140 lines
12 KiB
HTML
140 lines
12 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
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<link href="style.css" rel="stylesheet" type="text/css" />
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<title>LLDB Tutorial</title>
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<body>
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<div class="www_title">
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The <strong>LLDB</strong> Coding Conventions
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</div>
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<div id="container">
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<div id="content">
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<h1 class ="postheader">LLDB Coding Conventions</h1>
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<div class="postcontent">
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<p>The LLDB coding conventions differ in a few important respects from LLVM.</p>
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<p>
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Note that <a href="http://clang.llvm.org/docs/ClangFormat.html">clang-format</a> will deal with
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most of this for you, as such is suggested to run on patches before uploading. Note however that
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clang-format is not smart enough to detect instances of humans intentionally trying to line variables
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up on a particular column boundary, and it will reformat them to remove this "extraneous" whitespace.
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While this is usually the correct behavior, LLDB does have many uses of manually aligned types and
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fields, so please be aware of this behavior of clang-format when editing this type of code.
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</p>
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<p>
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<b>Important</b>: Where not explicitly outlined below, assume that the
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<a href="http://llvm.org/docs/CodingStandards.html">LLVM Coding Conventions</a> are to be followed.
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</p>
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<h3>Source code width:</h3>
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<p>lldb does not follow the 80 character line restriction llvm imposes. In our
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experience, trying to fit C++ code into an 80 character line results in code that
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is awkward to read, and the time spent trying to find good indentation points to
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avoid this would be much better spent on thinking about your code.
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<p>More importantly, the restriction induces coders to choose overly abbreviated names
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to make them better fit in 80 characters. In our opinion choosing good descriptive
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names is much more important than fitting in 80 characters.
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<p>In lldb the limit for code lines is 120 characters because it gets awkward to scan
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longer lines even on a fairly big monitor, and we've found at that length you seldom
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have to make code look ugly to get it to wrap.
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<p>However you will see some instances of longer lines. The most common occurrence is in
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the options tables for the CommandInterpreter, which contain the help strings as well as
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a bunch of important but hard to remember fields. These tables are much easier to read if
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all the fields line up vertically, and don't have help text interleaved in between the lines.
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This is another thing to keep in mind when running clang-format, as it will always wrap at
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120, so you will need to tweak its output when running against intentionally too-long lines.
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<h3>Indentation:</h3>
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<p>lldb uses 4 character indentation. We find this makes the code structure much easier to
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see when scanning code, and since we aren't trying to fit code into 80 characters, the
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benefit of not wasting 2 out of the 80 precious spaces per indentation level is moot.
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<p>We also use the Allman brace style rather than putting the initial brace at the end
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of the braced line. This makes the block structure of the code much easier to see on
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an initial scan, and most folks have big enough monitors nowadays that saving a few
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vertical lines isn't sufficiently important to outweigh this benefit.
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<p>Though the llvm coding conventions don't specify this, llvm/clang tend to declare and
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define methods by putting the return type and the method name on the same line. lldb
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puts the qualifiers and return type on a line by themselves and then the method name on
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the next line, i.e.:
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<code><pre><tt>
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virtual int
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MethodName ();
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</code></pre></tt>
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<p>When you are scanning a header file, that makes the method names stand out more easily,
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though at the cost of an extra line. When you have a editor that scrolls smoothly, it's
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easy to move through pages so the extra line is less important than the ease of picking
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out the method names, which is what you generally are scanning for.
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<h3> Names:</h3>
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<p>lldb's naming conventions are different and slightly more restrictive than the llvm
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ones. The goal is to make it easy to tell from immediate context the lifespan
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and what kind of entity a given name represents, which makes reading code you are not familiar
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with much easier. lldb uses the following conventions:
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<ul>
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<li> Macro definitions when needed are in all caps, nothing else should be in all caps. </li>
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<li>Types and classes are in CamelCase with an initial capital.</li>
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<li>Methods are also in CamelCase with an initial capital. The initial capital for methods
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has the handy benefit that it gets our method names into a different namespace
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than the standard C/C++ library functions, which tend to all be lower-cased.
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There are also places in lldb where we wrap clang objects in classes appropriate to lldb,
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and the difference from the llvm convention here actually makes it easier to tell
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whether you are using the clang object directly or are going through the lldb wrapper.</li>
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<li> All variables are written in lower case, with "_" as the word separator. We find that
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using a different capitalization and word separation convention makes variables and methods/types
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immediately visually distinct, resulting in code which is much easier to read.</li>
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<li> class ivars all start with "m_". It is important to be able to tell ivars from local
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variables, and this makes the distinction easily apparent. Some other coding conventions
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use an initial "_", but this seems much harder to spot. Also it allows:</li>
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<li> Class statics and other global variables start with "g_". You should be suspicious of all
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global variables, so having them stand out lexically is a good thing.</li>
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<li>We also use the suffixes "_sp" and "_up" for shared and unique pointer variables. Since
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these have very different lifecycle behaviors it is worthwhile to call them out
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specially. You will see some "_ap" suffixes around. There should be no auto_ptr variables
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left in lldb, but when we converted to unique_ptr's not all the names were changed.
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Feel free to change these to "_up" when you touch them for some other reason.</li>
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<li> enumerations that might end up being in the lldb SB API's should all be written like:
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<pre><code><tt>
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typedef enum EnumName
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{
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eEnumNameFirstValue,
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eEnumNameSecondValue,
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} EnumName;
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</pre></code></tt>
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<p>This redundancy is important because the enumerations that find their way through SWIG into
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Python will show up as lldb.eEnumNameFirstValue, so including the enum name
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in the value name disambiguates them in Python.
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<p>Since we've started allowing C++11 in lldb, we have started using "enum class" instead of straight
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enums. That is fine for enums that will only ever exist on the lldb_private side of lldb, but err on
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the side of caution here on't do that for any enums that might find their way into the SB API's, since then
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you will have to change them so we can get them through SWIG.</li>
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<p> Also, on a more general note, except when you are using a temporary whose lifespan is not
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far past its definition, never use one or two character names for ivars. Always use something
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descriptive, and as far as possible use the same name for the same kind of thing (or the name
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with an appropriate prefix.) That way if I'm looking at one use of a type, I can search on the
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variable name and see most of the other uses of the same type of thing. That makes it much easier
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to get quickly up to speed on how that type should be used.
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</li>
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</div>
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</body>
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</html>
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