Clarified some of my comments thanks to feedback and suggestions from Razvan Surdulescu. For actual COM compatibility, I added __stdcall to the method declarations as well. (Apparently, if you follow the guidelines of this article, it's easy to add bindings from your DLL to Delphi and VB!)
Somebody (unfortunately, I lost his e-mail address and name before I could write it down. if you're reading this or you know who sent it, please drop me a line) sent me another guideline. The gist of it is that you shouldn't use overloaded methods in your interfaces. Different compilers will order them in the vtable differently.
Ben Scott (bscott at iastate dot edu) submitted some excellent classes
that simplify usage of the concepts presented in this article. Simply
derive your interface classes from DLLInterface and your
implementations from DLLImpl
This article explains how to create C++ DLL APIs that will work across several compilers and configuration settings (Release, Debug, etc.).
Many platforms have an ABI for their preferred programming language. For example, BeOS's primary language is C++, so the C++ compiler must be able to generate code that remains binary compatible with the operating system's C++ system calls (and classes, etc.).
The Windows API and ABI were defined for C, so C++ compiler writers had free reign to implement the C++ ABI however they felt. Eventually, however, Microsoft created an object-oriented ABI for Windows called COM. To simplify COM usage, they made the vtables of their C++ ABI match the vtables required in COM interface. Since a Windows compiler that can't use COM is pretty limited, other compiler vendors enforced the mapping between COM vtables and C++ vtables.
There are several aspects to an ABI. This article only discusses the issues with using C++ in Windows. Other platforms have different requirements. (Fortunately, since most other platforms aren't as popular as Windows, they have only one or two compilers, and thus there isn't much of a problem.)
Let's say you want to create a portable windowing API and you want to stick the implementation in a DLL. I'm going to create a class called Window which can represent a window in several different windowing systems: Win32, MFC, wxWindows, Qt, Gtk, Aqua, X11, Swing (*gasp*), etc... We'll walk through several attempts at creating an interface until it works across different implementations, compilers, and compiler settings.
// Window.h
#include <string>
#ifdef WIN32
#ifdef EXPORTING
#define DLLIMPORT __declspec(dllexport)
#else
#define DLLIMPORT __declspec(dllimport)
#endif
#define CALL __stdcall
#else
#define DLLIMPORT
#define CALL
#endif
class DLLIMPORT Window {
public:
Window(std::string title);
~Window();
void setTitle(std::string title);
std::string getTitle();
// ...
private:
HWND m_window;
};
I'm not going to show the implementation, as I'm assuming you already know how to do that. There is one glaring problem with this interface: It assumes you're using the basic Win32 API. That is, it holds an HWND as a private member, which introduces a dependency between our Window class and the Win32 SDK. One possible solution is to use the pImpl idiom to remove the class's private members from the class definition. You can read more about that elsewhere [1], [2], [3], and [4]. Also, you cannot add new members to the class without breaking binary compatibility, as the size of the class changes.
Perhaps the most important problem with this approach is that the methods are non-virtual. Thus, they are implemented as specially named functions that take the 'this' pointer as their first argument. Unfortunately, I don't know of any two compilers that mangle method names in the same way. So don't think your DLL work with an executable compiled with another compiler!
For those of you experienced in object oriented programming, you know that every class can be broken into two concepts: an interface and a factory. A factory is a mechanism for creating objects, and an interface allows you to communicate with them. The next version of Window.h will separate these concepts. Notice that you no longer need to export the class (you have to export the factory function though!), as it is abstract: all method calls go through the object's vtable, not through a direct linking to the DLL. Only the call to the factory function calls directly into the DLL.
// Window.h
#include <string>
class Window {
public:
virtual ~Window() { }
virtual void setTitle(std::string title) = 0;
virtual std::string getTitle() = 0;
};
Window* DLLIMPORT CreateWindow(std::string title);
This is much better. The code that uses window objects doesn't care
what actual type the window object is, just that it implements the
Window interface. However, there is still a problem: Different
compilers mangle symbol names differently, so the
CreateWindow function in DLLs generated by different
compilers will have a different names. This means that if you compile
the windowing DLL with Visual C++ 6, you won't be able to use it in
Borland C++, and vice versa. Fortunately, the C++ standard lets us
disable symbol mangling on specified names, via extern
"C".
Some of you may have noticed another problem with this code. Different compilers implement the standard C++ library differently. In the less obvious case, some people replace their compiler's implementation of the library with another (such as STLPort). Since you can't depend on STL objects being binary compatible across compilers, you cannot safely use them in your DLL interfaces.
If a C++ ABI is ever created for Windows, it will need to specify exactly how to interface with every class in the standard library, but I don't see this happening anytime soon.
The final problem here is a minor one. By convention, COM methods and DLL functions use the __stdcall calling convention. We can fix this with the CALL macro I defined above. (You'll want to rename it in your project.)
// Window.h
class Window {
public:
virtual ~Window() { }
virtual void CALL setTitle(const char* title) = 0;
virtual const char* CALL getTitle() = 0;
};
extern "C" Window* CALL CreateWindow(const char* title);
We're almost there! This particular interface will probably work in a lot of
situations. However, the virtual destructor makes things a little
interesting... Since COM doesn't use virtual destructors, you can't depend
on different compilers to use them identically. However, you can replace the
virtual destructor with a virtual method which, in the implementation class,
is implemented by delete this; This way, both construction
and destruction are implemented on the same side of the DLL boundary. For
example, if you try to use a VC++ 6 debug DLL alongside a release executable,
you'll either crash or run into warnings like "Value of ESP not saved
across function call". This error occurs because the debug version of
the VC++ runtime library has a different allocator than the release
version. Since the two allocators are not compatible, we cannot
allocate memory on one side of the DLL boundary and delete it on the
other.
"But how is a virtual destructor different from another virtual
method?" Virtual destructors are not responsible for deallocating the
memory used by the object: They are simply called to perform
necessary cleanup before the object is deallocated. The executable
that uses your DLL will try to free the object's memory itself. On
the other hand, the destroy() method is
responsible for deallocating memory, so all new and delete calls stay
on the same side of the DLL boundary.
It's also a good idea to make the interface's destructor protected so that users of the interface can't inadvertently use delete on it.
// Window.h
class Window {
protected:
~Window() { } // use destroy()
public:
virtual void CALL destroy() = 0;
virtual void CALL setTitle(const char* title) = 0;
virtual const char* CALL getTitle() = 0;
};
extern "C" Window* CreateWindow(const char* title);
Since this code doesn't use any semantics not defined by COM, it should work
flawlessly across compilers and configuration settings. Unfortunately, it's
not ideal. You have to remember to delete objects with
object->destroy();, which isn't nearly as intuitive as
delete object;. Perhaps more importantly, you can no
longer use std::auto_ptr on objects of this type.
auto_ptr wants to delete the object it owns with
delete object;. Is there a way to make the syntax
delete object; actually call object->destroy();?
Yes. Here's where things get a little weird... You can overload
operator delete for the interface and have it call destroy().
Since operator delete takes a void pointer, you'll have to assume you
never call Window::operator delete on anything that isn't a Window. This
is a pretty safe assumption. Here's the operator implementation:
...
void operator delete(void* p) {
if (p) {
Window* w = static_cast<Window*>(p);
w->destroy();
}
}
...
Looks pretty good... You can now use auto_ptr again, and you still
have a stable binary interface. When you recompile and test your new
code (you are testing, right??), you'll notice that there is
a stack overflow in WindowImpl::destroy! What's going
on? If you remember how the destroy method is implemented, you'll see
that it simply executes delete this;. Since the interface
overloads operator delete, WindowImpl::destroy
calls Window::operator delete which calls
WindowImpl::destroy... ad infinitum. The solution to
this particular problem is to overload operator delete in the
implementation class to call the global operator delete:
...
void operator delete(void* p) {
::operator delete(p);
}
...
If your system has a lot of interfaces and implementations, you'll find that you'll want some way to automate undefining operator delete. Fortunately, this is possible too. Simply create a templated class called DefaultDelete and instead of deriving your implementation class from interface I, derive from class DefaultDelete<I>. Here's DefaultDelete's definition:
template<typename T>
class DefaultDelete : public T {
public:
void operator delete(void* p) {
::operator delete(p);
}
};
Here is the final version of the code.
// Window.h
class Window {
public:
virtual void CALL destroy() = 0;
virtual void CALL setTitle(const char* title) = 0;
virtual const char* CALL getTitle() = 0;
void operator delete(void* p) {
if (p) {
Window* w = static_cast<Window*>(p);
w->destroy();
}
}
};
extern "C" Window* CALL CreateWindow(const char* title);
// Window.cpp
#include <string>
#include <windows.h>
#include "DefaultDelete.h"
class WindowImpl : public DefaultDelete<Window> {
public:
WindowImpl(HWND window) {
m_window = window;
}
~WindowImpl() {
DestroyWindow(m_window);
}
void CALL destroy() {
delete this;
}
void CALL setTitle(const char* title) {
SetWindowText(m_window, title);
}
const char* CALL getTitle() {
char title[512];
GetWindowText(m_window, title, 512);
m_title = title; // save the title past the call
return m_title.c_str();
}
private:
HWND window;
std::string m_title;
};
Window* CALL CreateWindow(const char* title) {
// create the Win32 window object
HWND window = ::CreateWindow(..., title, ...);
return (window ? new WindowImpl(window) : 0);
}
// DefaultDelete.h
template<typename T>
class DefaultDelete : public T {
public:
void operator delete(void* p) {
::operator delete(p);
}
};
That's about it. In closure, I'll enumerate guidelines to keep in mind when creating C++ interface. You can look back on this as a reference or use it to help solidify your knowledge.
extern "C" to prevent
incompatible name mangling. Also, exported functions and methods
should use the __stdcall calling convention, as DLL
functions and COM
traditionally use that calling convention. This way, if a user of the
library is compiling with __cdecl by default, the calls
into the DLL will still use the correct convention.operator delete that calls destroy().I would really like feedback on this article. Was any part unclear? Do you want more details about a particular situation? Is any of the code wrong? Send e-mail to aegis@aegisknight.org.