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| 11 | .err { color:red; } |
| 12 | .snip { color:darkgreen; } |
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| 22 | |
| 23 | <!--=======================================================================--> |
| 24 | <h1>Expressive Diagnostics</h1> |
| 25 | <!--=======================================================================--> |
| 26 | |
| 27 | <p>In addition to being fast and functional, we aim to make Clang extremely user |
| 28 | friendly. As far as a command-line compiler goes, this basically boils down to |
| 29 | making the diagnostics (error and warning messages) generated by the compiler |
| 30 | be as useful as possible. There are several ways that we do this. This section |
| 31 | talks about the experience provided by the command line compiler, contrasting |
| 32 | Clang output to GCC 4.2's output in several examples. |
| 33 | <!-- |
| 34 | Other clients |
| 35 | that embed Clang and extract equivalent information through internal APIs.--> |
| 36 | </p> |
| 37 | |
| 38 | <h2>Column Numbers and Caret Diagnostics</h2> |
| 39 | |
| 40 | <p>First, all diagnostics produced by clang include full column number |
Douglas Gregor | d89df5a | 2011-06-01 22:45:49 +0000 | [diff] [blame] | 41 | information. The clang command-line compiler driver uses this information |
Benjamin Kramer | 665a8dc | 2012-01-15 15:26:07 +0000 | [diff] [blame] | 42 | to print "point diagnostics". |
Douglas Gregor | d89df5a | 2011-06-01 22:45:49 +0000 | [diff] [blame] | 43 | (IDEs can use the information to display in-line error markup.) |
| 44 | Precise error location in the source is a feature provided by many commercial |
| 45 | compilers, but is generally missing from open source |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 46 | compilers. This is nice because it makes it very easy to understand exactly |
Douglas Gregor | d89df5a | 2011-06-01 22:45:49 +0000 | [diff] [blame] | 47 | what is wrong in a particular piece of code</p> |
| 48 | |
Benjamin Kramer | 665a8dc | 2012-01-15 15:26:07 +0000 | [diff] [blame] | 49 | <p>The point (the blue "^" character) exactly shows where the problem is, even |
Douglas Gregor | d89df5a | 2011-06-01 22:45:49 +0000 | [diff] [blame] | 50 | inside of a string. This makes it really easy to jump to the problem and |
| 51 | helps when multiple instances of the same character occur on a line. (We'll |
| 52 | revisit this more in following examples.)</p> |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 53 | |
| 54 | <pre> |
| 55 | $ <b>gcc-4.2 -fsyntax-only -Wformat format-strings.c</b> |
| 56 | format-strings.c:91: warning: too few arguments for format |
| 57 | $ <b>clang -fsyntax-only format-strings.c</b> |
Benjamin Kramer | 665a8dc | 2012-01-15 15:26:07 +0000 | [diff] [blame] | 58 | format-strings.c:91:13: <span class="warn">warning:</span> '.*' specified field precision is missing a matching 'int' argument |
| 59 | <span class="snip"> printf("%.*d");</span> |
| 60 | <span class="point"> ^</span> |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 61 | </pre> |
| 62 | |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 63 | <h2>Range Highlighting for Related Text</h2> |
| 64 | |
| 65 | <p>Clang captures and accurately tracks range information for expressions, |
| 66 | statements, and other constructs in your program and uses this to make |
Douglas Gregor | d89df5a | 2011-06-01 22:45:49 +0000 | [diff] [blame] | 67 | diagnostics highlight related information. In the following somewhat |
| 68 | nonsensical example you can see that you don't even need to see the original source code to |
| 69 | understand what is wrong based on the Clang error. Because clang prints a |
Benjamin Kramer | 665a8dc | 2012-01-15 15:26:07 +0000 | [diff] [blame] | 70 | point, you know exactly <em>which</em> plus it is complaining about. The range |
Douglas Gregor | d89df5a | 2011-06-01 22:45:49 +0000 | [diff] [blame] | 71 | information highlights the left and right side of the plus which makes it |
| 72 | immediately obvious what the compiler is talking about. |
| 73 | Range information is very useful for |
| 74 | cases involving precedence issues and many other cases.</p> |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 75 | |
| 76 | <pre> |
| 77 | $ <b>gcc-4.2 -fsyntax-only t.c</b> |
| 78 | t.c:7: error: invalid operands to binary + (have 'int' and 'struct A') |
| 79 | $ <b>clang -fsyntax-only t.c</b> |
Benjamin Kramer | 665a8dc | 2012-01-15 15:26:07 +0000 | [diff] [blame] | 80 | t.c:7:39: <span class="err">error:</span> invalid operands to binary expression ('int' and 'struct A') |
| 81 | <span class="snip"> return y + func(y ? ((SomeA.X + 40) + SomeA) / 42 + SomeA.X : SomeA.X);</span> |
| 82 | <span class="point"> ~~~~~~~~~~~~~~ ^ ~~~~~</span> |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 83 | </pre> |
| 84 | |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 85 | <h2>Precision in Wording</h2> |
| 86 | |
| 87 | <p>A detail is that we have tried really hard to make the diagnostics that come |
| 88 | out of clang contain exactly the pertinent information about what is wrong and |
| 89 | why. In the example above, we tell you what the inferred types are for |
| 90 | the left and right hand sides, and we don't repeat what is obvious from the |
Benjamin Kramer | 665a8dc | 2012-01-15 15:26:07 +0000 | [diff] [blame] | 91 | point (e.g., that this is a "binary +").</p> |
Douglas Gregor | d89df5a | 2011-06-01 22:45:49 +0000 | [diff] [blame] | 92 | |
| 93 | <p>Many other examples abound. In the following example, not only do we tell you that there is a problem with the * |
| 94 | and point to it, we say exactly why and tell you what the type is (in case it is |
| 95 | a complicated subexpression, such as a call to an overloaded function). This |
| 96 | sort of attention to detail makes it much easier to understand and fix problems |
| 97 | quickly.</p> |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 98 | |
| 99 | <pre> |
| 100 | $ <b>gcc-4.2 -fsyntax-only t.c</b> |
| 101 | t.c:5: error: invalid type argument of 'unary *' |
| 102 | $ <b>clang -fsyntax-only t.c</b> |
Benjamin Kramer | 665a8dc | 2012-01-15 15:26:07 +0000 | [diff] [blame] | 103 | t.c:5:11: <span class="err">error:</span> indirection requires pointer operand ('int' invalid) |
| 104 | <span class="snip"> int y = *SomeA.X;</span> |
| 105 | <span class="point"> ^~~~~~~~</span> |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 106 | </pre> |
| 107 | |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 108 | <h2>No Pretty Printing of Expressions in Diagnostics</h2> |
| 109 | |
| 110 | <p>Since Clang has range highlighting, it never needs to pretty print your code |
Chris Lattner | b3de112 | 2011-08-03 15:54:40 +0000 | [diff] [blame] | 111 | back out to you. GCC can produce inscrutible error messages in some cases when |
| 112 | it tries to do this. In this example P and Q have type "int*":</p> |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 113 | |
| 114 | <pre> |
| 115 | $ <b>gcc-4.2 -fsyntax-only t.c</b> |
| 116 | #'exact_div_expr' not supported by pp_c_expression#'t.c:12: error: called object is not a function |
| 117 | $ <b>clang -fsyntax-only t.c</b> |
Benjamin Kramer | 665a8dc | 2012-01-15 15:26:07 +0000 | [diff] [blame] | 118 | t.c:12:8: <span class="err">error:</span> called object type 'int' is not a function or function pointer |
| 119 | <span class="snip"> (P-Q)();</span> |
| 120 | <span class="point"> ~~~~~^</span> |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 121 | </pre> |
| 122 | |
Chris Lattner | b3de112 | 2011-08-03 15:54:40 +0000 | [diff] [blame] | 123 | <p>This can be particularly bad in G++, which often emits errors |
| 124 | containing lowered vtable references. For example:</p> |
| 125 | |
| 126 | <pre> |
| 127 | $ <b>cat t.cc</b> |
| 128 | struct a { |
| 129 | virtual int bar(); |
| 130 | }; |
| 131 | |
| 132 | struct foo : public virtual a { |
| 133 | }; |
| 134 | |
| 135 | void test(foo *P) { |
| 136 | return P->bar() + *P; |
| 137 | } |
| 138 | $ <b>gcc-4.2 t.cc</b> |
| 139 | t.cc: In function 'void test(foo*)': |
| 140 | t.cc:9: error: no match for 'operator+' in '(((a*)P) + (*(long int*)(P->foo::<anonymous>.a::_vptr$a + -0x00000000000000020)))->a::bar() + * P' |
| 141 | t.cc:9: error: return-statement with a value, in function returning 'void' |
| 142 | $ <b>clang t.cc</b> |
Benjamin Kramer | 665a8dc | 2012-01-15 15:26:07 +0000 | [diff] [blame] | 143 | t.cc:9:18: <span class="err">error:</span> invalid operands to binary expression ('int' and 'foo') |
| 144 | <span class="snip"> return P->bar() + *P;</span> |
| 145 | <span class="point"> ~~~~~~~~ ^ ~~</span> |
Chris Lattner | b3de112 | 2011-08-03 15:54:40 +0000 | [diff] [blame] | 146 | </pre> |
| 147 | |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 148 | |
| 149 | <h2>Typedef Preservation and Selective Unwrapping</h2> |
| 150 | |
| 151 | <p>Many programmers use high-level user defined types, typedefs, and other |
| 152 | syntactic sugar to refer to types in their program. This is useful because they |
| 153 | can abbreviate otherwise very long types and it is useful to preserve the |
| 154 | typename in diagnostics. However, sometimes very simple typedefs can wrap |
| 155 | trivial types and it is important to strip off the typedef to understand what |
| 156 | is going on. Clang aims to handle both cases well.<p> |
| 157 | |
Douglas Gregor | d89df5a | 2011-06-01 22:45:49 +0000 | [diff] [blame] | 158 | <p>The following example shows where it is important to preserve |
| 159 | a typedef in C. Here the type printed by GCC isn't even valid, but if the error |
| 160 | were about a very long and complicated type (as often happens in C++) the error |
| 161 | message would be ugly just because it was long and hard to read.</p> |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 162 | |
| 163 | <pre> |
| 164 | $ <b>gcc-4.2 -fsyntax-only t.c</b> |
| 165 | t.c:15: error: invalid operands to binary / (have 'float __vector__' and 'const int *') |
| 166 | $ <b>clang -fsyntax-only t.c</b> |
Benjamin Kramer | 665a8dc | 2012-01-15 15:26:07 +0000 | [diff] [blame] | 167 | t.c:15:11: <span class="err">error:</span> can't convert between vector values of different size ('__m128' and 'int const *') |
| 168 | <span class="snip"> myvec[1]/P;</span> |
| 169 | <span class="point"> ~~~~~~~~^~</span> |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 170 | </pre> |
| 171 | |
Douglas Gregor | d89df5a | 2011-06-01 22:45:49 +0000 | [diff] [blame] | 172 | <p>The following example shows where it is useful for the compiler to expose |
| 173 | underlying details of a typedef. If the user was somehow confused about how the |
| 174 | system "pid_t" typedef is defined, Clang helpfully displays it with "aka".</p> |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 175 | |
| 176 | <pre> |
| 177 | $ <b>gcc-4.2 -fsyntax-only t.c</b> |
| 178 | t.c:13: error: request for member 'x' in something not a structure or union |
| 179 | $ <b>clang -fsyntax-only t.c</b> |
Benjamin Kramer | 665a8dc | 2012-01-15 15:26:07 +0000 | [diff] [blame] | 180 | t.c:13:9: <span class="err">error:</span> member reference base type 'pid_t' (aka 'int') is not a structure or union |
| 181 | <span class="snip"> myvar = myvar.x;</span> |
| 182 | <span class="point"> ~~~~~ ^</span> |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 183 | </pre> |
| 184 | |
Douglas Gregor | 969c689 | 2009-04-01 15:47:24 +0000 | [diff] [blame] | 185 | <p>In C++, type preservation includes retaining any qualification written into type names. For example, if we take a small snippet of code such as: |
| 186 | |
| 187 | <blockquote> |
| 188 | <pre> |
| 189 | namespace services { |
| 190 | struct WebService { }; |
| 191 | } |
| 192 | namespace myapp { |
| 193 | namespace servers { |
| 194 | struct Server { }; |
| 195 | } |
| 196 | } |
| 197 | |
| 198 | using namespace myapp; |
Benjamin Kramer | d8ee407 | 2011-08-06 03:04:45 +0000 | [diff] [blame] | 199 | void addHTTPService(servers::Server const &server, ::services::WebService const *http) { |
Douglas Gregor | 969c689 | 2009-04-01 15:47:24 +0000 | [diff] [blame] | 200 | server += http; |
| 201 | } |
| 202 | </pre> |
| 203 | </blockquote> |
| 204 | |
| 205 | <p>and then compile it, we see that Clang is both providing more accurate information and is retaining the types as written by the user (e.g., "servers::Server", "::services::WebService"): |
| 206 | |
| 207 | <pre> |
| 208 | $ <b>g++-4.2 -fsyntax-only t.cpp</b> |
| 209 | t.cpp:9: error: no match for 'operator+=' in 'server += http' |
| 210 | $ <b>clang -fsyntax-only t.cpp</b> |
Benjamin Kramer | 665a8dc | 2012-01-15 15:26:07 +0000 | [diff] [blame] | 211 | t.cpp:9:10: <span class="err">error:</span> invalid operands to binary expression ('servers::Server const' and '::services::WebService const *') |
| 212 | <span class="snip">server += http;</span> |
| 213 | <span class="point">~~~~~~ ^ ~~~~</span> |
Douglas Gregor | 969c689 | 2009-04-01 15:47:24 +0000 | [diff] [blame] | 214 | </pre> |
| 215 | |
| 216 | <p>Naturally, type preservation extends to uses of templates, and Clang retains information about how a particular template specialization (like <code>std::vector<Real></code>) was spelled within the source code. For example:</p> |
| 217 | |
| 218 | <pre> |
| 219 | $ <b>g++-4.2 -fsyntax-only t.cpp</b> |
| 220 | t.cpp:12: error: no match for 'operator=' in 'str = vec' |
| 221 | $ <b>clang -fsyntax-only t.cpp</b> |
Benjamin Kramer | 665a8dc | 2012-01-15 15:26:07 +0000 | [diff] [blame] | 222 | t.cpp:12:7: <span class="err">error:</span> incompatible type assigning 'vector<Real>', expected 'std::string' (aka 'class std::basic_string<char>') |
| 223 | <span class="snip">str = vec</span>; |
| 224 | <span class="point">^ ~~~</span> |
Douglas Gregor | 969c689 | 2009-04-01 15:47:24 +0000 | [diff] [blame] | 225 | </pre> |
| 226 | |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 227 | <h2>Fix-it Hints</h2> |
| 228 | |
Douglas Gregor | eff49c6 | 2009-04-01 16:24:40 +0000 | [diff] [blame] | 229 | <p>"Fix-it" hints provide advice for fixing small, localized problems |
| 230 | in source code. When Clang produces a diagnostic about a particular |
| 231 | problem that it can work around (e.g., non-standard or redundant |
| 232 | syntax, missing keywords, common mistakes, etc.), it may also provide |
| 233 | specific guidance in the form of a code transformation to correct the |
Douglas Gregor | d89df5a | 2011-06-01 22:45:49 +0000 | [diff] [blame] | 234 | problem. In the following example, Clang warns about the use of a GCC |
| 235 | extension that has been considered obsolete since 1993. The underlined |
| 236 | code should be removed, then replaced with the code below the |
Benjamin Kramer | 665a8dc | 2012-01-15 15:26:07 +0000 | [diff] [blame] | 237 | point line (".x =" or ".y =", respectively).</p> |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 238 | |
Douglas Gregor | eff49c6 | 2009-04-01 16:24:40 +0000 | [diff] [blame] | 239 | <pre> |
| 240 | $ <b>clang t.c</b> |
Benjamin Kramer | 665a8dc | 2012-01-15 15:26:07 +0000 | [diff] [blame] | 241 | t.c:5:28: <span class="warn">warning:</span> use of GNU old-style field designator extension |
| 242 | <span class="snip">struct point origin = { x: 0.0, y: 0.0 };</span> |
| 243 | <span class="err">~~</span> <span class="point">^</span> |
| 244 | <span class="snip">.x = </span> |
| 245 | t.c:5:36: <span class="warn">warning:</span> use of GNU old-style field designator extension |
| 246 | <span class="snip">struct point origin = { x: 0.0, y: 0.0 };</span> |
| 247 | <span class="err">~~</span> <span class="point">^</span> |
| 248 | <span class="snip">.y = </span> |
Douglas Gregor | eff49c6 | 2009-04-01 16:24:40 +0000 | [diff] [blame] | 249 | </pre> |
| 250 | |
Douglas Gregor | d89df5a | 2011-06-01 22:45:49 +0000 | [diff] [blame] | 251 | <p>"Fix-it" hints are most useful for |
Chris Lattner | 0e3b44b | 2009-09-30 20:19:10 +0000 | [diff] [blame] | 252 | working around common user errors and misconceptions. For example, C++ users |
| 253 | commonly forget the syntax for explicit specialization of class templates, |
Douglas Gregor | d89df5a | 2011-06-01 22:45:49 +0000 | [diff] [blame] | 254 | as in the error in the following example. Again, after describing the problem, |
| 255 | Clang provides the fix--add <code>template<></code>--as part of the |
| 256 | diagnostic.<p> |
Douglas Gregor | eff49c6 | 2009-04-01 16:24:40 +0000 | [diff] [blame] | 257 | |
| 258 | <pre> |
| 259 | $ <b>clang t.cpp</b> |
Benjamin Kramer | 665a8dc | 2012-01-15 15:26:07 +0000 | [diff] [blame] | 260 | t.cpp:9:3: <span class="err">error:</span> template specialization requires 'template<>' |
Douglas Gregor | eff49c6 | 2009-04-01 16:24:40 +0000 | [diff] [blame] | 261 | struct iterator_traits<file_iterator> { |
Benjamin Kramer | 665a8dc | 2012-01-15 15:26:07 +0000 | [diff] [blame] | 262 | <span class="point">^</span> |
| 263 | <span class="snip">template<> </span> |
Douglas Gregor | eff49c6 | 2009-04-01 16:24:40 +0000 | [diff] [blame] | 264 | </pre> |
| 265 | |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 266 | <h2>Automatic Macro Expansion</h2> |
| 267 | |
| 268 | <p>Many errors happen in macros that are sometimes deeply nested. With |
| 269 | traditional compilers, you need to dig deep into the definition of the macro to |
Douglas Gregor | d89df5a | 2011-06-01 22:45:49 +0000 | [diff] [blame] | 270 | understand how you got into trouble. The following simple example shows how |
| 271 | Clang helps you out by automatically printing instantiation information and |
| 272 | nested range information for diagnostics as they are instantiated through macros |
| 273 | and also shows how some of the other pieces work in a bigger example.</p> |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 274 | |
| 275 | <pre> |
| 276 | $ <b>gcc-4.2 -fsyntax-only t.c</b> |
| 277 | t.c: In function 'test': |
| 278 | t.c:80: error: invalid operands to binary < (have 'struct mystruct' and 'float') |
| 279 | $ <b>clang -fsyntax-only t.c</b> |
Benjamin Kramer | 665a8dc | 2012-01-15 15:26:07 +0000 | [diff] [blame] | 280 | t.c:80:3: <span class="err">error:</span> invalid operands to binary expression ('typeof(P)' (aka 'struct mystruct') and 'typeof(F)' (aka 'float')) |
| 281 | <span class="snip"> X = MYMAX(P, F);</span> |
| 282 | <span class="point"> ^~~~~~~~~~~</span> |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 283 | t.c:76:94: note: instantiated from: |
Benjamin Kramer | 665a8dc | 2012-01-15 15:26:07 +0000 | [diff] [blame] | 284 | <span class="snip">#define MYMAX(A,B) __extension__ ({ __typeof__(A) __a = (A); __typeof__(B) __b = (B); __a < __b ? __b : __a; })</span> |
| 285 | <span class="point"> ~~~ ^ ~~~</span> |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 286 | </pre> |
| 287 | |
Douglas Gregor | d89df5a | 2011-06-01 22:45:49 +0000 | [diff] [blame] | 288 | <p>Here's another real world warning that occurs in the "window" Unix package (which |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 289 | implements the "wwopen" class of APIs):</p> |
| 290 | |
| 291 | <pre> |
| 292 | $ <b>clang -fsyntax-only t.c</b> |
Benjamin Kramer | 665a8dc | 2012-01-15 15:26:07 +0000 | [diff] [blame] | 293 | t.c:22:2: <span class="warn">warning:</span> type specifier missing, defaults to 'int' |
| 294 | <span class="snip"> ILPAD();</span> |
| 295 | <span class="point"> ^</span> |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 296 | t.c:17:17: note: instantiated from: |
Benjamin Kramer | 665a8dc | 2012-01-15 15:26:07 +0000 | [diff] [blame] | 297 | <span class="snip">#define ILPAD() PAD((NROW - tt.tt_row) * 10) /* 1 ms per char */</span> |
| 298 | <span class="point"> ^</span> |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 299 | t.c:14:2: note: instantiated from: |
Benjamin Kramer | 665a8dc | 2012-01-15 15:26:07 +0000 | [diff] [blame] | 300 | <span class="snip"> register i; \</span> |
| 301 | <span class="point"> ^</span> |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 302 | </pre> |
| 303 | |
Douglas Gregor | d89df5a | 2011-06-01 22:45:49 +0000 | [diff] [blame] | 304 | <p>In practice, we've found that Clang's treatment of macros is actually more useful in multiply nested |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 305 | macros that in simple ones.</p> |
| 306 | |
Chris Lattner | 0e3b44b | 2009-09-30 20:19:10 +0000 | [diff] [blame] | 307 | <h2>Quality of Implementation and Attention to Detail</h2> |
| 308 | |
| 309 | <p>Finally, we have put a lot of work polishing the little things, because |
Douglas Gregor | d89df5a | 2011-06-01 22:45:49 +0000 | [diff] [blame] | 310 | little things add up over time and contribute to a great user experience.</p> |
| 311 | |
| 312 | <p>The following example shows a trivial little tweak, where we tell you to put the semicolon at |
| 313 | the end of the line that is missing it (line 4) instead of at the beginning of |
| 314 | the following line (line 5). This is particularly important with fixit hints |
Benjamin Kramer | 665a8dc | 2012-01-15 15:26:07 +0000 | [diff] [blame] | 315 | and point diagnostics, because otherwise you don't get the important context. |
Douglas Gregor | d89df5a | 2011-06-01 22:45:49 +0000 | [diff] [blame] | 316 | </p> |
Chris Lattner | 0e3b44b | 2009-09-30 20:19:10 +0000 | [diff] [blame] | 317 | |
| 318 | <pre> |
| 319 | $ <b>gcc-4.2 t.c</b> |
| 320 | t.c: In function 'foo': |
| 321 | t.c:5: error: expected ';' before '}' token |
| 322 | $ <b>clang t.c</b> |
Benjamin Kramer | 665a8dc | 2012-01-15 15:26:07 +0000 | [diff] [blame] | 323 | t.c:4:8: <span class="err">error:</span> expected ';' after expression |
| 324 | <span class="snip"> bar()</span> |
| 325 | <span class="point"> ^</span> |
| 326 | <span class="point"> ;</span> |
Chris Lattner | 0e3b44b | 2009-09-30 20:19:10 +0000 | [diff] [blame] | 327 | </pre> |
| 328 | |
Douglas Gregor | d89df5a | 2011-06-01 22:45:49 +0000 | [diff] [blame] | 329 | <p>The following example shows much better error recovery than GCC. The message coming out |
| 330 | of GCC is completely useless for diagnosing the problem. Clang tries much harder |
| 331 | and produces a much more useful diagnosis of the problem.</p> |
Chris Lattner | 0e3b44b | 2009-09-30 20:19:10 +0000 | [diff] [blame] | 332 | |
| 333 | <pre> |
| 334 | $ <b>gcc-4.2 t.c</b> |
| 335 | t.c:3: error: expected '=', ',', ';', 'asm' or '__attribute__' before '*' token |
| 336 | $ <b>clang t.c</b> |
Benjamin Kramer | 665a8dc | 2012-01-15 15:26:07 +0000 | [diff] [blame] | 337 | t.c:3:1: <span class="err">error:</span> unknown type name 'foo_t' |
| 338 | <span class="snip">foo_t *P = 0;</span> |
| 339 | <span class="point">^</span> |
Chris Lattner | 0e3b44b | 2009-09-30 20:19:10 +0000 | [diff] [blame] | 340 | </pre> |
| 341 | |
Douglas Gregor | d89df5a | 2011-06-01 22:45:49 +0000 | [diff] [blame] | 342 | <p>The following example shows that we recover from the simple case of |
| 343 | forgetting a ; after a struct definition much better than GCC.</p> |
Chris Lattner | 0e3b44b | 2009-09-30 20:19:10 +0000 | [diff] [blame] | 344 | |
Chris Lattner | 94634cc | 2010-02-02 01:35:23 +0000 | [diff] [blame] | 345 | <pre> |
| 346 | $ <b>cat t.cc</b> |
| 347 | template<class T> |
| 348 | class a {} |
| 349 | class temp {}; |
| 350 | a<temp> b; |
| 351 | struct b { |
| 352 | } |
| 353 | $ <b>gcc-4.2 t.cc</b> |
| 354 | t.cc:3: error: multiple types in one declaration |
| 355 | t.cc:4: error: non-template type 'a' used as a template |
| 356 | t.cc:4: error: invalid type in declaration before ';' token |
| 357 | t.cc:6: error: expected unqualified-id at end of input |
| 358 | $ <b>clang t.cc</b> |
Benjamin Kramer | 665a8dc | 2012-01-15 15:26:07 +0000 | [diff] [blame] | 359 | t.cc:2:11: <span class="err">error:</span> expected ';' after class |
| 360 | <span class="snip">class a {}</span> |
| 361 | <span class="point"> ^</span> |
| 362 | <span class="point"> ;</span> |
| 363 | t.cc:6:2: <span class="err">error:</span> expected ';' after struct |
| 364 | <span class="snip">}</span> |
| 365 | <span class="point"> ^</span> |
| 366 | <span class="point"> ;</span> |
Chris Lattner | 94634cc | 2010-02-02 01:35:23 +0000 | [diff] [blame] | 367 | </pre> |
| 368 | |
Chris Lattner | 0e3b44b | 2009-09-30 20:19:10 +0000 | [diff] [blame] | 369 | <p>While each of these details is minor, we feel that they all add up to provide |
| 370 | a much more polished experience.</p> |
| 371 | |
Chris Lattner | 7676601 | 2009-03-19 18:52:17 +0000 | [diff] [blame] | 372 | </div> |
| 373 | </body> |
| 374 | </html> |