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<chapter>
  <header>
    <copyright>
      <year>2002</year><year>2009</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
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    <title>OMG IDL</title>
    <prepared></prepared>
    <docno></docno>
    <date>2002-07-15</date>
    <rev></rev>
    <file>ch_basic_idl.xml</file>
  </header>

  <section>
    <title>OMG IDL - Overview</title>
    <p>The purpose of OMG IDL, <em>Interface Definition Language</em>, mapping
      is to act as translator between platforms and languages. An IDL 
      specification is supposed to describe data types, object types etc.</p>
    <p>Since the <c>C</c> and <c>Java</c> 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., <c>CORBA</c>, 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.</p>

    <section>
      <title>Reserved Compiler Names and Keywords</title>
      <p>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.</p>
      <p>IC reserves all identifiers starting with <c>OE_</c> and <c>oe_</c> 
        for internal use.</p>
      <p>Note also, that an identifier in IDL can  contain alphabetic,
        digits and underscore characters, but the first character 
        <em>must</em> be alphabetic.
        </p>
      <p>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.</p>
      <p>The OMG defines a set of reserved words, shown below, for use as keywords. 
        These may <em>not</em> be used as, for example, identifiers.</p>
      <table>
        <row>
          <cell align="left" valign="middle">abstract</cell>
          <cell align="left" valign="middle">double</cell>
          <cell align="left" valign="middle">local</cell>
          <cell align="left" valign="middle">raises</cell>
          <cell align="left" valign="middle">typedef</cell>
        </row>
        <row>
          <cell align="left" valign="middle">any</cell>
          <cell align="left" valign="middle">exception</cell>
          <cell align="left" valign="middle">long</cell>
          <cell align="left" valign="middle">readonly</cell>
          <cell align="left" valign="middle">unsigned</cell>
        </row>
        <row>
          <cell align="left" valign="middle">attribute</cell>
          <cell align="left" valign="middle">enum</cell>
          <cell align="left" valign="middle">module</cell>
          <cell align="left" valign="middle">sequence</cell>
          <cell align="left" valign="middle">union</cell>
        </row>
        <row>
          <cell align="left" valign="middle">boolean</cell>
          <cell align="left" valign="middle">factory</cell>
          <cell align="left" valign="middle">native</cell>
          <cell align="left" valign="middle">short</cell>
          <cell align="left" valign="middle">ValueBase</cell>
        </row>
        <row>
          <cell align="left" valign="middle">case</cell>
          <cell align="left" valign="middle">FALSE</cell>
          <cell align="left" valign="middle">Object</cell>
          <cell align="left" valign="middle">string</cell>
          <cell align="left" valign="middle">valuetype</cell>
        </row>
        <row>
          <cell align="left" valign="middle">char</cell>
          <cell align="left" valign="middle">fixed</cell>
          <cell align="left" valign="middle">octet</cell>
          <cell align="left" valign="middle">struct</cell>
          <cell align="left" valign="middle">void</cell>
        </row>
        <row>
          <cell align="left" valign="middle">const</cell>
          <cell align="left" valign="middle">float</cell>
          <cell align="left" valign="middle">oneway</cell>
          <cell align="left" valign="middle">supports</cell>
          <cell align="left" valign="middle">wchar</cell>
        </row>
        <row>
          <cell align="left" valign="middle">context</cell>
          <cell align="left" valign="middle">in</cell>
          <cell align="left" valign="middle">out</cell>
          <cell align="left" valign="middle">switch</cell>
          <cell align="left" valign="middle">wstring</cell>
        </row>
        <row>
          <cell align="left" valign="middle">custom</cell>
          <cell align="left" valign="middle">inout</cell>
          <cell align="left" valign="middle">private</cell>
          <cell align="left" valign="middle">TRUE</cell>
          <cell align="left" valign="middle"></cell>
        </row>
        <row>
          <cell align="left" valign="middle">default</cell>
          <cell align="left" valign="middle">interface</cell>
          <cell align="left" valign="middle">public</cell>
          <cell align="left" valign="middle">truncatable</cell>
          <cell align="left" valign="middle"></cell>
        </row>
        <tcaption>OMG IDL keywords</tcaption>
      </table>
      <p>The keywords listed above must be written exactly as shown. Any usage
        of identifiers that collide with a keyword is illegal. For example,
        <em>long</em> is a valid keyword; <em>Long</em> and <em>LONG</em> are
        illegal as keywords and identifiers. But, since the OMG must be able
        to expand the IDL grammar, it is possible to use <em>Escaped  Identifiers</em>. For example, it is not unlikely that <c>native</c>
        have been used in IDL-specifications as identifiers. One option is to
        change all occurrences to <c>myNative</c>. 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 <c>_</c>. The following IDL-code is illegal:</p>
      <code type="none">
typedef string native;
interface i {
   void foo(in native Arg);
   };
};
      </code>
      <p>With Escaped Identifiers the code will look like:</p>
      <code type="none">
typedef string _native;
interface i {
   void foo(in _native Arg);
   };
};
      </code>
    </section>
  </section>
</chapter>

