boost_1_45_0/libs/program_options/doc/changes.xml

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<section id="program_options.changes">
  <title>Changes since formal review</title>

  <para>During formal review, a large number of changes was suggested. To make
  using the new version easier, the implemented changes are described
  below.</para>

  <para>Let's start with an example. The following is a typical code for the
  reviewed version:<programlisting>
      options_description desc;
      desc.add_options()           
          ("magic", parameter&lt;int&gt;("value"), "magic value for the program")
          .default_value("43")

      variables_map vm;
      options_and_arguments oa1 = parse_command_line(ac, av, desc);
      store(oa1, vm, desc)

      variables_map vm2;
      ifstream ifs("main.cfg");
      options_and_arguments oa2 = parse_config_file(ifs, desc);
      store(oa1, vm2, desc);

      vm.next(&amp;vm2);
    </programlisting>The code for the current version would look like:
    <programlisting>
      options_description desc;
      desc.add_options()           
          ("magic", value&lt;int&gt;()->default_value(43), 
                   "magic value for the program")

      variables_map vm;

      store(parse_command_line(ac, av, desc), vm);

      ifstream ifs("main.cfg");
      store(parse_command_line(ifs, desc), vm);

      notify(vm);
    </programlisting>
  </para>
    
  <para>Let's examine all the changes in detail</para>

  <section>
    <title>Option description</title>      

    <itemizedlist>
      <listitem>
        <para>The <code>parameter</code> function was renamed to          
        <code>value</code>. Rationale: "paramater" is yet another term with no
        clear definition, while "value" is already used everywhere in
        docs.</para>
      </listitem>
      <listitem>
        <para>The default value is specified in different place, and should
        use the value of desired type, not string. Previous code was:
          <programlisting>
            ("magic", parameter&lt;int&gt;("value")).default_value("43")
          </programlisting>
          and the new code is
          <programlisting>
            ("magic", parameter&lt;int&gt;("value")->default_value(43));
          </programlisting>          
          Rationale: the new way is less restrictive. At the same time, the
          new design allows to implement other behaviour, like validation of
          the value, which require knowledge of the value type.         
        </para>
      </listitem>
        
      <listitem>
        <para>The number of token value can take on command line, which was
        specified using character suffix appended to value name, is now
        specified using more explicit member calls. Moreover, it's not longer
        possible to specify the "value name".
          For example:
<programlisting>("numbers", parameter&lt;int&gt;("n+"))</programlisting>
          has became
<programlisting>("numbers", value&lt;int&gt;()->multitoken())</programlisting>
          Rationale: such modifiers tend to make command line usage less
        clear. There's no need to make evil things too easy to do.
          The "value name" had only two roles: specifying modifiers, and
        telling what to output in automated help. The first role has became
        obsolete, and the second was questionable too. It was very unclear how
        to decide on the best "value name", and eventually the selection was randon.
        </para>
      </listitem>
    </itemizedlist>

  </section>

  <section>
    <title>Parsers</title>      

    
    <itemizedlist>
      <listitem>
        <para>The <code>options_and_argument</code> class was removed.</para>
      </listitem>
      <listitem>
        <para>The <code>cmdline</code> and <code>config_file</code> classes
        were removed from the public interface. Various command line styles
        are now declared in the <code>command_line_style</code> subnamespace.          
        </para>
      </listitem>

      <listitem>
        <para>New function <code>parse_environment</code> was added.</para>
      </listitem>

      <listitem>
        <para>Support for positional options was added</para>
      </listitem>

    </itemizedlist>
  </section>
    

  <section>
    <title>Storage</title>
     
    <itemizedlist>
      <listitem>
        <para>The <code>notify</code> function should be called after all
        sources are stored in a <code>variales_map</code> instance. This is
        done to property support priority of configuration sources.
        </para>
      </listitem>
    </itemizedlist>    
  </section>



</section>

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