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rubixibuc
rubixibuc

Reputation: 7397

For a lanuage to work with com object does it need to have an api and compiler developed specific for com?

In the documentation for com it says that it works literally with every language. Do you need to have a specific API for that language so it can interface with com, or can any language literally just use it out of box? Also do you need a special compiler? Sorry if this is a stupid question but I have never used it before, and I have been trying to find this answer. When I look at demos of com examples it all seems to access the objects in a c style syntax, are their bindings and apis for other languages (literally all)?

Upvotes: 0

Views: 68

Answers (1)

BrendanMcK
BrendanMcK

Reputation: 14498

The key thing about COM is that it is a "binary standard": which is to say that it doesn't care what the language used is, so long as the bits and bytes in memory end up in the right place.

COM basically specifies that all COM objects must have a specific layout in memory: the interface pointer points to a pointer that in turn points to a table of function pointers, which has at least three members, the first three of which are pointers to the IUnknown functions (AddRef, Release, QueryInterface), and the remainder are pointers to the other functions in the interface. COM also specifies how arguments are passed to these functions - so that the caller and callee agree on how the stack is used, and who pops off the values.

This requirement happily matches how C++ just happens to work on Windows; so most C++ classes that implement IUnknown will just happen to ends up as being valid COM classes: this is because Microsoft's implementation of C++ happens to use an object layout that matches what COM requires: the C++ object vtable pointer is the same as the COM pointer-to-table-of-function-pointers, the C++ table of function pointers is exactly what COM requires for its table-of-function-pointers, and so on. (This isn't entirely just a happy coincidence: COM was likely designed to take advantage of the most common way that C++ objects are implemented in memory which is the technique that MS's compiler uses. Note that C++ the language specification doesn't actually specify any particular object layout - so you could have a 3rd party C++ compiler that implemented C++ in a way that gave you classes that are not usable by COM. But no compiler vendor in their right mind would do that, since they would appear to be broken compared to the others!)

In plain C, you can create a COM object by creating suitable structs-containing-pointers manually. This works because C essentially allows you to specify binary-level memory layout for structs manually; you can create structs that you know will have the appropriate layout that COM is expecting.

In other languages, especially those that don't allow the user to specify memory layout explicitly, you need support from the language to allow for COM support. All the .Net languages - C#, VB.Net, and so on - use support in the .Net runtime that understands what COM expects, and produces the appropriate wrappers as needed to allow the interop to work.

So, long story short, it's not the case that any language under the sun will automatically work with COM; it's really the case that a couple of languages - namely C and C++ - are already aligned with COM's requirements; and most other languages will need some compiler support to make it work.

Upvotes: 1

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