Traps in the interface type when mixing calls

[Delphi]View PlainCopy 

1. Function ABC (A:integer): IUnknown;

This is a Delphi function declaration, it looks very simple, only one parameter, but the real situation? After being translated into binary code, there are actually 2 parameters for the function!

In order to explain the problem in more detail, first use Delphi to write a DLL, export an interface, the interface has a show method.

[Delphi]View PlainCopy  

2. Library Project1;
6. Uses
8. Windows;
12. {$R *.res}
16. Type
18. ITest = Interface
20. procedure Show (); stdcall;
22. end;
26. TTest = Class (Tinterfacedobject, ITest)
28. Public
30. procedure Show (); stdcall;
32. end;
36. function gettest:itest; stdcall;
38. Begin
40. Result: = TTest.  Create;
42. End
46. Exports
48. Gettest;
52. {TTest}
56. Procedure TTest.  Show;
58. Begin
60. OutputDebugString (' Hello World ');
62. End
66. Begin
68. End.

Callers are written in C + +

[CPP]View PlainCopy 

2. #include "stdafx.h"
4. #include <iostream>
6. #include <Windows.h>
10. Interface ITest: public IUnknown
12. {
14. virtual void __stdcall Show () = 0;
16. };
20. typedef itest* (WINAPI *getitest) ();
24. int _tmain (int argc, _tchar* argv[])
26. {
28. hmodule h = LoadLibrary (TEXT ("Project1.dll"));
30. if (h! = 0)
32. {
34. Getitest get = (getitest) GetProcAddress (H, "gettest");
36. ITest *test = Get ();
38. Test->show ();
40. Test->release ();
42. }
44. System ("pause");
46. return 0;
48. }

Directly eject a memory error after running

The error statement is in the DLL

[Delphi]View PlainCopy 

1. function gettest:itest; stdcall;
2. Begin
3. Result: = TTest.  Create;
4. End

You can see the problem by looking at the function in the form of disassembly code.

As you can see, when the function return value is an interface type, the return value is actually an implicit parameter and is a level two pointer type. The use of Dephi does not find the problem because it is automatically optimized.

In multi-language mixed programming, there is an empty memory error when you return an interface or an object directly.

The modified calling code

[CPP]View PlainCopy 

2. #include "stdafx.h"
4. #include <iostream>
6. #include <Windows.h>
10. Interface ITest: public IUnknown
12. {
14. virtual void __stdcall Show () = 0;
16. };
20. The correct function prototype
22. typedef VOID (WINAPI *getitest) (itest**);
26. int _tmain (int argc, _tchar* argv[])
28. {
30. hmodule h = LoadLibrary (TEXT ("Project1.dll"));
32. if (h! = 0)
34. {
36. Getitest get = (getitest) GetProcAddress (H, "gettest");
40. //Modified call method
42. ITest *test = nullptr;
44. Get (&test);
48. Test->show ();
50. Test->release ();
52. }
54. System ("pause");
56. return 0;
58. }

Finally, a little experience can be summed up, when the Delphi function return value is the interface type, the function will consider the first parameter is an interface buffer, used to accept the interface instance object.

Is it possible to return an interface pointer directly without changing the way the C + + side calls? The answer is yes, as long as you change the return data type to the underlying type

[Delphi]View PlainCopy 

1. function Gettest:pointer; stdcall;
2. Var
3. Temp:itest;
4. Begin
5. Temp: = TTest.  Create;
6. Temp. _addref;
7. Result: = Pointer (Temp);
8. End

Since the function return value is no longer an interface type, Delphi does not invoke the AddRef method of the interface to call the reference count + 1, so the AddRef method must be called manually after the interface is created.

Otherwise, the function will automatically release temp after the end, causing the return value to be a wild pointer.

http://blog.csdn.net/aqtata/article/details/19079737

Traps in the interface type when mixing calls