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135 lines
5.9 KiB
135 lines
5.9 KiB
======================
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Matching the Clang AST
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======================
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This document explains how to use Clang's LibASTMatchers to match interesting
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nodes of the AST and execute code that uses the matched nodes. Combined with
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:doc:`LibTooling`, LibASTMatchers helps to write code-to-code transformation
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tools or query tools.
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We assume basic knowledge about the Clang AST. See the :doc:`Introduction
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to the Clang AST <IntroductionToTheClangAST>` if you want to learn more
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about how the AST is structured.
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.. FIXME: create tutorial and link to the tutorial
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Introduction
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------------
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LibASTMatchers provides a domain specific language to create predicates on
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Clang's AST. This DSL is written in and can be used from C++, allowing users
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to write a single program to both match AST nodes and access the node's C++
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interface to extract attributes, source locations, or any other information
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provided on the AST level.
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AST matchers are predicates on nodes in the AST. Matchers are created by
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calling creator functions that allow building up a tree of matchers, where
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inner matchers are used to make the match more specific.
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For example, to create a matcher that matches all class or union declarations
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in the AST of a translation unit, you can call `recordDecl()
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<LibASTMatchersReference.html#recordDecl0Anchor>`_. To narrow the match down,
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for example to find all class or union declarations with the name "``Foo``",
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insert a `hasName <LibASTMatchersReference.html#hasName0Anchor>`_ matcher: the
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call ``recordDecl(hasName("Foo"))`` returns a matcher that matches classes or
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unions that are named "``Foo``", in any namespace. By default, matchers that
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accept multiple inner matchers use an implicit `allOf()
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<LibASTMatchersReference.html#allOf0Anchor>`_. This allows further narrowing
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down the match, for example to match all classes that are derived from
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"``Bar``": ``recordDecl(hasName("Foo"), isDerivedFrom("Bar"))``.
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How to create a matcher
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-----------------------
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With more than a thousand classes in the Clang AST, one can quickly get lost
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when trying to figure out how to create a matcher for a specific pattern. This
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section will teach you how to use a rigorous step-by-step pattern to build the
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matcher you are interested in. Note that there will always be matchers missing
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for some part of the AST. See the section about :ref:`how to write your own
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AST matchers <astmatchers-writing>` later in this document.
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.. FIXME: why is it linking back to the same section?!
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The precondition to using the matchers is to understand how the AST for what you
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want to match looks like. The
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:doc:`Introduction to the Clang AST <IntroductionToTheClangAST>` teaches you
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how to dump a translation unit's AST into a human readable format.
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.. FIXME: Introduce link to ASTMatchersTutorial.html
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.. FIXME: Introduce link to ASTMatchersCookbook.html
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In general, the strategy to create the right matchers is:
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#. Find the outermost class in Clang's AST you want to match.
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#. Look at the `AST Matcher Reference <LibASTMatchersReference.html>`_ for
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matchers that either match the node you're interested in or narrow down
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attributes on the node.
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#. Create your outer match expression. Verify that it works as expected.
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#. Examine the matchers for what the next inner node you want to match is.
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#. Repeat until the matcher is finished.
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.. _astmatchers-bind:
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Binding nodes in match expressions
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----------------------------------
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Matcher expressions allow you to specify which parts of the AST are interesting
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for a certain task. Often you will want to then do something with the nodes
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that were matched, like building source code transformations.
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To that end, matchers that match specific AST nodes (so called node matchers)
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are bindable; for example, ``recordDecl(hasName("MyClass")).bind("id")`` will
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bind the matched ``recordDecl`` node to the string "``id``", to be later
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retrieved in the `match callback
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<https://clang.llvm.org/doxygen/classclang_1_1ast__matchers_1_1MatchFinder_1_1MatchCallback.html>`_.
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.. FIXME: Introduce link to ASTMatchersTutorial.html
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.. FIXME: Introduce link to ASTMatchersCookbook.html
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Writing your own matchers
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-------------------------
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There are multiple different ways to define a matcher, depending on its type
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and flexibility.
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``VariadicDynCastAllOfMatcher<Base, Derived>``
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Those match all nodes of type *Base* if they can be dynamically casted to
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*Derived*. The names of those matchers are nouns, which closely resemble
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*Derived*. ``VariadicDynCastAllOfMatchers`` are the backbone of the matcher
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hierarchy. Most often, your match expression will start with one of them, and
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you can :ref:`bind <astmatchers-bind>` the node they represent to ids for later
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processing.
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``VariadicDynCastAllOfMatchers`` are callable classes that model variadic
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template functions in C++03. They take an arbitrary number of
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``Matcher<Derived>`` and return a ``Matcher<Base>``.
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``AST_MATCHER_P(Type, Name, ParamType, Param)``
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Most matcher definitions use the matcher creation macros. Those define both
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the matcher of type ``Matcher<Type>`` itself, and a matcher-creation function
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named *Name* that takes a parameter of type *ParamType* and returns the
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corresponding matcher.
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There are multiple matcher definition macros that deal with polymorphic return
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values and different parameter counts. See `ASTMatchersMacros.h
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<https://clang.llvm.org/doxygen/ASTMatchersMacros_8h.html>`_.
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.. _astmatchers-writing:
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Matcher creation functions
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^^^^^^^^^^^^^^^^^^^^^^^^^^
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Matchers are generated by nesting calls to matcher creation functions. Most of
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the time those functions are either created by using
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``VariadicDynCastAllOfMatcher`` or the matcher creation macros (see below).
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The free-standing functions are an indication that this matcher is just a
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combination of other matchers, as is for example the case with `callee
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<LibASTMatchersReference.html#callee1Anchor>`_.
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.. FIXME: "... macros (see below)" --- there isn't anything below
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