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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>
35 <li><a href="#lvalue-cast">Lvalue casts</a></li>
Daniel Dunbar5a410212010-09-02 21:35:16 +0000[diff] [blame^]36 <li><a href="#blocks-in-protected-scope">Jumps to within <tt>__block</tt> variable scope</a></li>
Douglas Gregorc41b6ff2010-06-30 22:01:08 +0000[diff] [blame]37 </ul>
38 </li>
39 <li><a href="#objective-c">Objective-C compatibility</a>
40 <ul>
41 <li><a href="#super-cast">Cast of super</a></li>
42 <li><a href="#sizeof-interface">Size of interfaces</a></li>
43 </ul>
44 </li>
45 <li><a href="#c++">C++ compatibility</a>
46 <ul>
47 <li><a href="#vla">Variable-length arrays</a></li>
48 <li><a href="#init_static_const">Initialization of non-integral static const data members within a class definition</a></li>
49 <li><a href="#dep_lookup">Unqualified lookup in templates</a></li>
50 <li><a href="#dep_lookup_bases">Unqualified lookup into dependent bases of class templates</a></li>
51 <li><a href="#undep_incomplete">Incomplete types in templates</a></li>
52 <li><a href="#bad_templates">Templates with no valid instantiations</a></li>
53 <li><a href="#default_init_const">Default initialization of const
54 variable of a class type requires user-defined default
55 constructor</a></li>
56 </ul>
57 </li>
58 <li><a href="#objective-c++">Objective-C++ compatibility</a>
59 <ul>
60 <li><a href="#implicit-downcasts">Implicit downcasts</a></li>
61 </ul>
Fariborz Jahanian36e738a2010-08-11 18:57:26 +0000[diff] [blame]62 <ul>
63 <li><a href="#Use of class as method name">Use of class as method name</a></li>
64 </ul>
Douglas Gregorc41b6ff2010-06-30 22:01:08 +0000[diff] [blame]65 </li>
66</ul>
67
68<!-- ======================================================================= -->
69<h2 id="c">C compatibility</h3>
70<!-- ======================================================================= -->
71
72<!-- ======================================================================= -->
73<h3 id="inline">C99 inline functions</h3>
74<!-- ======================================================================= -->
75<p>By default, Clang builds C code according to the C99 standard,
76which provides different inlining semantics than GCC's default
77behavior. For example, when compiling the following code with no optimization:</p>
78<pre>
79inline int add(int i, int j) { return i + j; }
80
81int main() {
82 int i = add(4, 5);
83 return i;
84}
85</pre>
86
87<p>In C99, this is an incomplete (incorrect) program because there is
88no external definition of the <code>add</code> function: the inline
89definition is only used for optimization, if the compiler decides to
90perform inlining. Therefore, we will get a (correct) link-time error
91with Clang, e.g.:</p>
92
93<pre>
94Undefined symbols:
95 "_add", referenced from:
96 _main in cc-y1jXIr.o
97</pre>
98
99<p>There are several ways to fix this problem:</p>
100
101<ul>
Douglas Gregorc41b6ff2010-06-30 22:01:08 +0000[diff] [blame]102 <li>Change <code>add</code> to a <code>static inline</code>
103 function. Static inline functions are always resolved within the
104 translation unit, so you won't have to add an external, non-inline
105 definition of the function elsewhere in your program.</li>
106
Douglas Gregorff6f66e2010-06-30 22:43:03 +0000[diff] [blame]107 <li>Provide an external (non-inline) definition of <code>add</code>
108 somewhere in your program.</li>
109
Douglas Gregorc41b6ff2010-06-30 22:01:08 +0000[diff] [blame]110 <li>Compile with the GNU89 dialect by adding
111 <code>-std=gnu89</code> to the set of Clang options. This option is
112 only recommended if the program source cannot be changed or if the
113 program also relies on additional C89-specific behavior that cannot
114 be changed.</li>
115</ul>
116
117<!-- ======================================================================= -->
118<h3 id="lvalue-cast">Lvalue casts</h3>
119<!-- ======================================================================= -->
120
Douglas Gregor6f1adba2010-06-30 22:38:37 +0000[diff] [blame]121<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]122different type. Clang produces an error on similar code, e.g.,</p>
123
124<pre>
125lvalue.c:2:3: error: assignment to cast is illegal, lvalue casts are not
126 supported
127 (int*)addr = val;
128 ^~~~~~~~~~ ~
129</pre>
130
131<p>To fix this problem, move the cast to the right-hand side. In this
132example, one could use:</p>
133
134<pre>
135 addr = (float *)val;
136</pre>
137
138<!-- ======================================================================= -->
Daniel Dunbar5a410212010-09-02 21:35:16 +0000[diff] [blame^]139<h3 id="blocks-in-protected-scope">Jumps to within <tt>__block</tt> variable scope</h3>
140<!-- ======================================================================= -->
141
142<p>Clang disallows jumps into the scope of a <tt>__block</tt> variable, similar
143to the manner in which both GCC and Clang disallow jumps into the scope of
144variables which have user defined constructors (in C++).</p>
145
146<p>Variables marked with <tt>__block</tt> require special runtime initialization
147before they can be used. A jump into the scope of a <tt>__block</tt> variable
148would bypass this initialization and therefore the variable cannot safely be
149used.</p>
150
151<p>For example, consider the following code fragment:</p>
152
153<pre>
154int f0(int c) {
155 if (c)
156 goto error;
157
158 __block int x;
159 x = 1;
160 return x;
161
162 error:
163 x = 0;
164 return x;
165}
166</pre>
167
168<p>GCC accepts this code, but it will crash at runtime along the error path,
169because the runtime setup for the storage backing the <tt>x</tt> variable will
170not have been initialized. Clang rejects this code with a hard error:</p>
171
172<pre>
173t.c:3:5: error: goto into protected scope
174 goto error;
175 ^
176t.c:5:15: note: jump bypasses setup of __block variable
177 __block int x;
178 ^
179</pre>
180
181<p>Some instances of this construct may be safe if the variable is never used
182after the jump target, however the protected scope checker does not check the
183uses of the varaible, only the scopes in which it is visible. You should rewrite
184your code to put the <tt>__block</tt> variables in a scope which is only visible
185where they are used.</p>
186
187<!-- ======================================================================= -->
Douglas Gregorc41b6ff2010-06-30 22:01:08 +0000[diff] [blame]188<h2 id="objective-c">Objective-C compatibility</h3>
189<!-- ======================================================================= -->
190
191<!-- ======================================================================= -->
192<h3 id="super-cast">Cast of super</h3>
193<!-- ======================================================================= -->
194
195<p>GCC treats the <code>super</code> identifier as an expression that
196can, among other things, be cast to a different type. Clang treats
197<code>super</code> as a context-sensitive keyword, and will reject a
198type-cast of <code>super</code>:</p>
199
200<pre>
201super.m:11:12: error: cannot cast 'super' (it isn't an expression)
202 [(Super*)super add:4];
203 ~~~~~~~~^
204</pre>
205
206<p>To fix this problem, remove the type cast, e.g.</p>
207<pre>
208 [super add:4];
209</pre>
210
211<!-- ======================================================================= -->
212<h3 id="sizeof-interface">Size of interfaces</h3>
213<!-- ======================================================================= -->
214
215<p>When using the "non-fragile" Objective-C ABI in use, the size of an
216Objective-C class may change over time as instance variables are added
217(or removed). For this reason, Clang rejects the application of the
218<code>sizeof</code> operator to an Objective-C class when using this
219ABI:</p>
220
221<pre>
222sizeof.m:4:14: error: invalid application of 'sizeof' to interface 'NSArray' in
223 non-fragile ABI
224 int size = sizeof(NSArray);
225 ^ ~~~~~~~~~
226</pre>
227
228<p>Code that relies on the size of an Objective-C class is likely to
229be broken anyway, since that size is not actually constant. To address
230this problem, use the Objective-C runtime API function
Benjamin Kramere6617502010-06-30 22:29:56 +0000[diff] [blame]231<code>class_getInstanceSize()</code>:</p>
Douglas Gregorc41b6ff2010-06-30 22:01:08 +0000[diff] [blame]232
233<pre>
234 class_getInstanceSize([NSArray class])
235</pre>
236
237<!-- ======================================================================= -->
238<h2 id="c++">C++ compatibility</h3>
239<!-- ======================================================================= -->
240
241<!-- ======================================================================= -->
242<h3 id="vla">Variable-length arrays</h3>
243<!-- ======================================================================= -->
244
245<p>GCC and C99 allow an array's size to be determined at run
246time. This extension is not permitted in standard C++. However, Clang
247supports such variable length arrays in very limited circumstances for
248compatibility with GNU C and C99 programs:</p>
249
250<ul>
251 <li>The element type of a variable length array must be a POD
252 ("plain old data") type, which means that it cannot have any
253 user-declared constructors or destructors, base classes, or any
254 members if non-POD type. All C types are POD types.</li>
255
256 <li>Variable length arrays cannot be used as the type of a non-type
257template parameter.</li> </ul>
258
259<p>If your code uses variable length arrays in a manner that Clang doesn't support, there are several ways to fix your code:
260
261<ol>
262<li>replace the variable length array with a fixed-size array if you can
263 determine a
264 reasonable upper bound at compile time; sometimes this is as
265 simple as changing <tt>int size = ...;</tt> to <tt>const int size
266 = ...;</tt> (if the definition of <tt>size</tt> is a compile-time
267 integral constant);</li>
268<li>use an <tt>std::string</tt> instead of a <tt>char []</tt>;</li>
269<li>use <tt>std::vector</tt> or some other suitable container type;
270 or</li>
271<li>allocate the array on the heap instead using <tt>new Type[]</tt> -
272 just remember to <tt>delete[]</tt> it.</li>
273</ol>
274
275<!-- ======================================================================= -->
276<h3 id="init_static_const">Initialization of non-integral static const data members within a class definition</h3>
277<!-- ======================================================================= -->
278
279The following code is ill-formed in C++'03:
280
281<pre>
282class SomeClass {
283 public:
284 static const double SomeConstant = 0.5;
285};
286
287const double SomeClass::SomeConstant;
288</pre>
289
290Clang errors with something similar to:
291
292<pre>
293.../your_file.h:42:42: error: 'SomeConstant' can only be initialized if it is a static const integral data member
294 static const double SomeConstant = 0.5;
295 ^ ~~~
296</pre>
297
298Only <i>integral</i> constant expressions are allowed as initializers
299within the class definition. See C++'03 [class.static.data] p4 for the
300details of this restriction. The fix here is straightforward: move
301the initializer to the definition of the static data member, which
302must exist outside of the class definition:
303
304<pre>
305class SomeClass {
306 public:
307 static const double SomeConstant;
308};
309
310const double SomeClass::SomeConstant<b> = 0.5</b>;
311</pre>
312
313Note that the forthcoming C++0x standard will allow this.
314
315<!-- ======================================================================= -->
316<h3 id="dep_lookup">Unqualified lookup in templates</h3>
317<!-- ======================================================================= -->
318
319<p>Some versions of GCC accept the following invalid code:
320
321<pre>
322template &lt;typename T&gt; T Squared(T x) {
323 return Multiply(x, x);
324}
325
326int Multiply(int x, int y) {
327 return x * y;
328}
329
330int main() {
331 Squared(5);
332}
333</pre>
334
335<p>Clang complains:
336
337<pre> <b>my_file.cpp:2:10: <span class="error">error:</span> use of undeclared identifier 'Multiply'</b>
338 return Multiply(x, x);
339 <span class="caret"> ^</span>
340
341 <b>my_file.cpp:10:3: <span class="note">note:</span> in instantiation of function template specialization 'Squared&lt;int&gt;' requested here</b>
342 Squared(5);
343 <span class="caret"> ^</span>
344</pre>
345
346<p>The C++ standard says that unqualified names like <q>Multiply</q>
347are looked up in two ways.
348
349<p>First, the compiler does <i>unqualified lookup</i> in the scope
350where the name was written. For a template, this means the lookup is
351done at the point where the template is defined, not where it's
352instantiated. Since <tt>Multiply</tt> hasn't been declared yet at
353this point, unqualified lookup won't find it.
354
355<p>Second, if the name is called like a function, then the compiler
356also does <i>argument-dependent lookup</i> (ADL). (Sometimes
357unqualified lookup can suppress ADL; see [basic.lookup.argdep]p3 for
358more information.) In ADL, the compiler looks at the types of all the
359arguments to the call. When it finds a class type, it looks up the
360name in that class's namespace; the result is all the declarations it
361finds in those namespaces, plus the declarations from unqualified
362lookup. However, the compiler doesn't do ADL until it knows all the
363argument types.
364
365<p>In our example, <tt>Multiply</tt> is called with dependent
366arguments, so ADL isn't done until the template is instantiated. At
367that point, the arguments both have type <tt>int</tt>, which doesn't
368contain any class types, and so ADL doesn't look in any namespaces.
369Since neither form of lookup found the declaration
370of <tt>Multiply</tt>, the code doesn't compile.
371
372<p>Here's another example, this time using overloaded operators,
373which obey very similar rules.
374
375<pre>#include &lt;iostream&gt;
376
377template&lt;typename T&gt;
378void Dump(const T&amp; value) {
379 std::cout &lt;&lt; value &lt;&lt; "\n";
380}
381
382namespace ns {
383 struct Data {};
384}
385
386std::ostream&amp; operator&lt;&lt;(std::ostream&amp; out, ns::Data data) {
387 return out &lt;&lt; "Some data";
388}
389
390void Use() {
391 Dump(ns::Data());
392}</pre>
393
394<p>Again, Clang complains about not finding a matching function:</p>
395
396<pre>
397<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>
398 std::cout &lt;&lt; value &lt;&lt; "\n";
399 <span class="caret">~~~~~~~~~ ^ ~~~~~</span>
400<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>
401 Dump(ns::Data());
402 <span class="caret">^</span>
403</pre>
404
405<p>Just like before, unqualified lookup didn't find any declarations
406with the name <tt>operator&lt;&lt;</tt>. Unlike before, the argument
407types both contain class types: one of them is an instance of the
408class template type <tt>std::basic_ostream</tt>, and the other is the
409type <tt>ns::Data</tt> that we declared above. Therefore, ADL will
410look in the namespaces <tt>std</tt> and <tt>ns</tt> for
411an <tt>operator&lt;&lt;</tt>. Since one of the argument types was
412still dependent during the template definition, ADL isn't done until
413the template is instantiated during <tt>Use</tt>, which means that
414the <tt>operator&lt;&lt;</tt> we want it to find has already been
415declared. Unfortunately, it was declared in the global namespace, not
416in either of the namespaces that ADL will look in!
417
418<p>There are two ways to fix this problem:</p>
419<ol><li>Make sure the function you want to call is declared before the
420template that might call it. This is the only option if none of its
421argument types contain classes. You can do this either by moving the
422template definition, or by moving the function definition, or by
423adding a forward declaration of the function before the template.</li>
424<li>Move the function into the same namespace as one of its arguments
425so that ADL applies.</li></ol>
426
427<p>For more information about argument-dependent lookup, see
428[basic.lookup.argdep]. For more information about the ordering of
429lookup in templates, see [temp.dep.candidate].
430
431<!-- ======================================================================= -->
432<h3 id="dep_lookup_bases">Unqualified lookup into dependent bases of class templates</h3>
433<!-- ======================================================================= -->
434
435Some versions of GCC accept the following invalid code:
436
437<pre>
438template &lt;typename T&gt; struct Base {
439 void DoThis(T x) {}
440 static void DoThat(T x) {}
441};
442
443template &lt;typename T&gt; struct Derived : public Base&lt;T&gt; {
444 void Work(T x) {
445 DoThis(x); // Invalid!
446 DoThat(x); // Invalid!
447 }
448};
449</pre>
450
451Clang correctly rejects it with the following errors
452(when <tt>Derived</tt> is eventually instantiated):
453
454<pre>
455my_file.cpp:8:5: error: use of undeclared identifier 'DoThis'
456 DoThis(x);
457 ^
458 this-&gt;
459my_file.cpp:2:8: note: must qualify identifier to find this declaration in dependent base class
460 void DoThis(T x) {}
461 ^
462my_file.cpp:9:5: error: use of undeclared identifier 'DoThat'
463 DoThat(x);
464 ^
465 this-&gt;
466my_file.cpp:3:15: note: must qualify identifier to find this declaration in dependent base class
467 static void DoThat(T x) {}
468</pre>
469
470Like we said <a href="#dep_lookup">above</a>, unqualified names like
471<tt>DoThis</tt> and <tt>DoThat</tt> are looked up when the template
472<tt>Derived</tt> is defined, not when it's instantiated. When we look
473up a name used in a class, we usually look into the base classes.
474However, we can't look into the base class <tt>Base&lt;T&gt;</tt>
475because its type depends on the template argument <tt>T</tt>, so the
476standard says we should just ignore it. See [temp.dep]p3 for details.
477
478<p>The fix, as Clang tells you, is to tell the compiler that we want a
479class member by prefixing the calls with <tt>this-&gt;</tt>:
480
481<pre>
482 void Work(T x) {
483 <b>this-&gt;</b>DoThis(x);
484 <b>this-&gt;</b>DoThat(x);
485 }
486</pre>
487
488Alternatively, you can tell the compiler exactly where to look:
489
490<pre>
491 void Work(T x) {
492 <b>Base&lt;T&gt;</b>::DoThis(x);
493 <b>Base&lt;T&gt;</b>::DoThat(x);
494 }
495</pre>
496
497This works whether the methods are static or not, but be careful:
498if <tt>DoThis</tt> is virtual, calling it this way will bypass virtual
499dispatch!
500
501<!-- ======================================================================= -->
502<h3 id="undep_incomplete">Incomplete types in templates</h3>
503<!-- ======================================================================= -->
504
505The following code is invalid, but compilers are allowed to accept it:
506
507<pre>
508 class IOOptions;
509 template &lt;class T&gt; bool read(T &amp;value) {
510 IOOptions opts;
511 return read(opts, value);
512 }
513
514 class IOOptions { bool ForceReads; };
515 bool read(const IOOptions &amp;opts, int &amp;x);
516 template bool read&lt;&gt;(int &amp;);
517</pre>
518
519The standard says that types which don't depend on template parameters
520must be complete when a template is defined if they affect the
521program's behavior. However, the standard also says that compilers
522are free to not enforce this rule. Most compilers enforce it to some
523extent; for example, it would be an error in GCC to
524write <tt>opts.ForceReads</tt> in the code above. In Clang, we feel
525that enforcing the rule consistently lets us provide a better
526experience, but unfortunately it also means we reject some code that
527other compilers accept.
528
529<p>We've explained the rule here in very imprecise terms; see
530[temp.res]p8 for details.
531
532<!-- ======================================================================= -->
533<h3 id="bad_templates">Templates with no valid instantiations</h3>
534<!-- ======================================================================= -->
535
536The following code contains a typo: the programmer
537meant <tt>init()</tt> but wrote <tt>innit()</tt> instead.
538
539<pre>
540 template &lt;class T&gt; class Processor {
541 ...
542 void init();
543 ...
544 };
545 ...
546 template &lt;class T&gt; void process() {
547 Processor&lt;T&gt; processor;
548 processor.innit(); // <-- should be 'init()'
549 ...
550 }
551</pre>
552
553Unfortunately, we can't flag this mistake as soon as we see it: inside
554a template, we're not allowed to make assumptions about "dependent
555types" like <tt>Processor&lt;T&gt;</tt>. Suppose that later on in
556this file the programmer adds an explicit specialization
557of <tt>Processor</tt>, like so:
558
559<pre>
560 template &lt;&gt; class Processor&lt;char*&gt; {
561 void innit();
562 };
563</pre>
564
565Now the program will work &mdash; as long as the programmer only ever
566instantiates <tt>process()</tt> with <tt>T = char*</tt>! This is why
567it's hard, and sometimes impossible, to diagnose mistakes in a
568template definition before it's instantiated.
569
570<p>The standard says that a template with no valid instantiations is
571ill-formed. Clang tries to do as much checking as possible at
572definition-time instead of instantiation-time: not only does this
573produce clearer diagnostics, but it also substantially improves
574compile times when using pre-compiled headers. The downside to this
575philosophy is that Clang sometimes fails to process files because they
576contain broken templates that are no longer used. The solution is
577simple: since the code is unused, just remove it.
578
579<!-- ======================================================================= -->
580<h3 id="default_init_const">Default initialization of const variable of a class type requires user-defined default constructor</h3>
581<!-- ======================================================================= -->
582
583If a <tt>class</tt> or <tt>struct</tt> has no user-defined default
584constructor, C++ doesn't allow you to default construct a <tt>const</tt>
585instance of it like this ([dcl.init], p9):
586
587<pre>
588class Foo {
589 public:
590 // The compiler-supplied default constructor works fine, so we
591 // don't bother with defining one.
592 ...
593};
594
595void Bar() {
596 const Foo foo; // Error!
597 ...
598}
599</pre>
600
601To fix this, you can define a default constructor for the class:
602
603<pre>
604class Foo {
605 public:
606 Foo() {}
607 ...
608};
609
610void Bar() {
611 const Foo foo; // Now the compiler is happy.
612 ...
613}
614</pre>
615
616<!-- ======================================================================= -->
617<h2 id="objective-c++">Objective-C++ compatibility</h3>
618<!-- ======================================================================= -->
619
620<!-- ======================================================================= -->
621<h3 id="implicit-downcasts">Implicit downcasts</h3>
622<!-- ======================================================================= -->
623
624<p>Due to a bug in its implementation, GCC allows implicit downcasts
625(from base class to a derived class) when calling functions. Such code is
626inherently unsafe, since the object might not actually be an instance
627of the derived class, and is rejected by Clang. For example, given
628this code:</p>
629
630<pre>
631@interface Base @end
632@interface Derived : Base @end
633
634void f(Derived *);
635void g(Base *base) {
636 f(base);
637}
638</pre>
639
640<p>Clang produces the following error:</p>
641
642<pre>
643downcast.mm:6:3: error: no matching function for call to 'f'
644 f(base);
645 ^
Douglas Gregor92bc0272010-07-01 03:50:01 +0000[diff] [blame]646downcast.mm:4:6: note: candidate function not viable: cannot convert from
647 superclass 'Base *' to subclass 'Derived *' for 1st argument
Douglas Gregorc41b6ff2010-06-30 22:01:08 +0000[diff] [blame]648void f(Derived *);
649 ^
650</pre>
651
652<p>If the downcast is actually correct (e.g., because the code has
653already checked that the object has the appropriate type), add an
654explicit cast:</p>
655
656<pre>
657 f((Derived *)base);
658</pre>
659
Fariborz Jahanian36e738a2010-08-11 18:57:26 +0000[diff] [blame]660<!-- ======================================================================= -->
661<h3 id="Use of class as method name">Use of class as method name</h3>
662<!-- ======================================================================= -->
663
664<p>Use of 'class' name to declare a method is allowed in objective-c++ mode to
665be compatible with GCC. However, use of property dot syntax notation to call
666this method is not allowed in clang++, as [I class] is a suitable syntax that
667will work. So, this test will fail in clang++.
668
669<pre>
670@interface I {
671int cls;
672}
673+ (int)class;
674@end
675
676@implementation I
677- (int) Meth { return I.class; }
678@end
679<pre>
680
681
Douglas Gregorc41b6ff2010-06-30 22:01:08 +0000[diff] [blame]682</div>
683</body>
684</html>