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18<!-- ======================================================================= -->
19<h1>Clang's C++ Compatibility</h1>
20<!-- ======================================================================= -->
21
22<ul>
23<li><a href="#intro">Introduction</a></li>
24<li><a href="#vla">Variable-length arrays</a></li>
25<li><a href="#init_static_const">Initialization of non-integral static const data members within a class definition</a></li>
John McCall489722f2010-03-17 07:10:56 +0000[diff] [blame]26<li><a href="#dep_lookup">Unqualified lookup in templates</a></li>
27<li><a href="#dep_lookup_bases">Unqualified lookup into dependent bases of class templates</a></li>
John McCall5dd52ac2010-04-09 01:07:07 +0000[diff] [blame]28<li><a href="#bad_templates">Templates with no valid instantiations</a></li>
Rafael Espindola9b2fc952010-03-17 04:31:53 +0000[diff] [blame]29<li><a href="#default_init_const">Default initialization of const variable of a class type requires user-defined default constructor</a></li>
30</ul>
31
32<!-- ======================================================================= -->
33<h2 id="intro">Introduction</h2>
34<!-- ======================================================================= -->
35
36<p>Clang strives to strictly conform to the C++ standard. That means
37it will reject invalid C++ code that another compiler may accept.
38This page helps you decide whether a Clang error message means a
39C++-conformance bug in your code and how you can fix it.</p>
40
41<!-- ======================================================================= -->
42<h2 id="vla">Variable-length arrays</h2>
43<!-- ======================================================================= -->
44
Douglas Gregor0fddb972010-05-22 16:17:30 +0000[diff] [blame]45<p>GCC and C99 allow an array's size to be determined at run
46time. This extension is not permitted in standard C++. However, Clang
47supports such variable length arrays in very limited circumstances for
48compatibility with GNU C and C99 programs:</p>
49
50<ul>
51 <li>The element type of a variable length array must be a POD
52 ("plain old data") type, which means that it cannot have any
53 user-declared constructors or destructors, base classes, or any
54 members if non-POD type. All C types are POD types.</li>
55
Douglas Gregora481ec42010-05-23 19:57:01 +0000[diff] [blame]56 <li>Variable length arrays cannot be used as the type of a non-type
57template parameter.</li> </ul>
Douglas Gregor0fddb972010-05-22 16:17:30 +0000[diff] [blame]58
59<p>If your code uses variable length arrays in a manner that Clang doesn't support, there are several ways to fix your code:
Rafael Espindola9b2fc952010-03-17 04:31:53 +0000[diff] [blame]60
61<ol>
Douglas Gregor0fddb972010-05-22 16:17:30 +0000[diff] [blame]62<li>replace the variable length array with a fixed-size array if you can
63 determine a
Rafael Espindola9b2fc952010-03-17 04:31:53 +0000[diff] [blame]64 reasonable upper bound at compile time; sometimes this is as
65 simple as changing <tt>int size = ...;</tt> to <tt>const int size
66 = ...;</tt> (if the definition of <tt>size</tt> is a compile-time
67 integral constant);</li>
68<li>use an <tt>std::string</tt> instead of a <tt>char []</tt>;</li>
69<li>use <tt>std::vector</tt> or some other suitable container type;
70 or</li>
71<li>allocate the array on the heap instead using <tt>new Type[]</tt> -
Chandler Carruth6243e332010-03-17 05:46:21 +0000[diff] [blame]72 just remember to <tt>delete[]</tt> it.</li>
Rafael Espindola9b2fc952010-03-17 04:31:53 +0000[diff] [blame]73</ol>
74
75<!-- ======================================================================= -->
76<h2 id="init_static_const">Initialization of non-integral static const data members within a class definition</h2>
77<!-- ======================================================================= -->
78
79The following code is ill-formed in C++'03:
80
81<pre>
82class SomeClass {
83 public:
84 static const double SomeConstant = 0.5;
85};
86
87const double SomeClass::SomeConstant;
88</pre>
89
90Clang errors with something similar to:
91
92<pre>
93.../your_file.h:42:42: error: 'SomeConstant' can only be initialized if it is a static const integral data member
94 static const double SomeConstant = 0.5;
95 ^ ~~~
96</pre>
97
98Only <i>integral</i> constant expressions are allowed as initializers
99within the class definition. See C++'03 [class.static.data] p4 for the
100details of this restriction. The fix here is straightforward: move
101the initializer to the definition of the static data member, which
102must exist outside of the class definition:
103
104<pre>
105class SomeClass {
106 public:
107 static const double SomeConstant;
108};
109
110const double SomeClass::SomeConstant<b> = 0.5</b>;
111</pre>
112
John McCall489722f2010-03-17 07:10:56 +0000[diff] [blame]113Note that the forthcoming C++0x standard will allow this.
114
Rafael Espindola9b2fc952010-03-17 04:31:53 +0000[diff] [blame]115<!-- ======================================================================= -->
John McCall489722f2010-03-17 07:10:56 +0000[diff] [blame]116<h2 id="dep_lookup">Unqualified lookup in templates</h2>
Rafael Espindola9b2fc952010-03-17 04:31:53 +0000[diff] [blame]117<!-- ======================================================================= -->
118
119Some versions of GCC accept the following invalid code:
120
121<pre>
John McCall489722f2010-03-17 07:10:56 +0000[diff] [blame]122template &lt;typename T&gt; struct Foo {
123 void Work(T x) {
124 func(x);
125 }
126};
127...
128void func(int x);
129...
130template struct Foo&lt;int&gt;; // or anything else that instantiates Foo&lt;int&gt;::Work
131</pre>
Rafael Espindola9b2fc952010-03-17 04:31:53 +0000[diff] [blame]132
John McCall489722f2010-03-17 07:10:56 +0000[diff] [blame]133The standard says that unqualified names like <tt>func</tt> are looked up
134when the template is defined, not when it's instantiated. Since
135<tt>void func(int)</tt> was not declared yet when <tt>Foo</tt> was
136defined, it's not considered. The fix is usually to
137declare <tt>func</tt> before <tt>Foo</tt>.
138
139<p>This is complicated by <i>argument-dependent lookup</i> (ADL),
140which is done when unqualified names are called as functions,
141like <tt>func(x)</tt> above. The standard says that ADL is performed
142in both places if any of the arguments are type-dependent, like
143<tt>x</tt> is in this example. However, ADL does nothing for builtin
144types like <tt>int</tt>, so the example is still invalid. See
145[basic.lookup.argdep] for more information.
146
147<!-- ======================================================================= -->
148<h2 id="dep_lookup_bases">Unqualified lookup into dependent bases of class templates</h2>
149<!-- ======================================================================= -->
150
151Some versions of GCC accept the following invalid code:
152
153<pre>
154template &lt;typename T&gt; struct Base {
155 void DoThis(T x) {}
Rafael Espindola9b2fc952010-03-17 04:31:53 +0000[diff] [blame]156 static void DoThat(T x) {}
157};
158
John McCall489722f2010-03-17 07:10:56 +0000[diff] [blame]159template &lt;typename T&gt; struct Derived : public Base&lt;T&gt; {
Rafael Espindola9b2fc952010-03-17 04:31:53 +0000[diff] [blame]160 void Work(T x) {
161 DoThis(x); // Invalid!
162 DoThat(x); // Invalid!
163 }
164};
Rafael Espindola9b2fc952010-03-17 04:31:53 +0000[diff] [blame]165</pre>
166
John McCall489722f2010-03-17 07:10:56 +0000[diff] [blame]167Clang correctly rejects it with the following errors
168(when <tt>Derived</tt> is eventually instantiated):
Rafael Espindola9b2fc952010-03-17 04:31:53 +0000[diff] [blame]169
170<pre>
John McCall489722f2010-03-17 07:10:56 +0000[diff] [blame]171my_file.cpp:8:5: error: use of undeclared identifier 'DoThis'
Rafael Espindola9b2fc952010-03-17 04:31:53 +0000[diff] [blame]172 DoThis(x);
173 ^
174 this-&gt;
John McCall489722f2010-03-17 07:10:56 +0000[diff] [blame]175my_file.cpp:2:8: note: must qualify identifier to find this declaration in dependent base class
Rafael Espindola9b2fc952010-03-17 04:31:53 +0000[diff] [blame]176 void DoThis(T x) {}
177 ^
John McCall489722f2010-03-17 07:10:56 +0000[diff] [blame]178my_file.cpp:9:5: error: use of undeclared identifier 'DoThat'
Rafael Espindola9b2fc952010-03-17 04:31:53 +0000[diff] [blame]179 DoThat(x);
180 ^
181 this-&gt;
John McCall489722f2010-03-17 07:10:56 +0000[diff] [blame]182my_file.cpp:3:15: note: must qualify identifier to find this declaration in dependent base class
Rafael Espindola9b2fc952010-03-17 04:31:53 +0000[diff] [blame]183 static void DoThat(T x) {}
184</pre>
185
John McCall489722f2010-03-17 07:10:56 +0000[diff] [blame]186Like we said <a href="#dep_lookup">above</a>, unqualified names like
187<tt>DoThis</tt> and <tt>DoThat</tt> are looked up when the template
188<tt>Derived</tt> is defined, not when it's instantiated. When we look
189up a name used in a class, we usually look into the base classes.
190However, we can't look into the base class <tt>Base&lt;T&gt;</tt>
191because its type depends on the template argument <tt>T</tt>, so the
192standard says we should just ignore it. See [temp.dep]p3 for details.
193
194<p>The fix, as Clang tells you, is to tell the compiler that we want a
195class member by prefixing the calls with <tt>this-&gt;</tt>:
Rafael Espindola9b2fc952010-03-17 04:31:53 +0000[diff] [blame]196
197<pre>
Rafael Espindola9b2fc952010-03-17 04:31:53 +0000[diff] [blame]198 void Work(T x) {
199 <b>this-&gt;</b>DoThis(x);
200 <b>this-&gt;</b>DoThat(x);
201 }
Rafael Espindola9b2fc952010-03-17 04:31:53 +0000[diff] [blame]202</pre>
203
John McCall489722f2010-03-17 07:10:56 +0000[diff] [blame]204Alternatively, you can tell the compiler exactly where to look:
Rafael Espindola9b2fc952010-03-17 04:31:53 +0000[diff] [blame]205
206<pre>
207 void Work(T x) {
John McCall489722f2010-03-17 07:10:56 +0000[diff] [blame]208 <b>Base&lt;T&gt;</b>::DoThis(x);
Rafael Espindola9b2fc952010-03-17 04:31:53 +0000[diff] [blame]209 <b>Base&lt;T&gt;</b>::DoThat(x);
210 }
211</pre>
212
John McCall489722f2010-03-17 07:10:56 +0000[diff] [blame]213This works whether the methods are static or not, but be careful:
214if <tt>DoThis</tt> is virtual, calling it this way will bypass virtual
215dispatch!
216
Rafael Espindola9b2fc952010-03-17 04:31:53 +0000[diff] [blame]217<!-- ======================================================================= -->
John McCall5dd52ac2010-04-09 01:07:07 +0000[diff] [blame]218<h2 id="bad_templates">Templates with no valid instantiations</h2>
219<!-- ======================================================================= -->
220
221The following code contains a typo: the programmer
222meant <tt>init()</tt> but wrote <tt>innit()</tt> instead.
223
224<pre>
225 template &lt;class T&gt; class Processor {
226 ...
227 void init();
228 ...
229 };
230 ...
231 template &lt;class T&gt; void process() {
232 Processor&lt;T&gt; processor;
233 processor.innit(); // <-- should be 'init()'
234 ...
235 }
236</pre>
237
238Unfortunately, we can't flag this mistake as soon as we see it: inside
239a template, we're not allowed to make assumptions about "dependent
240types" like <tt>Processor&lt;T&gt;</tt>. Suppose that later on in
241this file the programmer adds an explicit specialization
242of <tt>Processor</tt>, like so:
243
244<pre>
245 template &lt;&gt; class Processor&lt;char*&gt; {
246 void innit();
247 };
248</pre>
249
250Now the program will work &mdash; as long as the programmer only ever
251instantiates <tt>process()</tt> with <tt>T = char*</tt>! This is why
252it's hard, and sometimes impossible, to diagnose mistakes in a
253template definition before it's instantiated.
254
255<p>The standard says that a template with no valid instantiations is
256ill-formed. Clang tries to do as much checking as possible at
257definition-time instead of instantiation-time: not only does this
258produce clearer diagnostics, but it also substantially improves
259compile times when using pre-compiled headers. The downside to this
260philosophy is that Clang sometimes fails to process files because they
261contain broken templates that are no longer used. The solution is
262simple: since the code is unused, just remove it.
263
264<!-- ======================================================================= -->
Rafael Espindola9b2fc952010-03-17 04:31:53 +0000[diff] [blame]265<h2 id="default_init_const">Default initialization of const variable of a class type requires user-defined default constructor</h2>
266<!-- ======================================================================= -->
267
268If a <tt>class</tt> or <tt>struct</tt> has no user-defined default
269constructor, C++ doesn't allow you to default construct a <tt>const</tt>
270instance of it like this ([dcl.init], p9):
271
272<pre>
273class Foo {
274 public:
275 // The compiler-supplied default constructor works fine, so we
276 // don't bother with defining one.
277 ...
278};
279
280void Bar() {
281 const Foo foo; // Error!
282 ...
283}
284</pre>
285
286To fix this, you can define a default constructor for the class:
287
288<pre>
289class Foo {
290 public:
291 Foo() {}
292 ...
293};
294
295void Bar() {
296 const Foo foo; // Now the compiler is happy.
297 ...
298}
299</pre>
300
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