

Output:

ptr[0] = 00000000
ptr[1] = 00000000
ptr[0] = 0013F390
ptr[1] = 0013F394
pointerToStructObj1->foo = 4

Edit

To use Debug::WriteLine, add using namespace System::Diagnostics.

The debugger doesn't know how to display the contents of a void*, so it just displays blank. It does display a null pointer differently, though: null shows up as <undefined value>, non-null shows up as just blank.

My philosophy on C++/CLI is: If you're going to write managed code, write managed code. Consider replacing your vector with a managed List. If you still need unmanaged objects, I strongly urge you to consider writing a managed class with properly typed pointers, rather than a void* array.

To implement such a class, create whatever fields you need, just be sure that they're pointers, not direct. (vector<foo>* instead of vector<foo>.) Create the objects with new in the constructor, and delete them in the destructor (which is called on Dispose) & finalizer.

edited May 16, 2013 at 16:12

answered May 15, 2013 at 15:48

David Yaw

28.1k5 gold badges65 silver badges95 bronze badges

Sign up to request clarification or add additional context in comments.

10 Comments

Mai Saad Over a year ago

thanks alot for your reply , it was very helpful , but i`ve some concerns :

Mai Saad Over a year ago

1- why normal casting dont work i.e (struct*)(ptr[0])...it says cant convert from void* to struct* ?! 2- do we need to include certain header to use Debug::WriteLine as it didnt work with me !! 3- do you have any idea why using watch tool , I see nothing in ptr value also we can show its value on console(I used cout) - By the way , I used pointer because , my structures are unmanaged and it contains vector so I couldnt define its content in ref class as use it directly , is there a way other than pointer??

Mai Saad Over a year ago

forget about normal casting question (qs 1) as after changing place of code , it works probably .

Mai Saad Over a year ago

@ David : you are right , void* array isn

t good , what about collecting unmanaged vars in unmanaged class without any structs then create ptr to that class as member variable when needed and make new/delete for class in constructor/destructor , isn

t this better than creating *,new,delete to every unmanaged obj in a managed class ??

Mai Saad Over a year ago

@ David : another question , if i

ve 2 classes a and b , then in class a i created member variable * to b , made new b in constructor and delete in destructor , and class a calls class b method passing the previous member variable then in class b to access the member variable of b in member functions of b why can

t i access them directly and i shall access through the passed pointer ?? <br/>Thanks.

|

Collectives™ on Stack Overflow

how to assign pointer in a managed array of pointers c++/cli?

1 Answer 1

Edit

10 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Edit

10 Comments

Your Answer

Sign up or log in

Post as a guest

Related