Question

I have a fairly large C++ application (on Windows, no other platforms planned), which currently stores all settings (even some kind of addresses) in the Windows registry.

Sometimes this is inconvenient, because the users have difficulties changing entries in the registry. I would like to have settings versioned, so settings always match the current code. At the moment we version reg-files, but you are never sure, if all reg-files have been added on the target machines. With C# you can define default values in app.config, but don’t overwrite existing settings. I don’t know, if such a mechanism or library exists for C++.

I would like to have the following “features”:

• Settings can be versioned
• Simple update on target machines (can be done by user)
• Ensure that on update only new settings are added and no existing settings are overwritten with default values
• Simple change of settings for user
• Same workflow under Win XP and Win 7

As far as I see it, there are 3 possibilities to store settings on Windows:

• Registry
• Ini file
• XML file

Only one application of our suite uses Qt at the moment, but Boost would be available.

For addresses, we will put them in some kind of XML address book, but for the other settings we are not sure, what’s the best practise.

Answer 1

As comments mention, tree-based key/value structures are a common solution and libraries are easy to find.

Boost’s property_tree is an excellent choice, as it is well-tested and can easily be exported as XML or JSON

Regarding your requirements:

• Settings can be versioned

Yes! Make “version” a top-level key. Make it easily comparable with other versions.

You can also categorize your settings into various tree nodes and give each node a version.

• Simple update on target machines (can be done by user)

Have your application do that when it runs. See below.

• Ensure that on update only new settings are added and no existing settings are overwritten with default values
• Simple change of settings for user
• Same workflow under Win XP and Win 7

As settings change from one version to another, usually these changes fall into three categories. New properties are needed, old settings are abandoned, and some settings change their expected format. E.g. “32 Fahrenheit” becomes “0 Celsius”

When your application initializes:

• Load the existing configuration file, regardless of its version.
• If the version does not match what’s current for the application:
  • Create a new blank property tree for the new configuration
  • For each node in the tree, have a set of expected property names, and a function pointer or similar to get this setting if it’s absent in the old file’s tree. If a setting changes its format, give it a new name.
  • Search for each setting in the old tree, copying it if it’s found, and using the result of the supplied function if it’s not.
  • Save your new settings file.

Your “missing setting” functions can:

• Return a constant default value.
• Query the tree for a different setting and convert it (with a default value if the old setting isn’t found either)
• Ask the user