131 lines
6.1 KiB
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131 lines
6.1 KiB
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<a name="Hash-Nodes"></a>
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<div class="header">
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<p>
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Next: <a href="Macro-Expansion.html#Macro-Expansion" accesskey="n" rel="next">Macro Expansion</a>, Previous: <a href="Lexer.html#Lexer" accesskey="p" rel="prev">Lexer</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
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<a name="Hash-Nodes-1"></a>
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<h2 class="unnumbered">Hash Nodes</h2>
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<a name="index-hash-table"></a>
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<a name="index-identifiers"></a>
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<a name="index-macros"></a>
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<a name="index-assertions"></a>
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<a name="index-named-operators"></a>
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<p>When cpplib encounters an “identifier”, it generates a hash code for
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it and stores it in the hash table. By “identifier” we mean tokens
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with type <code>CPP_NAME</code>; this includes identifiers in the usual C
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sense, as well as keywords, directive names, macro names and so on. For
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example, all of <code>pragma</code>, <code>int</code>, <code>foo</code> and
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<code>__GNUC__</code> are identifiers and hashed when lexed.
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</p>
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<p>Each node in the hash table contain various information about the
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identifier it represents. For example, its length and type. At any one
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time, each identifier falls into exactly one of three categories:
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</p>
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<ul>
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<li> Macros
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<p>These have been declared to be macros, either on the command line or
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with <code>#define</code>. A few, such as <code>__TIME__</code> are built-ins
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entered in the hash table during initialization. The hash node for a
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normal macro points to a structure with more information about the
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macro, such as whether it is function-like, how many arguments it takes,
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and its expansion. Built-in macros are flagged as special, and instead
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contain an enum indicating which of the various built-in macros it is.
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</p>
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</li><li> Assertions
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<p>Assertions are in a separate namespace to macros. To enforce this, cpp
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actually prepends a <code>#</code> character before hashing and entering it in
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the hash table. An assertion’s node points to a chain of answers to
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that assertion.
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</p>
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</li><li> Void
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<p>Everything else falls into this category—an identifier that is not
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currently a macro, or a macro that has since been undefined with
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<code>#undef</code>.
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</p>
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<p>When preprocessing C++, this category also includes the named operators,
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such as <code>xor</code>. In expressions these behave like the operators they
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represent, but in contexts where the spelling of a token matters they
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are spelt differently. This spelling distinction is relevant when they
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are operands of the stringizing and pasting macro operators <code>#</code> and
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<code>##</code>. Named operator hash nodes are flagged, both to catch the
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spelling distinction and to prevent them from being defined as macros.
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</p></li></ul>
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<p>The same identifiers share the same hash node. Since each identifier
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token, after lexing, contains a pointer to its hash node, this is used
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to provide rapid lookup of various information. For example, when
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parsing a <code>#define</code> statement, CPP flags each argument’s identifier
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hash node with the index of that argument. This makes duplicated
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argument checking an O(1) operation for each argument. Similarly, for
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each identifier in the macro’s expansion, lookup to see if it is an
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argument, and which argument it is, is also an O(1) operation. Further,
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each directive name, such as <code>endif</code>, has an associated directive
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enum stored in its hash node, so that directive lookup is also O(1).
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Macro-Expansion.html#Macro-Expansion" accesskey="n" rel="next">Macro Expansion</a>, Previous: <a href="Lexer.html#Lexer" accesskey="p" rel="prev">Lexer</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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