Trying to call a function via a pointer to a smart pointer to a COM interface

Question

I followed the Microsoft tutorial here (with a little help from SO) to call a COM object from C++ code.

Step 9 of the tutorial says:

To call the managed DLL, add the following code to the _tmain function:

// Initialize COM.
HRESULT hr = CoInitialize(NULL);

// Create the interface pointer.
ICalculatorPtr pICalc(__uuidof(ManagedClass));

When I used these lines of code in my file, they worked fine, and I called functions on the COM interface successfully.

Now, I need to access pICalc in 2 static functions, so I thought of making it a static class variable (I am aware that static has 2 different meanings in this sentence).

This is my code:

In MyCPlusPlusClass.h:

static ICalculatorPtr* pICalc;

In MyCPlusPlusClass.cpp:

//Pointer definition
ICalculatorPtr* MyCPlusPlusClass::pICalc;

and in a static function:

pICalc = new ICalculatorPtr(__uuidof(ManagedClass));

but when I tried to call a function with

(*pICalc)->SomeICalcFunction();

I got

_com_issue_error(Int32) at _com_ptr_t ...

I'm mostly a C# programmer, so am I just making a stupid syntax mistake in C++?

EDIT: In the .tlh file, there is

struct __declspec(uuid("xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx")) _ManagedClass

_COM_SMARTPTR_TYPEDEF(ICalc, __uuidof(ICalc));
_COM_SMARTPTR_TYPEDEF(_ManagedClass, __uuidof(_ManagedClass));

virtual HRESULT __stdcall SomeICalcFunction (BSTR * pRetVal) = 0;

Can you find ManagedDLL.tlh, ManagedDLL.tli and paste content of this files ? — grisha, Jul 10 '14 at 09:18
Are you 100% sure that you call the static function with pICalc initialisation before calling SomeICalcFunction? — Wojtek Surowka, Jul 10 '14 at 09:25
@Wojteksurowka - yes, because I call SomeICalcFunction inside the static initialisation function, i.e. it is the next line after initialising pICalc. — user1725145, Jul 10 '14 at 09:31
@user2451677 - I added some relevant lines from the .tlh file. Please note, that it worked perfectly when I created and used the interface pointer locally, like in the tutorial. — user1725145, Jul 10 '14 at 09:32

score 1 · Accepted Answer · answered Jul 10 '14 at 09:42

Ok, I think, that you must do the following.

In MyCPlusPlusClass.h:

change

static ICalcPtr* pICalc;

to

static ICalcPtr pICalc;

In MyCPlusPlusClass.cpp:

change

//Pointer definition
ICalcPtr* MyCPlusPlusClass::pICalc;

to

ICalcPtr MyCPlusPlusClass::pICalc;

In all of your static functions:

if(pICalc == NULL)
{
    if(FAILED(pICalc.CreateInstance(__uuidof(ManagedClass))))
        std::cout << "Can't create pICalc" << std::endl;
}

And then use:

pICalc->SomeICalcFunction();

I really dont understand why you need static functions. Make all them non-static and pICalc non-static too. That will be more cleaner.

@SList, you need to paste all your C++ code, because constructor of _com_ptr_t (it's the class of pICalc) internally calls CreateInstance(). This means that ICalcPtr pICalc(__uuidof(ManagedClass)) and ICalcPtr pICalc; pICalc.CreateInstance(__uuidof(ManagedClass)) are the same things. You can check this by changing your local variable creation. — grisha, Jul 10 '14 at 10:22

Trying to call a function via a pointer to a smart pointer to a COM interface

1 Answers1