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6 <title>Language Compatibility</title>
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16<div id="content">
17
18<!-- ======================================================================= -->
19<h1>Language Compatibility</h1>
20<!-- ======================================================================= -->
21
22<p>Clang strives to both conform to current language standards (C99,
23 C++98) and also to implement many widely-used extensions available
24 in other compilers, so that most correct code will "just work" when
25 compiler with Clang. However, Clang is more strict than other
26 popular compilers, and may reject incorrect code that other
27 compilers allow. This page documents common compatibility and
28 portability issues with Clang to help you understand and fix the
29 problem in your code when Clang emits an error message.</p>
30
31<ul>
32 <li><a href="#c">C compatibility</a>
33 <ul>
34 <li><a href="#inline">C99 inline functions</a></li>
Chris Lattnera02d1832010-09-16 18:17:55 +0000[diff] [blame]35 <li><a href="#vector_builtins">"missing" vector __builtin functions</a></li>
Douglas Gregorc41b6ff2010-06-30 22:01:08 +0000[diff] [blame]36 <li><a href="#lvalue-cast">Lvalue casts</a></li>
Daniel Dunbar5a410212010-09-02 21:35:16 +0000[diff] [blame]37 <li><a href="#blocks-in-protected-scope">Jumps to within <tt>__block</tt> variable scope</a></li>
Daniel Dunbar15952c92010-11-09 22:45:16 +0000[diff] [blame]38 <li><a href="#block-variable-initialization">Non-initialization of <tt>__block</tt> variables</a></li>
Chris Lattner9b743f42010-11-10 23:51:50 +0000[diff] [blame]39 <li><a href="#inline-asm">Inline assembly</a></li>
Douglas Gregorc41b6ff2010-06-30 22:01:08 +0000[diff] [blame]40 </ul>
41 </li>
42 <li><a href="#objective-c">Objective-C compatibility</a>
43 <ul>
44 <li><a href="#super-cast">Cast of super</a></li>
45 <li><a href="#sizeof-interface">Size of interfaces</a></li>
Argyrios Kyrtzidis3b5b92a2010-09-13 17:48:07 +0000[diff] [blame]46 <li><a href="#objc_objs-cast">Internal Objective-C types</a></li>
Fariborz Jahanianddfa6c32010-10-22 22:35:51 +0000[diff] [blame]47 <li><a href="#c_variables-class">C variables in @class or @protocol</a></li>
Douglas Gregorc41b6ff2010-06-30 22:01:08 +0000[diff] [blame]48 </ul>
49 </li>
50 <li><a href="#c++">C++ compatibility</a>
51 <ul>
52 <li><a href="#vla">Variable-length arrays</a></li>
Douglas Gregorc41b6ff2010-06-30 22:01:08 +0000[diff] [blame]53 <li><a href="#dep_lookup">Unqualified lookup in templates</a></li>
54 <li><a href="#dep_lookup_bases">Unqualified lookup into dependent bases of class templates</a></li>
55 <li><a href="#undep_incomplete">Incomplete types in templates</a></li>
56 <li><a href="#bad_templates">Templates with no valid instantiations</a></li>
57 <li><a href="#default_init_const">Default initialization of const
58 variable of a class type requires user-defined default
59 constructor</a></li>
Douglas Gregora66d3bb2010-11-10 20:24:21 +0000[diff] [blame]60 <li><a href="#param_name_lookup">Parameter name lookup</a></li>
Douglas Gregorc41b6ff2010-06-30 22:01:08 +0000[diff] [blame]61 </ul>
62 </li>
63 <li><a href="#objective-c++">Objective-C++ compatibility</a>
64 <ul>
65 <li><a href="#implicit-downcasts">Implicit downcasts</a></li>
66 </ul>
Fariborz Jahanian36e738a2010-08-11 18:57:26 +0000[diff] [blame]67 <ul>
68 <li><a href="#Use of class as method name">Use of class as method name</a></li>
69 </ul>
Douglas Gregorc41b6ff2010-06-30 22:01:08 +0000[diff] [blame]70 </li>
71</ul>
72
73<!-- ======================================================================= -->
74<h2 id="c">C compatibility</h3>
75<!-- ======================================================================= -->
76
77<!-- ======================================================================= -->
78<h3 id="inline">C99 inline functions</h3>
79<!-- ======================================================================= -->
80<p>By default, Clang builds C code according to the C99 standard,
John McCall5ae84f22011-02-03 10:16:40 +0000[diff] [blame]81which provides different semantics for the <code>inline</code> keyword
82than GCC's default behavior. For example, consider the following
83code:</p>
Douglas Gregorc41b6ff2010-06-30 22:01:08 +0000[diff] [blame]84<pre>
85inline int add(int i, int j) { return i + j; }
86
87int main() {
88 int i = add(4, 5);
89 return i;
90}
91</pre>
92
John McCall5ae84f22011-02-03 10:16:40 +0000[diff] [blame]93<p>In C99, <code>inline</code> means that a function's definition is
94provided only for inlining, and that there is another definition
95(without <code>inline</code>) somewhere else in the program. That
96means that this program is incomplete, because if <code>add</code>
97isn't inlined (for example, when compiling without optimization), then
98<code>main</code> will have an unresolved reference to that other
99definition. Therefore we'll get a (correct) link-time error like this:</p>
Douglas Gregorc41b6ff2010-06-30 22:01:08 +0000[diff] [blame]100
101<pre>
102Undefined symbols:
103 "_add", referenced from:
104 _main in cc-y1jXIr.o
105</pre>
106
John McCall5ae84f22011-02-03 10:16:40 +0000[diff] [blame]107<p>By contrast, GCC's default behavior follows the GNU89 dialect,
108which is the C89 standard plus a lot of extensions. C89 doesn't have
109an <code>inline</code> keyword, but GCC recognizes it as an extension
110and just treats it as a hint to the optimizer.</p>
111
Douglas Gregorc41b6ff2010-06-30 22:01:08 +0000[diff] [blame]112<p>There are several ways to fix this problem:</p>
113
114<ul>
Douglas Gregorc41b6ff2010-06-30 22:01:08 +0000[diff] [blame]115 <li>Change <code>add</code> to a <code>static inline</code>
John McCall5ae84f22011-02-03 10:16:40 +0000[diff] [blame]116 function. This is usually the right solution if only one
117 translation unit needs to use the function. <code>static
118 inline</code> functions are always resolved within the translation
119 unit, so you won't have to add a non-<code>inline</code> definition
120 of the function elsewhere in your program.</li>
121
122 <li>Remove the <code>inline</code> keyword from this definition of
123 <code>add</code>. The <code>inline</code> keyword is not required
124 for a function to be inlined, nor does it guarantee that it will be.
125 Some compilers ignore it completely. Clang treats it as a mild
126 suggestion from the programmer.</li>
127
128 <li>Provide an external (non-<code>inline</code>) definition
129 of <code>add</code> somewhere else in your program. The two
130 definitions must be equivalent!</li>
131
Douglas Gregorc41b6ff2010-06-30 22:01:08 +0000[diff] [blame]132 <li>Compile with the GNU89 dialect by adding
133 <code>-std=gnu89</code> to the set of Clang options. This option is
134 only recommended if the program source cannot be changed or if the
135 program also relies on additional C89-specific behavior that cannot
136 be changed.</li>
137</ul>
138
John McCall5ae84f22011-02-03 10:16:40 +0000[diff] [blame]139<p>All of this only applies to C code; the meaning of <code>inline</code>
140in C++ is very different from its meaning in either GNU89 or C99.</p>
Chris Lattnera02d1832010-09-16 18:17:55 +0000[diff] [blame]141
142<!-- ======================================================================= -->
143<h3 id="vector_builtins">"missing" vector __builtin functions</h3>
144<!-- ======================================================================= -->
145
146<p>The Intel and AMD manuals document a number "<tt>&lt;*mmintrin.h&gt;</tt>"
147header files, which define a standardized API for accessing vector operations
148on X86 CPUs. These functions have names like <tt>_mm_xor_ps</tt> and
149<tt>_mm256_addsub_pd</tt>. Compilers have leeway to implement these functions
150however they want. Since Clang supports an excellent set of <a
151href="../docs/LanguageExtensions.html#vectors">native vector operations</a>,
152the Clang headers implement these interfaces in terms of the native vector
153operations.
154</p>
155
156<p>In contrast, GCC implements these functions mostly as a 1-to-1 mapping to
157builtin function calls, like <tt>__builtin_ia32_paddw128</tt>. These builtin
158functions are an internal implementation detail of GCC, and are not portable to
159the Intel compiler, the Microsoft compiler, or Clang. If you get build errors
160mentioning these, the fix is simple: switch to the *mmintrin.h functions.</p>
161
162<p>The same issue occurs for NEON and Altivec for the ARM and PowerPC
163architectures respectively. For these, make sure to use the &lt;arm_neon.h&gt;
164and &lt;altivec.h&gt; headers.</p>
165
Eric Christophera473c952010-10-25 21:17:59 +0000[diff] [blame]166<p>For x86 architectures this <a href="builtins.py">script</a> should help with
167the manual migration process. It will rewrite your source files in place to
168use the APIs instead of builtin function calls. Just call it like this:</p>
169
170<pre>
171 builtins.py *.c *.h
172</pre>
173
174<p>and it will rewrite all of the .c and .h files in the current directory to
175use the API calls instead of calls like <tt>__builtin_ia32_paddw128</tt>.</p>
Chris Lattnera02d1832010-09-16 18:17:55 +0000[diff] [blame]176
Douglas Gregorc41b6ff2010-06-30 22:01:08 +0000[diff] [blame]177<!-- ======================================================================= -->
178<h3 id="lvalue-cast">Lvalue casts</h3>
179<!-- ======================================================================= -->
180
Douglas Gregor6f1adba2010-06-30 22:38:37 +0000[diff] [blame]181<p>Old versions of GCC permit casting the left-hand side of an assignment to a
Douglas Gregorc41b6ff2010-06-30 22:01:08 +0000[diff] [blame]182different type. Clang produces an error on similar code, e.g.,</p>
183
184<pre>
185lvalue.c:2:3: error: assignment to cast is illegal, lvalue casts are not
186 supported
187 (int*)addr = val;
188 ^~~~~~~~~~ ~
189</pre>
190
191<p>To fix this problem, move the cast to the right-hand side. In this
192example, one could use:</p>
193
194<pre>
195 addr = (float *)val;
196</pre>
197
198<!-- ======================================================================= -->
Daniel Dunbar5a410212010-09-02 21:35:16 +0000[diff] [blame]199<h3 id="blocks-in-protected-scope">Jumps to within <tt>__block</tt> variable scope</h3>
200<!-- ======================================================================= -->
201
John McCall504b3692011-02-03 10:56:31 +0000[diff] [blame^]202<p>Clang disallows jumps into the scope of a <tt>__block</tt>
203variable. Variables marked with <tt>__block</tt> require special
204runtime initialization. A jump into the scope of a <tt>__block</tt>
205variable bypasses this initialization, leaving the variable's metadata
206in an invalid state. Consider the following code fragment:</p>
Daniel Dunbar5a410212010-09-02 21:35:16 +0000[diff] [blame]207
208<pre>
John McCall504b3692011-02-03 10:56:31 +0000[diff] [blame^]209int fetch_object_state(struct MyObject *c) {
210 if (!c->active) goto error;
Daniel Dunbar5a410212010-09-02 21:35:16 +0000[diff] [blame]211
John McCall504b3692011-02-03 10:56:31 +0000[diff] [blame^]212 __block int result;
213 run_specially_somehow(^{ result = c->state; });
214 return result;
Daniel Dunbar5a410212010-09-02 21:35:16 +0000[diff] [blame]215
216 error:
John McCall504b3692011-02-03 10:56:31 +0000[diff] [blame^]217 fprintf(stderr, "error while fetching object state");
218 return -1;
Daniel Dunbar5a410212010-09-02 21:35:16 +0000[diff] [blame]219}
220</pre>
221
John McCall504b3692011-02-03 10:56:31 +0000[diff] [blame^]222<p>GCC accepts this code, but it produces code that will usually crash
223when <code>result</code> goes out of scope if the jump is taken. (It's
224possible for this bug to go undetected because it often won't crash if
225the stack is fresh, i.e. still zeroed.) Therefore, Clang rejects this
226code with a hard error:</p>
Daniel Dunbar5a410212010-09-02 21:35:16 +0000[diff] [blame]227
228<pre>
229t.c:3:5: error: goto into protected scope
230 goto error;
231 ^
232t.c:5:15: note: jump bypasses setup of __block variable
John McCall504b3692011-02-03 10:56:31 +0000[diff] [blame^]233 __block int result;
Daniel Dunbar5a410212010-09-02 21:35:16 +0000[diff] [blame]234 ^
235</pre>
236
John McCall504b3692011-02-03 10:56:31 +0000[diff] [blame^]237<p>The fix is to rewrite the code to not require jumping into a
238<tt>__block</tt> variable's scope, e.g. by limiting that scope:</p>
239
240<pre>
241 {
242 __block int result;
243 run_specially_somehow(^{ result = c->state; });
244 return result;
245 }
246</pre>
Daniel Dunbar5a410212010-09-02 21:35:16 +0000[diff] [blame]247
248<!-- ======================================================================= -->
Daniel Dunbar15952c92010-11-09 22:45:16 +0000[diff] [blame]249<h3 id="block-variable-initialization">Non-initialization of <tt>__block</tt>
250variables</h3>
251<!-- ======================================================================= -->
252
253<p>In the following example code, the <tt>x</tt> variable is used before it is
254defined:</p>
255<pre>
256int f0() {
257 __block int x;
258 return ^(){ return x; }();
259}
260</pre>
261
262<p>By an accident of implementation, GCC and llvm-gcc unintentionally always
263zero initialized <tt>__block</tt> variables. However, any program which depends
264on this behavior is relying on unspecified compiler behavior. Programs must
265explicitly initialize all local block variables before they are used, as with
266other local variables.</p>
267
268<p>Clang does not zero initialize local block variables, and programs which rely
269on such behavior will most likely break when built with Clang.</p>
270
Chris Lattner9b743f42010-11-10 23:51:50 +0000[diff] [blame]271
272<!-- ======================================================================= -->
273<h3 id="inline-asm">Inline assembly</h3>
274<!-- ======================================================================= -->
275
276<p>In general, Clang is highly compatible with the GCC inline assembly
277extensions, allowing the same set of constraints, modifiers and operands as GCC
278inline assembly.</p>
279
280<p>On targets that use the integrated assembler (such as most X86 targets),
281inline assembly is run through the integrated assembler instead of your system
282assembler (which is most commonly "gas", the GNU assembler). The LLVM
283integrated assembler is extremely compatible with GAS, but there are a couple of
284minor places where it is more picky, particularly due to outright GAS bugs.</p>
285
286<p>One specific example is that the assembler rejects ambiguous X86 instructions
287that don't have suffixes. For example:</p>
288
289<pre>
290 asm("add %al, (%rax)");
291 asm("addw $4, (%rax)");
292 asm("add $4, (%rax)");
293</pre>
294
295<p>Both clang and GAS accept the first instruction: because the first
296instruction uses the 8-bit <tt>%al</tt> register as an operand, it is clear that
297it is an 8-bit add. The second instruction is accepted by both because the "w"
298suffix indicates that it is a 16-bit add. The last instruction is accepted by
299GAS even though there is nothing that specifies the size of the instruction (and
300the assembler randomly picks a 32-bit add). Because it is ambiguous, Clang
301rejects the instruction with this error message:
302</p>
303
304<pre>
305&lt;inline asm&gt;:3:1: error: ambiguous instructions require an explicit suffix (could be 'addb', 'addw', 'addl', or 'addq')
306add $4, (%rax)
307^
3081 error generated.
309</pre>
310
311<p>To fix this compatibility issue, add an explicit suffix to the instruction:
312this makes your code more clear and is compatible with both GCC and Clang.</p>
313
Daniel Dunbar15952c92010-11-09 22:45:16 +0000[diff] [blame]314<!-- ======================================================================= -->
Douglas Gregorc41b6ff2010-06-30 22:01:08 +0000[diff] [blame]315<h2 id="objective-c">Objective-C compatibility</h3>
316<!-- ======================================================================= -->
317
318<!-- ======================================================================= -->
319<h3 id="super-cast">Cast of super</h3>
320<!-- ======================================================================= -->
321
322<p>GCC treats the <code>super</code> identifier as an expression that
323can, among other things, be cast to a different type. Clang treats
324<code>super</code> as a context-sensitive keyword, and will reject a
325type-cast of <code>super</code>:</p>
326
327<pre>
328super.m:11:12: error: cannot cast 'super' (it isn't an expression)
329 [(Super*)super add:4];
330 ~~~~~~~~^
331</pre>
332
333<p>To fix this problem, remove the type cast, e.g.</p>
334<pre>
335 [super add:4];
336</pre>
337
338<!-- ======================================================================= -->
339<h3 id="sizeof-interface">Size of interfaces</h3>
340<!-- ======================================================================= -->
341
342<p>When using the "non-fragile" Objective-C ABI in use, the size of an
343Objective-C class may change over time as instance variables are added
344(or removed). For this reason, Clang rejects the application of the
345<code>sizeof</code> operator to an Objective-C class when using this
346ABI:</p>
347
348<pre>
349sizeof.m:4:14: error: invalid application of 'sizeof' to interface 'NSArray' in
350 non-fragile ABI
351 int size = sizeof(NSArray);
352 ^ ~~~~~~~~~
353</pre>
354
355<p>Code that relies on the size of an Objective-C class is likely to
356be broken anyway, since that size is not actually constant. To address
357this problem, use the Objective-C runtime API function
Benjamin Kramere6617502010-06-30 22:29:56 +0000[diff] [blame]358<code>class_getInstanceSize()</code>:</p>
Douglas Gregorc41b6ff2010-06-30 22:01:08 +0000[diff] [blame]359
360<pre>
361 class_getInstanceSize([NSArray class])
362</pre>
363
364<!-- ======================================================================= -->
Argyrios Kyrtzidis3b5b92a2010-09-13 17:48:07 +0000[diff] [blame]365<h3 id="objc_objs-cast">Internal Objective-C types</h3>
366<!-- ======================================================================= -->
367
368<p>GCC allows using pointers to internal Objective-C objects, <tt>struct objc_object*</tt>,
369<tt>struct objc_selector*</tt>, and <tt>struct objc_class*</tt> in place of the types
370<tt>id</tt>, <tt>SEL</tt>, and <tt>Class</tt> respectively. Clang treats the
371internal Objective-C structures as implementation detail and won't do implicit conversions:
372
373<pre>
374t.mm:11:2: error: no matching function for call to 'f'
375 f((struct objc_object *)p);
376 ^
377t.mm:5:6: note: candidate function not viable: no known conversion from 'struct objc_object *' to 'id' for 1st argument
378void f(id x);
379 ^
380</pre>
381
382<p>Code should use types <tt>id</tt>, <tt>SEL</tt>, and <tt>Class</tt>
383instead of the internal types.</p>
384
385<!-- ======================================================================= -->
Fariborz Jahanianddfa6c32010-10-22 22:35:51 +0000[diff] [blame]386<h3 id="c_variables-class">C variables in @class or @protocol</h3>
387<!-- ======================================================================= -->
388
389<p>GCC allows declaration of C variables in a @class or @protocol, but not
390C functions. Clang does not allow variable or C function declarations. External
391declarations, however, is allowed. Variables may only be declared in an
392@implementation.
393
394<pre>
395@interface XX
396int x; // not allowed in clang
397int one=1; // not allowed in clang
398extern int OK;
399@end
400
401</pre>
402
403<!-- ======================================================================= -->
Douglas Gregorc41b6ff2010-06-30 22:01:08 +0000[diff] [blame]404<h2 id="c++">C++ compatibility</h3>
405<!-- ======================================================================= -->
406
407<!-- ======================================================================= -->
408<h3 id="vla">Variable-length arrays</h3>
409<!-- ======================================================================= -->
410
411<p>GCC and C99 allow an array's size to be determined at run
412time. This extension is not permitted in standard C++. However, Clang
413supports such variable length arrays in very limited circumstances for
414compatibility with GNU C and C99 programs:</p>
415
416<ul>
417 <li>The element type of a variable length array must be a POD
418 ("plain old data") type, which means that it cannot have any
419 user-declared constructors or destructors, base classes, or any
420 members if non-POD type. All C types are POD types.</li>
421
422 <li>Variable length arrays cannot be used as the type of a non-type
423template parameter.</li> </ul>
424
425<p>If your code uses variable length arrays in a manner that Clang doesn't support, there are several ways to fix your code:
426
427<ol>
428<li>replace the variable length array with a fixed-size array if you can
429 determine a
430 reasonable upper bound at compile time; sometimes this is as
431 simple as changing <tt>int size = ...;</tt> to <tt>const int size
432 = ...;</tt> (if the definition of <tt>size</tt> is a compile-time
433 integral constant);</li>
434<li>use an <tt>std::string</tt> instead of a <tt>char []</tt>;</li>
435<li>use <tt>std::vector</tt> or some other suitable container type;
436 or</li>
437<li>allocate the array on the heap instead using <tt>new Type[]</tt> -
438 just remember to <tt>delete[]</tt> it.</li>
439</ol>
440
441<!-- ======================================================================= -->
Douglas Gregorc41b6ff2010-06-30 22:01:08 +0000[diff] [blame]442<h3 id="dep_lookup">Unqualified lookup in templates</h3>
443<!-- ======================================================================= -->
444
445<p>Some versions of GCC accept the following invalid code:
446
447<pre>
448template &lt;typename T&gt; T Squared(T x) {
449 return Multiply(x, x);
450}
451
452int Multiply(int x, int y) {
453 return x * y;
454}
455
456int main() {
457 Squared(5);
458}
459</pre>
460
461<p>Clang complains:
462
463<pre> <b>my_file.cpp:2:10: <span class="error">error:</span> use of undeclared identifier 'Multiply'</b>
464 return Multiply(x, x);
465 <span class="caret"> ^</span>
466
467 <b>my_file.cpp:10:3: <span class="note">note:</span> in instantiation of function template specialization 'Squared&lt;int&gt;' requested here</b>
468 Squared(5);
469 <span class="caret"> ^</span>
470</pre>
471
472<p>The C++ standard says that unqualified names like <q>Multiply</q>
473are looked up in two ways.
474
475<p>First, the compiler does <i>unqualified lookup</i> in the scope
476where the name was written. For a template, this means the lookup is
477done at the point where the template is defined, not where it's
478instantiated. Since <tt>Multiply</tt> hasn't been declared yet at
479this point, unqualified lookup won't find it.
480
481<p>Second, if the name is called like a function, then the compiler
482also does <i>argument-dependent lookup</i> (ADL). (Sometimes
483unqualified lookup can suppress ADL; see [basic.lookup.argdep]p3 for
484more information.) In ADL, the compiler looks at the types of all the
485arguments to the call. When it finds a class type, it looks up the
486name in that class's namespace; the result is all the declarations it
487finds in those namespaces, plus the declarations from unqualified
488lookup. However, the compiler doesn't do ADL until it knows all the
489argument types.
490
491<p>In our example, <tt>Multiply</tt> is called with dependent
492arguments, so ADL isn't done until the template is instantiated. At
493that point, the arguments both have type <tt>int</tt>, which doesn't
494contain any class types, and so ADL doesn't look in any namespaces.
495Since neither form of lookup found the declaration
496of <tt>Multiply</tt>, the code doesn't compile.
497
498<p>Here's another example, this time using overloaded operators,
499which obey very similar rules.
500
501<pre>#include &lt;iostream&gt;
502
503template&lt;typename T&gt;
504void Dump(const T&amp; value) {
505 std::cout &lt;&lt; value &lt;&lt; "\n";
506}
507
508namespace ns {
509 struct Data {};
510}
511
512std::ostream&amp; operator&lt;&lt;(std::ostream&amp; out, ns::Data data) {
513 return out &lt;&lt; "Some data";
514}
515
516void Use() {
517 Dump(ns::Data());
518}</pre>
519
520<p>Again, Clang complains about not finding a matching function:</p>
521
522<pre>
523<b>my_file.cpp:5:13: <span class="error">error:</span> invalid operands to binary expression ('ostream' (aka 'basic_ostream&lt;char&gt;') and 'ns::Data const')</b>
524 std::cout &lt;&lt; value &lt;&lt; "\n";
525 <span class="caret">~~~~~~~~~ ^ ~~~~~</span>
526<b>my_file.cpp:17:3: <span class="note">note:</span> in instantiation of function template specialization 'Dump&lt;ns::Data&gt;' requested here</b>
527 Dump(ns::Data());
528 <span class="caret">^</span>
529</pre>
530
531<p>Just like before, unqualified lookup didn't find any declarations
532with the name <tt>operator&lt;&lt;</tt>. Unlike before, the argument
533types both contain class types: one of them is an instance of the
534class template type <tt>std::basic_ostream</tt>, and the other is the
535type <tt>ns::Data</tt> that we declared above. Therefore, ADL will
536look in the namespaces <tt>std</tt> and <tt>ns</tt> for
537an <tt>operator&lt;&lt;</tt>. Since one of the argument types was
538still dependent during the template definition, ADL isn't done until
539the template is instantiated during <tt>Use</tt>, which means that
540the <tt>operator&lt;&lt;</tt> we want it to find has already been
541declared. Unfortunately, it was declared in the global namespace, not
542in either of the namespaces that ADL will look in!
543
544<p>There are two ways to fix this problem:</p>
545<ol><li>Make sure the function you want to call is declared before the
546template that might call it. This is the only option if none of its
547argument types contain classes. You can do this either by moving the
548template definition, or by moving the function definition, or by
549adding a forward declaration of the function before the template.</li>
550<li>Move the function into the same namespace as one of its arguments
551so that ADL applies.</li></ol>
552
553<p>For more information about argument-dependent lookup, see
554[basic.lookup.argdep]. For more information about the ordering of
555lookup in templates, see [temp.dep.candidate].
556
557<!-- ======================================================================= -->
558<h3 id="dep_lookup_bases">Unqualified lookup into dependent bases of class templates</h3>
559<!-- ======================================================================= -->
560
561Some versions of GCC accept the following invalid code:
562
563<pre>
564template &lt;typename T&gt; struct Base {
565 void DoThis(T x) {}
566 static void DoThat(T x) {}
567};
568
569template &lt;typename T&gt; struct Derived : public Base&lt;T&gt; {
570 void Work(T x) {
571 DoThis(x); // Invalid!
572 DoThat(x); // Invalid!
573 }
574};
575</pre>
576
577Clang correctly rejects it with the following errors
578(when <tt>Derived</tt> is eventually instantiated):
579
580<pre>
581my_file.cpp:8:5: error: use of undeclared identifier 'DoThis'
582 DoThis(x);
583 ^
584 this-&gt;
585my_file.cpp:2:8: note: must qualify identifier to find this declaration in dependent base class
586 void DoThis(T x) {}
587 ^
588my_file.cpp:9:5: error: use of undeclared identifier 'DoThat'
589 DoThat(x);
590 ^
591 this-&gt;
592my_file.cpp:3:15: note: must qualify identifier to find this declaration in dependent base class
593 static void DoThat(T x) {}
594</pre>
595
596Like we said <a href="#dep_lookup">above</a>, unqualified names like
597<tt>DoThis</tt> and <tt>DoThat</tt> are looked up when the template
598<tt>Derived</tt> is defined, not when it's instantiated. When we look
599up a name used in a class, we usually look into the base classes.
600However, we can't look into the base class <tt>Base&lt;T&gt;</tt>
601because its type depends on the template argument <tt>T</tt>, so the
602standard says we should just ignore it. See [temp.dep]p3 for details.
603
604<p>The fix, as Clang tells you, is to tell the compiler that we want a
605class member by prefixing the calls with <tt>this-&gt;</tt>:
606
607<pre>
608 void Work(T x) {
609 <b>this-&gt;</b>DoThis(x);
610 <b>this-&gt;</b>DoThat(x);
611 }
612</pre>
613
614Alternatively, you can tell the compiler exactly where to look:
615
616<pre>
617 void Work(T x) {
618 <b>Base&lt;T&gt;</b>::DoThis(x);
619 <b>Base&lt;T&gt;</b>::DoThat(x);
620 }
621</pre>
622
623This works whether the methods are static or not, but be careful:
624if <tt>DoThis</tt> is virtual, calling it this way will bypass virtual
625dispatch!
626
627<!-- ======================================================================= -->
628<h3 id="undep_incomplete">Incomplete types in templates</h3>
629<!-- ======================================================================= -->
630
631The following code is invalid, but compilers are allowed to accept it:
632
633<pre>
634 class IOOptions;
635 template &lt;class T&gt; bool read(T &amp;value) {
636 IOOptions opts;
637 return read(opts, value);
638 }
639
640 class IOOptions { bool ForceReads; };
641 bool read(const IOOptions &amp;opts, int &amp;x);
642 template bool read&lt;&gt;(int &amp;);
643</pre>
644
645The standard says that types which don't depend on template parameters
646must be complete when a template is defined if they affect the
647program's behavior. However, the standard also says that compilers
648are free to not enforce this rule. Most compilers enforce it to some
649extent; for example, it would be an error in GCC to
650write <tt>opts.ForceReads</tt> in the code above. In Clang, we feel
651that enforcing the rule consistently lets us provide a better
652experience, but unfortunately it also means we reject some code that
653other compilers accept.
654
655<p>We've explained the rule here in very imprecise terms; see
656[temp.res]p8 for details.
657
658<!-- ======================================================================= -->
659<h3 id="bad_templates">Templates with no valid instantiations</h3>
660<!-- ======================================================================= -->
661
662The following code contains a typo: the programmer
663meant <tt>init()</tt> but wrote <tt>innit()</tt> instead.
664
665<pre>
666 template &lt;class T&gt; class Processor {
667 ...
668 void init();
669 ...
670 };
671 ...
672 template &lt;class T&gt; void process() {
673 Processor&lt;T&gt; processor;
674 processor.innit(); // <-- should be 'init()'
675 ...
676 }
677</pre>
678
679Unfortunately, we can't flag this mistake as soon as we see it: inside
680a template, we're not allowed to make assumptions about "dependent
681types" like <tt>Processor&lt;T&gt;</tt>. Suppose that later on in
682this file the programmer adds an explicit specialization
683of <tt>Processor</tt>, like so:
684
685<pre>
686 template &lt;&gt; class Processor&lt;char*&gt; {
687 void innit();
688 };
689</pre>
690
691Now the program will work &mdash; as long as the programmer only ever
692instantiates <tt>process()</tt> with <tt>T = char*</tt>! This is why
693it's hard, and sometimes impossible, to diagnose mistakes in a
694template definition before it's instantiated.
695
696<p>The standard says that a template with no valid instantiations is
697ill-formed. Clang tries to do as much checking as possible at
698definition-time instead of instantiation-time: not only does this
699produce clearer diagnostics, but it also substantially improves
700compile times when using pre-compiled headers. The downside to this
701philosophy is that Clang sometimes fails to process files because they
702contain broken templates that are no longer used. The solution is
703simple: since the code is unused, just remove it.
704
705<!-- ======================================================================= -->
706<h3 id="default_init_const">Default initialization of const variable of a class type requires user-defined default constructor</h3>
707<!-- ======================================================================= -->
708
709If a <tt>class</tt> or <tt>struct</tt> has no user-defined default
710constructor, C++ doesn't allow you to default construct a <tt>const</tt>
711instance of it like this ([dcl.init], p9):
712
713<pre>
714class Foo {
715 public:
716 // The compiler-supplied default constructor works fine, so we
717 // don't bother with defining one.
718 ...
719};
720
721void Bar() {
722 const Foo foo; // Error!
723 ...
724}
725</pre>
726
727To fix this, you can define a default constructor for the class:
728
729<pre>
730class Foo {
731 public:
732 Foo() {}
733 ...
734};
735
736void Bar() {
737 const Foo foo; // Now the compiler is happy.
738 ...
739}
740</pre>
741
742<!-- ======================================================================= -->
Douglas Gregora66d3bb2010-11-10 20:24:21 +0000[diff] [blame]743<h3 id="param_name_lookup">Parameter name lookup</h3>
744<!-- ======================================================================= -->
745
746<p>Due to a bug in its implementation, GCC allows the redeclaration of function parameter names within a function prototype in C++ code, e.g.</p>
747<blockquote>
748<pre>
749void f(int a, int a);
750</pre>
751</blockquote>
752<p>Clang diagnoses this error (where the parameter name has been redeclared). To fix this problem, rename one of the parameters.</p>
753
754<!-- ======================================================================= -->
Douglas Gregorc41b6ff2010-06-30 22:01:08 +0000[diff] [blame]755<h2 id="objective-c++">Objective-C++ compatibility</h3>
756<!-- ======================================================================= -->
757
758<!-- ======================================================================= -->
759<h3 id="implicit-downcasts">Implicit downcasts</h3>
760<!-- ======================================================================= -->
761
762<p>Due to a bug in its implementation, GCC allows implicit downcasts
763(from base class to a derived class) when calling functions. Such code is
764inherently unsafe, since the object might not actually be an instance
765of the derived class, and is rejected by Clang. For example, given
766this code:</p>
767
768<pre>
769@interface Base @end
770@interface Derived : Base @end
771
772void f(Derived *);
773void g(Base *base) {
774 f(base);
775}
776</pre>
777
778<p>Clang produces the following error:</p>
779
780<pre>
781downcast.mm:6:3: error: no matching function for call to 'f'
782 f(base);
783 ^
Douglas Gregor92bc0272010-07-01 03:50:01 +0000[diff] [blame]784downcast.mm:4:6: note: candidate function not viable: cannot convert from
785 superclass 'Base *' to subclass 'Derived *' for 1st argument
Douglas Gregorc41b6ff2010-06-30 22:01:08 +0000[diff] [blame]786void f(Derived *);
787 ^
788</pre>
789
790<p>If the downcast is actually correct (e.g., because the code has
791already checked that the object has the appropriate type), add an
792explicit cast:</p>
793
794<pre>
795 f((Derived *)base);
796</pre>
797
Fariborz Jahanian36e738a2010-08-11 18:57:26 +0000[diff] [blame]798<!-- ======================================================================= -->
799<h3 id="Use of class as method name">Use of class as method name</h3>
800<!-- ======================================================================= -->
801
802<p>Use of 'class' name to declare a method is allowed in objective-c++ mode to
803be compatible with GCC. However, use of property dot syntax notation to call
804this method is not allowed in clang++, as [I class] is a suitable syntax that
805will work. So, this test will fail in clang++.
806
807<pre>
808@interface I {
809int cls;
810}
811+ (int)class;
812@end
813
814@implementation I
815- (int) Meth { return I.class; }
816@end
817<pre>
818
819
Douglas Gregorc41b6ff2010-06-30 22:01:08 +0000[diff] [blame]820</div>
821</body>
822</html>