Parse trees

Parse Tree in ATL

We use Parse Tree in ATL to create a register script. The registry script in ATL is responsible for providing data-driven access to the system's registry.  The data-driven access is more efficient than API driven because it takes only one or two lines of code to enter a key in the script.

The ATL Control Wizard automatically generates a registry script in the COM environment. You can check out the script in the .rgs file.

With the help of parser trees, we can add multiple keys and subkeys in the script.
 

A parser tree has the following form:

<root-key>{<registry expression>}+

Where

<root-key> ::=
 HKEY_CLASSES_ROOT | HKEY_CURRENT_USER |
 HKEY_LOCAL_MACHINE | HKEY_USERS 

All these names mentioned in the root key expressions are names of the key of the user with their subkeys.
 

<registry-expression> ::=
  <Add-Key> | <Delete-Key>

<Add-Key> ::=
  [ForceRemove | NoRemove | val] <Key-Name> [<Key-Value>] [ { <Add-Key> } ]

<Delete-Key> ::=
  Delete <Key-Name>

<Key-Name> ::=
  '<AlphaNumeric>+'

<AlphaNumeric> ::=
  any non-null character.

<Key-Value> ::=
  <Key-Type> <Key-Name>

<Key-Type> ::=
  s | d

You can add multiple keys in the parse tree, and the important thing is registrar will opens the handle of a subkey until the parser is completed, which is more efficient than parsing one key at a time.

Example

HKEY_CLASSES_ROOT
{
    'MyKey'
    {
        'SubKey'
        {
            'newkey'
        }
    }
}

The registrar will first open the HKEY_CLASSES_ROOT/Mykey. It observes there is a subkey in MyKey, and then the registrar will not close the MyKey instead, it will keep the handle open and creates SubKey with Parent Handle.

Check out this problem - Duplicate Subtree In Binary Tree

Frequently Asked Questions

What is the other name of the parse tree?

A parse tree is also known as a derivate tree.

A parse tree follows which Grammar?

A parse tree follows context-free grammar.

What does ATL stand for?

ATL stands for the active template library.

Why do we need ATL?

We need ATL to provide efficient and easy-to-implement COM objects.

Conclusion

In this blog, we learned about the parse tree and how to use the parse tree in ATL to create a registrar script in ATL.

To learn more about ATL, check out the following articles.

• How does ATL implement contained windows?
• How to implement ATL objects?
• Introduction to ATL Window Classes
• ATL Control Containment

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