FAQ 35.10 Isn t OMG IDL pretty much the same as C?

FAQ 35.10 Isn't OMG IDL pretty much the same as C++?

No.

IDL does have a C++-like syntax, and it includes these familiar features:

1. Basic data types such as short, long, float, char and others

2. Modules, which are roughly equivalent to namespaces

3. Exceptions (but see FAQ 35.13)

4. Strings, arrays, enumerations, typedefs, and so on

Despite these surface similarities, OMG IDL is not a programming language and it should not be compared as such with C++. OMG IDL is a platform-neutral specification language. OMG IDL can handle only interface issues parameter types, return types, member function names. It cannot handle implementation issues such as member data, local variables, implementation code, and so on.

Furthermore, it is important to note that CORBA data types do not necessarily correspond to the similarly named C++ data types. For example, a long in IDL may or may not be equivalent to the C++ type long. A long in IDL corresponds to the C++ type CORBA::Long. For these reasons, C++ programmers who are implementing CORBA classes should be careful to use the CORBA data types rather than the C++ data types when necessary.

As another paradigm shift, C++ programmers who are new to CORBA tend to think of the CORBA data types (such as CORBA::Long) as aberrations that should be avoided. In reality the opposite is true: in a sense the CORBA data types are more portable than the C++ data types. For example, sizeof(CORBA::Long) is guaranteed to be 4 bytes on all hardware platforms, whereas sizeof(long) might not be exactly 4 bytes.

In addition, CORBA is responsible for transmitting data values between various machines as necessary and dealing with any data transformation issues. For example, different machines use different byte orderings for different data types (little-endian vs. big-endian), and different programming languages may represent data types differently than C/C++ does. CORBA manages all this transparently if the communicating objects are on different machines it uses a process called marshaling to convert the source machine's binary data to the IIOP (the standard usually allows others) representation, and a reverse process called demarshaling to convert the IIOP representation at the destination machine back into the appropriate binary data format. One performance trick used by some ORBs is recognizing situations where these processes can be safely eliminated (such as when everything is on the same hardware and the networking can be avoided). Other performance tricks, such as receiver-makes-right semantics, avoid unnecessary message translations between machines using the same byte order and are part of the specification.

Finally, C++ programmers need to be aware that IDL does not support overloading, overriding, or pointers. So even though IDL looks a lot like C++, there are significant differences.