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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 (up to C11
23  and C++11) and also to implement many widely-used extensions available
24  in other compilers, so that most correct code will "just work" when
25  compiled 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>
35      <li><a href="#vector_builtins">"missing" vector __builtin functions</a></li>
36      <li><a href="#lvalue-cast">Lvalue casts</a></li>
37      <li><a href="#blocks-in-protected-scope">Jumps to within <tt>__block</tt> variable scope</a></li>
38      <li><a href="#block-variable-initialization">Non-initialization of <tt>__block</tt> variables</a></li>
39      <li><a href="#inline-asm">Inline assembly</a></li>
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>
46      <li><a href="#objc_objs-cast">Internal Objective-C types</a></li>
47      <li><a href="#c_variables-class">C variables in @class or @protocol</a></li>
48    </ul>
49  </li>
50  <li><a href="#cxx">C++ compatibility</a>
51    <ul>
52      <li><a href="#vla">Variable-length arrays</a></li>
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>
60      <li><a href="#param_name_lookup">Parameter name lookup</a></li>
61    </ul>
62  </li>
63  <li><a href="#cxx11">C++11 compatibility</a>
64    <ul>
65      <li><a href="#deleted-special-func">Deleted special member
66  functions</a></li>
67    </ul>
68  </li>
69  <li><a href="#objective-cxx">Objective-C++ compatibility</a>
70    <ul>
71      <li><a href="#implicit-downcasts">Implicit downcasts</a></li>
72    </ul>
73    <ul>
74      <li><a href="#class-as-property-name">Using <code>class</code> as a property name</a></li>
75    </ul>
76  </li>
77</ul>
78
79<!-- ======================================================================= -->
80<h2 id="c">C compatibility</h2>
81<!-- ======================================================================= -->
82
83<!-- ======================================================================= -->
84<h3 id="inline">C99 inline functions</h3>
85<!-- ======================================================================= -->
86<p>By default, Clang builds C code in GNU C11 mode, so it uses standard C99
87semantics for the <code>inline</code> keyword. These semantics are different
88from those in GNU C89 mode, which is the default mode in versions of GCC
89prior to 5.0. For example, consider the following code:</p>
90<pre>
91inline int add(int i, int j) { return i + j; }
92
93int main() {
94  int i = add(4, 5);
95  return i;
96}
97</pre>
98
99<p>In C99, <code>inline</code> means that a function's definition is
100provided only for inlining, and that there is another definition
101(without <code>inline</code>) somewhere else in the program.  That
102means that this program is incomplete, because if <code>add</code>
103isn't inlined (for example, when compiling without optimization), then
104<code>main</code> will have an unresolved reference to that other
105definition.  Therefore we'll get a (correct) link-time error like this:</p>
106
107<pre>
108Undefined symbols:
109  "_add", referenced from:
110      _main in cc-y1jXIr.o
111</pre>
112
113<p>By contrast, GNU C89 mode (used by default in older versions of GCC) is the
114C89 standard plus a lot of extensions. C89 doesn't have an <code>inline</code>
115keyword, but GCC recognizes it as an extension and just treats it as a hint to
116the optimizer.</p>
117
118<p>There are several ways to fix this problem:</p>
119
120<ul>
121  <li>Change <code>add</code> to a <code>static inline</code>
122  function.  This is usually the right solution if only one
123  translation unit needs to use the function.  <code>static
124  inline</code> functions are always resolved within the translation
125  unit, so you won't have to add a non-<code>inline</code> definition
126  of the function elsewhere in your program.</li>
127
128  <li>Remove the <code>inline</code> keyword from this definition of
129  <code>add</code>.  The <code>inline</code> keyword is not required
130  for a function to be inlined, nor does it guarantee that it will be.
131  Some compilers ignore it completely.  Clang treats it as a mild
132  suggestion from the programmer.</li>
133
134  <li>Provide an external (non-<code>inline</code>) definition
135  of <code>add</code> somewhere else in your program.  The two
136  definitions must be equivalent!</li>
137
138  <li>Compile in the GNU C89 dialect by adding
139  <code>-std=gnu89</code> to the set of Clang options. This option is
140  only recommended if the program source cannot be changed or if the
141  program also relies on additional C89-specific behavior that cannot
142  be changed.</li>
143</ul>
144
145<p>All of this only applies to C code; the meaning of <code>inline</code>
146in C++ is very different from its meaning in either GNU89 or C99.</p>
147
148<!-- ======================================================================= -->
149<h3 id="vector_builtins">"missing" vector __builtin functions</h3>
150<!-- ======================================================================= -->
151
152<p>The Intel and AMD manuals document a number "<tt>&lt;*mmintrin.h&gt;</tt>"
153header files, which define a standardized API for accessing vector operations
154on X86 CPUs.  These functions have names like <tt>_mm_xor_ps</tt> and
155<tt>_mm256_addsub_pd</tt>.  Compilers have leeway to implement these functions
156however they want.  Since Clang supports an excellent set of <a
157href="../docs/LanguageExtensions.html#vectors">native vector operations</a>,
158the Clang headers implement these interfaces in terms of the native vector
159operations.
160</p>
161
162<p>In contrast, GCC implements these functions mostly as a 1-to-1 mapping to
163builtin function calls, like <tt>__builtin_ia32_paddw128</tt>.  These builtin
164functions are an internal implementation detail of GCC, and are not portable to
165the Intel compiler, the Microsoft compiler, or Clang.  If you get build errors
166mentioning these, the fix is simple: switch to the *mmintrin.h functions.</p>
167
168<p>The same issue occurs for NEON and Altivec for the ARM and PowerPC
169architectures respectively.  For these, make sure to use the &lt;arm_neon.h&gt;
170and &lt;altivec.h&gt; headers.</p>
171
172<p>For x86 architectures this <a href="builtins.py">script</a> should help with
173the manual migration process.  It will rewrite your source files in place to
174use the APIs instead of builtin function calls. Just call it like this:</p>
175
176<pre>
177  builtins.py *.c *.h
178</pre>
179
180<p>and it will rewrite all of the .c and .h files in the current directory to
181use the API calls instead of calls like <tt>__builtin_ia32_paddw128</tt>.</p>
182
183<!-- ======================================================================= -->
184<h3 id="lvalue-cast">Lvalue casts</h3>
185<!-- ======================================================================= -->
186
187<p>Old versions of GCC permit casting the left-hand side of an assignment to a
188different type. Clang produces an error on similar code, e.g.,</p>
189
190<pre>
191<b>lvalue.c:2:3: <span class="error">error:</span> assignment to cast is illegal, lvalue casts are not supported</b>
192  (int*)addr = val;
193<span class="caret">  ^~~~~~~~~~ ~</span>
194</pre>
195
196<p>To fix this problem, move the cast to the right-hand side. In this
197example, one could use:</p>
198
199<pre>
200  addr = (float *)val;
201</pre>
202
203<!-- ======================================================================= -->
204<h3 id="blocks-in-protected-scope">Jumps to within <tt>__block</tt> variable scope</h3>
205<!-- ======================================================================= -->
206
207<p>Clang disallows jumps into the scope of a <tt>__block</tt>
208variable.  Variables marked with <tt>__block</tt> require special
209runtime initialization. A jump into the scope of a <tt>__block</tt>
210variable bypasses this initialization, leaving the variable's metadata
211in an invalid state.  Consider the following code fragment:</p>
212
213<pre>
214int fetch_object_state(struct MyObject *c) {
215  if (!c->active) goto error;
216
217  __block int result;
218  run_specially_somehow(^{ result = c->state; });
219  return result;
220
221 error:
222  fprintf(stderr, "error while fetching object state");
223  return -1;
224}
225</pre>
226
227<p>GCC accepts this code, but it produces code that will usually crash
228when <code>result</code> goes out of scope if the jump is taken.  (It's
229possible for this bug to go undetected because it often won't crash if
230the stack is fresh, i.e. still zeroed.)  Therefore, Clang rejects this
231code with a hard error:</p>
232
233<pre>
234<b>t.c:3:5: <span class="error">error:</span> goto into protected scope</b>
235    goto error;
236<span class="caret">    ^</span>
237<b>t.c:5:15: <span class="note">note:</note></b> jump bypasses setup of __block variable
238  __block int result;
239<span class="caret">              ^</span>
240</pre>
241
242<p>The fix is to rewrite the code to not require jumping into a
243<tt>__block</tt> variable's scope, e.g. by limiting that scope:</p>
244
245<pre>
246  {
247    __block int result;
248    run_specially_somehow(^{ result = c->state; });
249    return result;
250  }
251</pre>
252
253<!-- ======================================================================= -->
254<h3 id="block-variable-initialization">Non-initialization of <tt>__block</tt>
255variables</h3>
256<!-- ======================================================================= -->
257
258<p>In the following example code, the <tt>x</tt> variable is used before it is
259defined:</p>
260<pre>
261int f0() {
262  __block int x;
263  return ^(){ return x; }();
264}
265</pre>
266
267<p>By an accident of implementation, GCC and llvm-gcc unintentionally always
268zero initialized <tt>__block</tt> variables. However, any program which depends
269on this behavior is relying on unspecified compiler behavior. Programs must
270explicitly initialize all local block variables before they are used, as with
271other local variables.</p>
272
273<p>Clang does not zero initialize local block variables, and programs which rely
274on such behavior will most likely break when built with Clang.</p>
275
276
277<!-- ======================================================================= -->
278<h3 id="inline-asm">Inline assembly</h3>
279<!-- ======================================================================= -->
280
281<p>In general, Clang is highly compatible with the GCC inline assembly
282extensions, allowing the same set of constraints, modifiers and operands as GCC
283inline assembly.</p>
284
285<p>On targets that use the integrated assembler (such as most X86 targets),
286inline assembly is run through the integrated assembler instead of your system
287assembler (which is most commonly "gas", the GNU assembler).  The LLVM
288integrated assembler is extremely compatible with GAS, but there are a couple of
289minor places where it is more picky, particularly due to outright GAS bugs.</p>
290
291<p>One specific example is that the assembler rejects ambiguous X86 instructions
292that don't have suffixes.  For example:</p>
293
294<pre>
295  asm("add %al, (%rax)");
296  asm("addw $4, (%rax)");
297  asm("add $4, (%rax)");
298</pre>
299
300<p>Both clang and GAS accept the first instruction: because the first
301instruction uses the 8-bit <tt>%al</tt> register as an operand, it is clear that
302it is an 8-bit add.  The second instruction is accepted by both because the "w"
303suffix indicates that it is a 16-bit add.  The last instruction is accepted by
304GAS even though there is nothing that specifies the size of the instruction (and
305the assembler randomly picks a 32-bit add).  Because it is ambiguous, Clang
306rejects the instruction with this error message:
307</p>
308
309<pre>
310<b>&lt;inline asm&gt;:3:1: <span class="error">error:</span> ambiguous instructions require an explicit suffix (could be 'addb', 'addw', 'addl', or 'addq')</b>
311add $4, (%rax)
312<span class="caret">^</span>
313</pre>
314
315<p>To fix this compatibility issue, add an explicit suffix to the instruction:
316this makes your code more clear and is compatible with both GCC and Clang.</p>
317
318<!-- ======================================================================= -->
319<h2 id="objective-c">Objective-C compatibility</h2>
320<!-- ======================================================================= -->
321
322<!-- ======================================================================= -->
323<h3 id="super-cast">Cast of super</h3>
324<!-- ======================================================================= -->
325
326<p>GCC treats the <code>super</code> identifier as an expression that
327can, among other things, be cast to a different type. Clang treats
328<code>super</code> as a context-sensitive keyword, and will reject a
329type-cast of <code>super</code>:</p>
330
331<pre>
332<b>super.m:11:12: <span class="error">error:</span> cannot cast 'super' (it isn't an expression)</b>
333  [(Super*)super add:4];
334<span class="caret">   ~~~~~~~~^</span>
335</pre>
336
337<p>To fix this problem, remove the type cast, e.g.</p>
338<pre>
339  [super add:4];
340</pre>
341
342<!-- ======================================================================= -->
343<h3 id="sizeof-interface">Size of interfaces</h3>
344<!-- ======================================================================= -->
345
346<p>When using the "non-fragile" Objective-C ABI in use, the size of an
347Objective-C class may change over time as instance variables are added
348(or removed). For this reason, Clang rejects the application of the
349<code>sizeof</code> operator to an Objective-C class when using this
350ABI:</p>
351
352<pre>
353<b>sizeof.m:4:14: <span class="error">error:</span> invalid application of 'sizeof' to interface 'NSArray' in non-fragile ABI</b>
354  int size = sizeof(NSArray);
355<span class="caret">             ^     ~~~~~~~~~</span>
356</pre>
357
358<p>Code that relies on the size of an Objective-C class is likely to
359be broken anyway, since that size is not actually constant. To address
360this problem, use the Objective-C runtime API function
361<code>class_getInstanceSize()</code>:</p>
362
363<pre>
364  class_getInstanceSize([NSArray class])
365</pre>
366
367<!-- ======================================================================= -->
368<h3 id="objc_objs-cast">Internal Objective-C types</h3>
369<!-- ======================================================================= -->
370
371<p>GCC allows using pointers to internal Objective-C objects, <tt>struct objc_object*</tt>,
372<tt>struct objc_selector*</tt>, and <tt>struct objc_class*</tt> in place of the types
373<tt>id</tt>, <tt>SEL</tt>, and <tt>Class</tt> respectively. Clang treats the
374internal Objective-C structures as implementation detail and won't do implicit conversions:
375
376<pre>
377<b>t.mm:11:2: <span class="error">error:</span> no matching function for call to 'f'</b>
378        f((struct objc_object *)p);
379<span class="caret">        ^</span>
380<b>t.mm:5:6: <span class="note">note:</note></b> candidate function not viable: no known conversion from 'struct objc_object *' to 'id' for 1st argument
381void f(id x);
382<span class="caret">     ^</span>
383</pre>
384
385<p>Code should use types <tt>id</tt>, <tt>SEL</tt>, and <tt>Class</tt>
386instead of the internal types.</p>
387
388<!-- ======================================================================= -->
389<h3 id="c_variables-class">C variables in @interface or @protocol</h3>
390<!-- ======================================================================= -->
391
392<p>GCC allows the declaration of C variables in
393an <code>@interface</code> or <code>@protocol</code>
394declaration. Clang does not allow variable declarations to appear
395within these declarations unless they are marked <code>extern</code>.</p>
396
397<p>Variables may still be declared in an @implementation.</p>
398
399<pre>
400@interface XX
401int a;         // not allowed in clang
402int b = 1;     // not allowed in clang
403extern int c;  // allowed
404@end
405
406</pre>
407
408<!-- ======================================================================= -->
409<h2 id="cxx">C++ compatibility</h2>
410<!-- ======================================================================= -->
411
412<!-- ======================================================================= -->
413<h3 id="vla">Variable-length arrays</h3>
414<!-- ======================================================================= -->
415
416<p>GCC and C99 allow an array's size to be determined at run
417time. This extension is not permitted in standard C++. However, Clang
418supports such variable length arrays for compatibility with GNU C and
419C99 programs.</p>
420
421<p>If you would prefer not to use this extension, you can disable it with
422<tt>-Werror=vla</tt>. There are several ways to fix your code:
423
424<ol>
425<li>replace the variable length array with a fixed-size array if you can
426    determine a reasonable upper bound at compile time; sometimes this is as
427    simple as changing <tt>int size = ...;</tt> to <tt>const int size
428    = ...;</tt> (if the initializer is a compile-time constant);</li>
429<li>use <tt>std::vector</tt> or some other suitable container type;
430    or</li>
431<li>allocate the array on the heap instead using <tt>new Type[]</tt> -
432    just remember to <tt>delete[]</tt> it.</li>
433</ol>
434
435<!-- ======================================================================= -->
436<h3 id="dep_lookup">Unqualified lookup in templates</h3>
437<!-- ======================================================================= -->
438
439<p>Some versions of GCC accept the following invalid code:
440
441<pre>
442template &lt;typename T&gt; T Squared(T x) {
443  return Multiply(x, x);
444}
445
446int Multiply(int x, int y) {
447  return x * y;
448}
449
450int main() {
451  Squared(5);
452}
453</pre>
454
455<p>Clang complains:
456
457<pre>
458<b>my_file.cpp:2:10: <span class="error">error:</span> call to function 'Multiply' that is neither visible in the template definition nor found by argument-dependent lookup</b>
459  return Multiply(x, x);
460<span class="caret">         ^</span>
461<b>my_file.cpp:10:3: <span class="note">note:</span></b> in instantiation of function template specialization 'Squared&lt;int&gt;' requested here
462  Squared(5);
463<span class="caret">  ^</span>
464<b>my_file.cpp:5:5: <span class="note">note:</span></b> 'Multiply' should be declared prior to the call site
465int Multiply(int x, int y) {
466<span class="caret">    ^</span>
467</pre>
468
469<p>The C++ standard says that unqualified names like <q>Multiply</q>
470are looked up in two ways.
471
472<p>First, the compiler does <i>unqualified lookup</i> in the scope
473where the name was written.  For a template, this means the lookup is
474done at the point where the template is defined, not where it's
475instantiated.  Since <tt>Multiply</tt> hasn't been declared yet at
476this point, unqualified lookup won't find it.
477
478<p>Second, if the name is called like a function, then the compiler
479also does <i>argument-dependent lookup</i> (ADL).  (Sometimes
480unqualified lookup can suppress ADL; see [basic.lookup.argdep]p3 for
481more information.)  In ADL, the compiler looks at the types of all the
482arguments to the call.  When it finds a class type, it looks up the
483name in that class's namespace; the result is all the declarations it
484finds in those namespaces, plus the declarations from unqualified
485lookup.  However, the compiler doesn't do ADL until it knows all the
486argument types.
487
488<p>In our example, <tt>Multiply</tt> is called with dependent
489arguments, so ADL isn't done until the template is instantiated.  At
490that point, the arguments both have type <tt>int</tt>, which doesn't
491contain any class types, and so ADL doesn't look in any namespaces.
492Since neither form of lookup found the declaration
493of <tt>Multiply</tt>, the code doesn't compile.
494
495<p>Here's another example, this time using overloaded operators,
496which obey very similar rules.
497
498<pre>#include &lt;iostream&gt;
499
500template&lt;typename T&gt;
501void Dump(const T&amp; value) {
502  std::cout &lt;&lt; value &lt;&lt; "\n";
503}
504
505namespace ns {
506  struct Data {};
507}
508
509std::ostream&amp; operator&lt;&lt;(std::ostream&amp; out, ns::Data data) {
510  return out &lt;&lt; "Some data";
511}
512
513void Use() {
514  Dump(ns::Data());
515}</pre>
516
517<p>Again, Clang complains:</p>
518
519<pre>
520<b>my_file2.cpp:5:13: <span class="error">error:</span> call to function 'operator&lt;&lt;' that is neither visible in the template definition nor found by argument-dependent lookup</b>
521  std::cout &lt;&lt; value &lt;&lt; "\n";
522<span class="caret">            ^</span>
523<b>my_file2.cpp:17:3: <span class="note">note:</span></b> in instantiation of function template specialization 'Dump&lt;ns::Data&gt;' requested here
524  Dump(ns::Data());
525<span class="caret">  ^</span>
526<b>my_file2.cpp:12:15: <span class="note">note:</span></b> 'operator&lt;&lt;' should be declared prior to the call site or in namespace 'ns'
527std::ostream&amp; operator&lt;&lt;(std::ostream&amp; out, ns::Data 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
561<p>Some 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>
581<b>my_file.cpp:8:5: <span class="error">error:</span> use of undeclared identifier 'DoThis'</b>
582    DoThis(x);
583<span class="caret">    ^</span>
584    this-&gt;
585<b>my_file.cpp:2:8: <span class="note">note:</note></b> must qualify identifier to find this declaration in dependent base class
586  void DoThis(T x) {}
587<span class="caret">       ^</span>
588<b>my_file.cpp:9:5: <span class="error">error:</span> use of undeclared identifier 'DoThat'</b>
589    DoThat(x);
590<span class="caret">    ^</span>
591    this-&gt;
592<b>my_file.cpp:3:15: <span class="note">note:</note></b> 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
631<p>The 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
662<p>The 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
709<p>If 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
742An upcoming change to the C++ standard is expected to weaken this rule to only
743apply when the compiler-supplied default constructor would leave a member
744uninitialized. Clang implements the more relaxed rule in version 3.8 onwards.
745
746<!-- ======================================================================= -->
747<h3 id="param_name_lookup">Parameter name lookup</h3>
748<!-- ======================================================================= -->
749
750<p>Some versions of GCC allow the redeclaration of function parameter names within a function prototype in C++ code, e.g.</p>
751<blockquote>
752<pre>
753void f(int a, int a);
754</pre>
755</blockquote>
756<p>Clang diagnoses this error (where the parameter name has been redeclared). To fix this problem, rename one of the parameters.</p>
757
758<!-- ======================================================================= -->
759<h2 id="cxx11">C++11 compatibility</h2>
760<!-- ======================================================================= -->
761
762<!-- ======================================================================= -->
763<h3 id="deleted-special-func">Deleted special member functions</h3>
764<!-- ======================================================================= -->
765
766<p>In C++11, the explicit declaration of a move constructor or a move
767assignment operator within a class deletes the implicit declaration
768of the copy constructor and copy assignment operator. This change came
769fairly late in the C++11 standardization process, so early
770implementations of C++11 (including Clang before 3.0, GCC before 4.7,
771and Visual Studio 2010) do not implement this rule, leading them to
772accept this ill-formed code:</p>
773
774<pre>
775struct X {
776  X(X&amp;&amp;); <i>// deletes implicit copy constructor:</i>
777  <i>// X(const X&amp;) = delete;</i>
778};
779
780void f(X x);
781void g(X x) {
782  f(x); <i>// error: X has a deleted copy constructor</i>
783}
784</pre>
785
786<p>This affects some early C++11 code, including Boost's popular <a
787href="http://www.boost.org/doc/libs/release/libs/smart_ptr/shared_ptr.htm"><tt>shared_ptr</tt></a>
788up to version 1.47.0. The fix for Boost's <tt>shared_ptr</tt> is
789<a href="https://svn.boost.org/trac/boost/changeset/73202">available here</a>.</p>
790
791<!-- ======================================================================= -->
792<h2 id="objective-cxx">Objective-C++ compatibility</h2>
793<!-- ======================================================================= -->
794
795<!-- ======================================================================= -->
796<h3 id="implicit-downcasts">Implicit downcasts</h3>
797<!-- ======================================================================= -->
798
799<p>Due to a bug in its implementation, GCC allows implicit downcasts
800of Objective-C pointers (from a base class to a derived class) when
801calling functions. Such code is inherently unsafe, since the object
802might not actually be an instance of the derived class, and is
803rejected by Clang. For example, given this code:</p>
804
805<pre>
806@interface Base @end
807@interface Derived : Base @end
808
809void f(Derived *p);
810void g(Base *p) {
811  f(p);
812}
813</pre>
814
815<p>Clang produces the following error:</p>
816
817<pre>
818<b>downcast.mm:6:3: <span class="error">error:</span> no matching function for call to 'f'</b>
819  f(p);
820<span class="caret">  ^</span>
821<b>downcast.mm:4:6: <span class="note">note:</note></b> candidate function not viable: cannot convert from
822      superclass 'Base *' to subclass 'Derived *' for 1st argument
823void f(Derived *p);
824<span class="caret">     ^</span>
825</pre>
826
827<p>If the downcast is actually correct (e.g., because the code has
828already checked that the object has the appropriate type), add an
829explicit cast:</p>
830
831<pre>
832  f((Derived *)base);
833</pre>
834
835<!-- ======================================================================= -->
836<h3 id="class-as-property-name">Using <code>class</code> as a property name</h3>
837<!-- ======================================================================= -->
838
839<p>In C and Objective-C, <code>class</code> is a normal identifier and
840can be used to name fields, ivars, methods, and so on.  In
841C++, <code>class</code> is a keyword.  For compatibility with existing
842code, Clang permits <code>class</code> to be used as part of a method
843selector in Objective-C++, but this does not extend to any other part
844of the language.  In particular, it is impossible to use property dot
845syntax in Objective-C++ with the property name <code>class</code>, so
846the following code will fail to parse:</p>
847
848<pre>
849@interface I {
850int cls;
851}
852+ (int)class;
853@end
854
855@implementation  I
856- (int) Meth { return I.class; }
857@end
858</pre>
859
860<p>Use explicit message-send syntax instead, i.e. <code>[I class]</code>.</p>
861
862</div>
863</body>
864</html>
865