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263 lines
8.8 KiB
ReStructuredText
263 lines
8.8 KiB
ReStructuredText
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.. _qpdf-job:
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QPDFJob: a Job-Based Interface
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==============================
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All of the functionality from the :command:`qpdf` command-line
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executable is available from inside the C++ library using the
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``QPDFJob`` class. There are several ways to access this functionality:
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- Command-line options
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- Run the :command:`qpdf` command line
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- Use from the C++ API with ``QPDFJob::initializeFromArgv``
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- Use from the C API with ``qpdfjob_run_from_argv`` from
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:file:`qpdfjob-c.h`. If you are calling from a Windows-style main
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and have an argv array of ``wchar_t``, you can use
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``qpdfjob_run_from_wide_argv``.
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- The job JSON file format
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- Use from the CLI with the :qpdf:ref:`--job-json-file` parameter
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- Use from the C++ API with ``QPDFJob::initializeFromJson``
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- Use from the C API with ``qpdfjob_run_from_json`` from :file:`qpdfjob-c.h`
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- Note: this is unrelated to :qpdf:ref:`--json` but can be combined
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with it. For more information on qpdf JSON (vs. QPDFJob JSON), see
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:ref:`json`.
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- The ``QPDFJob`` C++ API
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If you can understand how to use the :command:`qpdf` CLI, you can
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understand the ``QPDFJob`` class and the JSON file. qpdf guarantees
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that all of the above methods are in sync. Here's how it works:
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.. list-table:: QPDFJob Interfaces
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:widths: 30 30 30
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:header-rows: 1
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- - CLI
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- JSON
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- C++
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- - ``--some-option``
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- ``"someOption": ""``
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- ``config()->someOption()``
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- - ``--some-option=value``
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- ``"someOption": "value"``
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- ``config()->someOption("value")``
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- - positional argument
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- ``"otherOption": "value"``
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- ``config()->otherOption("value")``
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In the JSON file, the JSON structure is an object (dictionary) whose
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keys are command-line flags converted to camelCase. Positional
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arguments have some corresponding key, which you can find by running
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``qpdf`` with the :qpdf:ref:`--job-json-help` flag. For example, input
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and output files are named by positional arguments on the CLI. In the
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JSON, they appear in the ``"inputFile"`` and ``"outputFile"`` keys.
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The following are equivalent:
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.. It would be nice to have an automated test that these are all the
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same, but we have so few live examples that it's not worth it for
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now.
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CLI:
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::
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qpdf infile.pdf outfile.pdf \
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--pages . other.pdf --password=x 1-5 -- \
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--encrypt user owner 256 --print=low -- \
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--object-streams=generate
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Job JSON:
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.. code-block:: json
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{
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"inputFile": "infile.pdf",
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"outputFile": "outfile.pdf",
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"pages": [
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{
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"file": "."
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},
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{
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"file": "other.pdf",
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"password": "x",
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"range": "1-5"
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}
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],
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"encrypt": {
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"userPassword": "user",
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"ownerPassword": "owner",
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"256bit": {
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"print": "low"
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}
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},
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"objectStreams": "generate"
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}
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C++ code:
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.. code-block:: c++
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#include <qpdf/QPDFJob.hh>
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#include <qpdf/QPDFUsage.hh>
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#include <iostream>
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int main(int argc, char* argv[])
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{
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try
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{
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QPDFJob j;
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j.config()
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->inputFile("infile.pdf")
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->outputFile("outfile.pdf")
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->pages()
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->pageSpec(".", "1-z")
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->pageSpec("other.pdf", "1-5", "x")
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->endPages()
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->encrypt(256, "user", "owner")
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->print("low")
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->endEncrypt()
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->objectStreams("generate")
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->checkConfiguration();
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j.run();
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}
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catch (QPDFUsage& e)
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{
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std::cerr << "configuration error: " << e.what() << std::endl;
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return 2;
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}
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catch (std::exception& e)
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{
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std::cerr << "other error: " << e.what() << std::endl;
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return 2;
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}
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return 0;
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}
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Note the ``QPDFUsage`` exception above. This is thrown whenever a
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configuration error occurs. These exactly correspond to usage messages
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issued by the :command:`qpdf` CLI for things like omitting an output
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file, specifying `--pages` multiple times, or other invalid
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combinations of options. ``QPDFUsage`` is thrown by the argv and JSON
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interfaces as well as the native ``QPDFJob`` interface.
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It is also possible to mix and match command-line options and JSON
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from the CLI. For example, you could create a file called
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:file:`my-options.json` containing the following:
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.. code-block:: json
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{
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"encrypt": {
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"userPassword": "",
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"ownerPassword": "owner",
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"256bit": {
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}
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},
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"objectStreams": "generate"
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}
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and use it with other options to create 256-bit encrypted (but
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unrestricted) files with object streams while specifying other
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parameters on the command line, such as
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::
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qpdf infile.pdf outfile.pdf --job-json-file=my-options.json
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.. _qpdfjob-design:
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See also :file:`examples/qpdf-job.cc` in the source distribution as
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well as comments in ``QPDFJob.hh``.
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QPDFJob Design
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--------------
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This section describes some of the design rationale and history behind
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``QPDFJob``.
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Documentation of ``QPDFJob`` is divided among three places:
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- "HOW TO ADD A COMMAND-LINE ARGUMENT" in :file:`README-maintainer`
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provides a quick reminder of how to add a command-line argument.
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- The source file :file:`generate_auto_job` has a detailed explanation
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about how ``QPDFJob`` and ``generate_auto_job`` work together.
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- This chapter of the manual has other details.
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Prior to qpdf version 10.6.0, the qpdf CLI executable had a lot of
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functionality built into it that was not callable from the library as
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such. This created a number of problems:
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- Some of the logic in :file:`qpdf.cc` was pretty complex, such as
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image optimization, generating JSON output, and many of the page
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manipulations. While those things could all be coded using the C++
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API, there would be a lot of duplicated code.
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- Page splitting and merging will get more complicated over time as
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qpdf supports a wider range of document-level options. It would be
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nice to be able to expose this to library users instead of baking it
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all into the CLI.
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- Users of other languages who just wanted an interface to do things
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that the CLI could do didn't have a good way to do it, such as just
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handing a library call a set of command-line options or an
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equivalent JSON object that could be passed in as a string.
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- The qpdf CLI itself was almost 8,000 lines of code. It needed to be
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refactored, cleaned up, and split.
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- Exposing a new feature via the command-line required making lots of
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small edits to lots of small bits of code, and it was easy to forget
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something. Adding a code generator, while complex in some ways,
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greatly reduces the chances of error when extending qpdf.
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Here are a few notes on some design decisions about QPDFJob and its
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various interfaces.
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- Bare command-line options (flags with no parameter) map to config
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functions that take no options and to JSON keys whose values are
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required to be the empty string. The rationale is that we can later
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change these bare options to options that take an optional parameter
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without breaking backward compatibility in the CLI or the JSON.
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Options that take optional parameters generate two config functions:
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one has no arguments, and one that has a ``char const*`` argument.
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This means that adding an optional parameter to a previously bare
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option also doesn't break binary compatibility.
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- Adding a new argument to :file:`job.yml` automatically triggers
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almost everything by declaring and referencing things that you have
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to implement. This way, once you get the code to compile and link,
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you know you haven't forgotten anything. There are two tricky cases:
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- If an argument handler has to do something special, like call a
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nested config method or select an option table, you have to
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implement it manually. This is discussed in
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:file:`generate_auto_job`.
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- When you add an option that has optional parameters or choices,
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both of the handlers described above are declared, but only the
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one that takes an argument is referenced. You have to remember to
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implement the one that doesn't take an argument or else people
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will get a linker error if they try to call it. The assumption is
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that things with optional parameters started out as bare, so the
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argument-less version is already there.
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- If you have to add a new option that requires its own option table,
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you will have to do some extra work including adding a new nested
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Config class, adding a config member variable to ``ArgParser`` in
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:file:`QPDFJob_argv.cc` and ``Handlers`` in :file:`QPDFJob_json.cc`,
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and make sure that manually implemented handlers are consistent with
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each other. It is best to add explicit test cases for all the
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various ways to get to the option.
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