## 2 OMG IDL

### 2.1 OMG IDL - Overview

The purpose of OMG IDL, Interface Definition Language, mapping is to act as translator between platforms and languages. An IDL specification is supposed to describe data types, object types etc.

Since the `C` and `Java` IC backends only supports a subset of the IDL types supported by the other backends, the mapping is divided into different parts. For more information about IDL to Erlang mapping, i.e., `CORBA`, plain Erlang and generic Erlang Server, see the Orber User's Guide. How to use the plain Erlang and generic Erlang Server is found in this User's Guide.

#### 2.1.1 Reserved Compiler Names and Keywords

The use of some names is strongly discouraged due to ambiguities. However, the use of some names is prohibited when using the Erlang mapping , as they are strictly reserved for IC.

IC reserves all identifiers starting with `OE_` and `oe_` for internal use.

Note also, that an identifier in IDL can contain alphabetic, digits and underscore characters, but the first character must be alphabetic.

Using underscores in IDL names can lead to ambiguities due to the name mapping described above. It is advisable to avoid the use of underscores in identifiers.

The OMG defines a set of reserved words, shown below, for use as keywords. These may not be used as, for example, identifiers.

OMG IDL keywords
abstract | double | local | raises | typedef
any | exception | long | readonly | unsigned
attribute | enum | module | sequence | union
boolean | factory | native | short | ValueBase
case | FALSE | Object | string | valuetype
char | fixed | octet | struct | void
const | float | oneway | supports | wchar
context | in | out | switch | wstring
custom | inout | private | TRUE |
default | interface | public | truncatable |

The keywords listed above must be written exactly as shown. Any usage of identifiers that collide with a keyword is illegal. For example, long is a valid keyword; Long and LONG are illegal as keywords and identifiers. But, since the OMG must be able to expand the IDL grammar, it is possible to use Escaped Identifiers. For example, it is not unlikely that `native` have been used in IDL-specifications as identifiers. One option is to change all occurances to `myNative`. Usually, it is necessary to change programming language code that depends upon that IDL as well. Since Escaped Identifiers just disable type checking (i.e. if it is a reserved word or not) and leaves everything else unchanged, it is only necessary to update the IDL-specification. To escape an identifier, simply prefix it with `_`. The following IDL-code is illegal:

    typedef string native;
    interface i {
       void foo(in native Arg);
       };
    };

With Escaped Identifiers the code will look like:

    typedef string _native;
    interface i {
       void foo(in _native Arg);
       };
    };

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