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New language-compatibility page, including language-compatibility information for the various language dialects Clang supports in a single, easy-to-find page llvm-svn: 107325
2010-07-01 06:01:08 +08:00
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<!-- ======================================================================= -->
<h1>Language Compatibility</h1>
<!-- ======================================================================= -->
<p>Clang strives to both conform to current language standards (C99,
C++98) and also to implement many widely-used extensions available
in other compilers, so that most correct code will "just work" when
compiler with Clang. However, Clang is more strict than other
popular compilers, and may reject incorrect code that other
compilers allow. This page documents common compatibility and
portability issues with Clang to help you understand and fix the
problem in your code when Clang emits an error message.</p>
<ul>
<li><a href="#c">C compatibility</a>
<ul>
<li><a href="#inline">C99 inline functions</a></li>
<li><a href="#lvalue-cast">Lvalue casts</a></li>
</ul>
</li>
<li><a href="#objective-c">Objective-C compatibility</a>
<ul>
<li><a href="#super-cast">Cast of super</a></li>
<li><a href="#sizeof-interface">Size of interfaces</a></li>
</ul>
</li>
<li><a href="#c++">C++ compatibility</a>
<ul>
<li><a href="#vla">Variable-length arrays</a></li>
<li><a href="#init_static_const">Initialization of non-integral static const data members within a class definition</a></li>
<li><a href="#dep_lookup">Unqualified lookup in templates</a></li>
<li><a href="#dep_lookup_bases">Unqualified lookup into dependent bases of class templates</a></li>
<li><a href="#undep_incomplete">Incomplete types in templates</a></li>
<li><a href="#bad_templates">Templates with no valid instantiations</a></li>
<li><a href="#default_init_const">Default initialization of const
variable of a class type requires user-defined default
constructor</a></li>
</ul>
</li>
<li><a href="#objective-c++">Objective-C++ compatibility</a>
<ul>
<li><a href="#implicit-downcasts">Implicit downcasts</a></li>
</ul>
</li>
</ul>
<!-- ======================================================================= -->
<h2 id="c">C compatibility</h3>
<!-- ======================================================================= -->
<!-- ======================================================================= -->
<h3 id="inline">C99 inline functions</h3>
<!-- ======================================================================= -->
<p>By default, Clang builds C code according to the C99 standard,
which provides different inlining semantics than GCC's default
behavior. For example, when compiling the following code with no optimization:</p>
<pre>
inline int add(int i, int j) { return i + j; }
int main() {
int i = add(4, 5);
return i;
}
</pre>
<p>In C99, this is an incomplete (incorrect) program because there is
no external definition of the <code>add</code> function: the inline
definition is only used for optimization, if the compiler decides to
perform inlining. Therefore, we will get a (correct) link-time error
with Clang, e.g.:</p>
<pre>
Undefined symbols:
"_add", referenced from:
_main in cc-y1jXIr.o
</pre>
<p>There are several ways to fix this problem:</p>
<ul>
<li>Provide an external (non-inline) definition of <code>add</code>
somewhere in your program.</li>
<li>Change <code>add</code> to a <code>static inline</code>
function. Static inline functions are always resolved within the
translation unit, so you won't have to add an external, non-inline
definition of the function elsewhere in your program.</li>
<li>Compile with the GNU89 dialect by adding
<code>-std=gnu89</code> to the set of Clang options. This option is
only recommended if the program source cannot be changed or if the
program also relies on additional C89-specific behavior that cannot
be changed.</li>
</ul>
<!-- ======================================================================= -->
<h3 id="lvalue-cast">Lvalue casts</h3>
<!-- ======================================================================= -->
<p>GCC permits casting the left-hand side of an assignment to a
different type. Clang produces an error on similar code, e.g.,</p>
<pre>
lvalue.c:2:3: error: assignment to cast is illegal, lvalue casts are not
supported
(int*)addr = val;
^~~~~~~~~~ ~
</pre>
<p>To fix this problem, move the cast to the right-hand side. In this
example, one could use:</p>
<pre>
addr = (float *)val;
</pre>
<!-- ======================================================================= -->
<h2 id="objective-c">Objective-C compatibility</h3>
<!-- ======================================================================= -->
<!-- ======================================================================= -->
<h3 id="super-cast">Cast of super</h3>
<!-- ======================================================================= -->
<p>GCC treats the <code>super</code> identifier as an expression that
can, among other things, be cast to a different type. Clang treats
<code>super</code> as a context-sensitive keyword, and will reject a
type-cast of <code>super</code>:</p>
<pre>
super.m:11:12: error: cannot cast 'super' (it isn't an expression)
[(Super*)super add:4];
~~~~~~~~^
</pre>
<p>To fix this problem, remove the type cast, e.g.</p>
<pre>
[super add:4];
</pre>
<!-- ======================================================================= -->
<h3 id="sizeof-interface">Size of interfaces</h3>
<!-- ======================================================================= -->
<p>When using the "non-fragile" Objective-C ABI in use, the size of an
Objective-C class may change over time as instance variables are added
(or removed). For this reason, Clang rejects the application of the
<code>sizeof</code> operator to an Objective-C class when using this
ABI:</p>
<pre>
sizeof.m:4:14: error: invalid application of 'sizeof' to interface 'NSArray' in
non-fragile ABI
int size = sizeof(NSArray);
^ ~~~~~~~~~
</pre>
<p>Code that relies on the size of an Objective-C class is likely to
be broken anyway, since that size is not actually constant. To address
this problem, use the Objective-C runtime API function
<code>clang_getInstanceSize()</code>:</p>
<pre>
class_getInstanceSize([NSArray class])
</pre>
<!-- ======================================================================= -->
<h2 id="c++">C++ compatibility</h3>
<!-- ======================================================================= -->
<!-- ======================================================================= -->
<h3 id="vla">Variable-length arrays</h3>
<!-- ======================================================================= -->
<p>GCC and C99 allow an array's size to be determined at run
time. This extension is not permitted in standard C++. However, Clang
supports such variable length arrays in very limited circumstances for
compatibility with GNU C and C99 programs:</p>
<ul>
<li>The element type of a variable length array must be a POD
("plain old data") type, which means that it cannot have any
user-declared constructors or destructors, base classes, or any
members if non-POD type. All C types are POD types.</li>
<li>Variable length arrays cannot be used as the type of a non-type
template parameter.</li> </ul>
<p>If your code uses variable length arrays in a manner that Clang doesn't support, there are several ways to fix your code:
<ol>
<li>replace the variable length array with a fixed-size array if you can
determine a
reasonable upper bound at compile time; sometimes this is as
simple as changing <tt>int size = ...;</tt> to <tt>const int size
= ...;</tt> (if the definition of <tt>size</tt> is a compile-time
integral constant);</li>
<li>use an <tt>std::string</tt> instead of a <tt>char []</tt>;</li>
<li>use <tt>std::vector</tt> or some other suitable container type;
or</li>
<li>allocate the array on the heap instead using <tt>new Type[]</tt> -
just remember to <tt>delete[]</tt> it.</li>
</ol>
<!-- ======================================================================= -->
<h3 id="init_static_const">Initialization of non-integral static const data members within a class definition</h3>
<!-- ======================================================================= -->
The following code is ill-formed in C++'03:
<pre>
class SomeClass {
public:
static const double SomeConstant = 0.5;
};
const double SomeClass::SomeConstant;
</pre>
Clang errors with something similar to:
<pre>
.../your_file.h:42:42: error: 'SomeConstant' can only be initialized if it is a static const integral data member
static const double SomeConstant = 0.5;
^ ~~~
</pre>
Only <i>integral</i> constant expressions are allowed as initializers
within the class definition. See C++'03 [class.static.data] p4 for the
details of this restriction. The fix here is straightforward: move
the initializer to the definition of the static data member, which
must exist outside of the class definition:
<pre>
class SomeClass {
public:
static const double SomeConstant;
};
const double SomeClass::SomeConstant<b> = 0.5</b>;
</pre>
Note that the forthcoming C++0x standard will allow this.
<!-- ======================================================================= -->
<h3 id="dep_lookup">Unqualified lookup in templates</h3>
<!-- ======================================================================= -->
<p>Some versions of GCC accept the following invalid code:
<pre>
template &lt;typename T&gt; T Squared(T x) {
return Multiply(x, x);
}
int Multiply(int x, int y) {
return x * y;
}
int main() {
Squared(5);
}
</pre>
<p>Clang complains:
<pre> <b>my_file.cpp:2:10: <span class="error">error:</span> use of undeclared identifier 'Multiply'</b>
return Multiply(x, x);
<span class="caret"> ^</span>
<b>my_file.cpp:10:3: <span class="note">note:</span> in instantiation of function template specialization 'Squared&lt;int&gt;' requested here</b>
Squared(5);
<span class="caret"> ^</span>
</pre>
<p>The C++ standard says that unqualified names like <q>Multiply</q>
are looked up in two ways.
<p>First, the compiler does <i>unqualified lookup</i> in the scope
where the name was written. For a template, this means the lookup is
done at the point where the template is defined, not where it's
instantiated. Since <tt>Multiply</tt> hasn't been declared yet at
this point, unqualified lookup won't find it.
<p>Second, if the name is called like a function, then the compiler
also does <i>argument-dependent lookup</i> (ADL). (Sometimes
unqualified lookup can suppress ADL; see [basic.lookup.argdep]p3 for
more information.) In ADL, the compiler looks at the types of all the
arguments to the call. When it finds a class type, it looks up the
name in that class's namespace; the result is all the declarations it
finds in those namespaces, plus the declarations from unqualified
lookup. However, the compiler doesn't do ADL until it knows all the
argument types.
<p>In our example, <tt>Multiply</tt> is called with dependent
arguments, so ADL isn't done until the template is instantiated. At
that point, the arguments both have type <tt>int</tt>, which doesn't
contain any class types, and so ADL doesn't look in any namespaces.
Since neither form of lookup found the declaration
of <tt>Multiply</tt>, the code doesn't compile.
<p>Here's another example, this time using overloaded operators,
which obey very similar rules.
<pre>#include &lt;iostream&gt;
template&lt;typename T&gt;
void Dump(const T&amp; value) {
std::cout &lt;&lt; value &lt;&lt; "\n";
}
namespace ns {
struct Data {};
}
std::ostream&amp; operator&lt;&lt;(std::ostream&amp; out, ns::Data data) {
return out &lt;&lt; "Some data";
}
void Use() {
Dump(ns::Data());
}</pre>
<p>Again, Clang complains about not finding a matching function:</p>
<pre>
<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>
std::cout &lt;&lt; value &lt;&lt; "\n";
<span class="caret">~~~~~~~~~ ^ ~~~~~</span>
<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>
Dump(ns::Data());
<span class="caret">^</span>
</pre>
<p>Just like before, unqualified lookup didn't find any declarations
with the name <tt>operator&lt;&lt;</tt>. Unlike before, the argument
types both contain class types: one of them is an instance of the
class template type <tt>std::basic_ostream</tt>, and the other is the
type <tt>ns::Data</tt> that we declared above. Therefore, ADL will
look in the namespaces <tt>std</tt> and <tt>ns</tt> for
an <tt>operator&lt;&lt;</tt>. Since one of the argument types was
still dependent during the template definition, ADL isn't done until
the template is instantiated during <tt>Use</tt>, which means that
the <tt>operator&lt;&lt;</tt> we want it to find has already been
declared. Unfortunately, it was declared in the global namespace, not
in either of the namespaces that ADL will look in!
<p>There are two ways to fix this problem:</p>
<ol><li>Make sure the function you want to call is declared before the
template that might call it. This is the only option if none of its
argument types contain classes. You can do this either by moving the
template definition, or by moving the function definition, or by
adding a forward declaration of the function before the template.</li>
<li>Move the function into the same namespace as one of its arguments
so that ADL applies.</li></ol>
<p>For more information about argument-dependent lookup, see
[basic.lookup.argdep]. For more information about the ordering of
lookup in templates, see [temp.dep.candidate].
<!-- ======================================================================= -->
<h3 id="dep_lookup_bases">Unqualified lookup into dependent bases of class templates</h3>
<!-- ======================================================================= -->
Some versions of GCC accept the following invalid code:
<pre>
template &lt;typename T&gt; struct Base {
void DoThis(T x) {}
static void DoThat(T x) {}
};
template &lt;typename T&gt; struct Derived : public Base&lt;T&gt; {
void Work(T x) {
DoThis(x); // Invalid!
DoThat(x); // Invalid!
}
};
</pre>
Clang correctly rejects it with the following errors
(when <tt>Derived</tt> is eventually instantiated):
<pre>
my_file.cpp:8:5: error: use of undeclared identifier 'DoThis'
DoThis(x);
^
this-&gt;
my_file.cpp:2:8: note: must qualify identifier to find this declaration in dependent base class
void DoThis(T x) {}
^
my_file.cpp:9:5: error: use of undeclared identifier 'DoThat'
DoThat(x);
^
this-&gt;
my_file.cpp:3:15: note: must qualify identifier to find this declaration in dependent base class
static void DoThat(T x) {}
</pre>
Like we said <a href="#dep_lookup">above</a>, unqualified names like
<tt>DoThis</tt> and <tt>DoThat</tt> are looked up when the template
<tt>Derived</tt> is defined, not when it's instantiated. When we look
up a name used in a class, we usually look into the base classes.
However, we can't look into the base class <tt>Base&lt;T&gt;</tt>
because its type depends on the template argument <tt>T</tt>, so the
standard says we should just ignore it. See [temp.dep]p3 for details.
<p>The fix, as Clang tells you, is to tell the compiler that we want a
class member by prefixing the calls with <tt>this-&gt;</tt>:
<pre>
void Work(T x) {
<b>this-&gt;</b>DoThis(x);
<b>this-&gt;</b>DoThat(x);
}
</pre>
Alternatively, you can tell the compiler exactly where to look:
<pre>
void Work(T x) {
<b>Base&lt;T&gt;</b>::DoThis(x);
<b>Base&lt;T&gt;</b>::DoThat(x);
}
</pre>
This works whether the methods are static or not, but be careful:
if <tt>DoThis</tt> is virtual, calling it this way will bypass virtual
dispatch!
<!-- ======================================================================= -->
<h3 id="undep_incomplete">Incomplete types in templates</h3>
<!-- ======================================================================= -->
The following code is invalid, but compilers are allowed to accept it:
<pre>
class IOOptions;
template &lt;class T&gt; bool read(T &amp;value) {
IOOptions opts;
return read(opts, value);
}
class IOOptions { bool ForceReads; };
bool read(const IOOptions &amp;opts, int &amp;x);
template bool read&lt;&gt;(int &amp;);
</pre>
The standard says that types which don't depend on template parameters
must be complete when a template is defined if they affect the
program's behavior. However, the standard also says that compilers
are free to not enforce this rule. Most compilers enforce it to some
extent; for example, it would be an error in GCC to
write <tt>opts.ForceReads</tt> in the code above. In Clang, we feel
that enforcing the rule consistently lets us provide a better
experience, but unfortunately it also means we reject some code that
other compilers accept.
<p>We've explained the rule here in very imprecise terms; see
[temp.res]p8 for details.
<!-- ======================================================================= -->
<h3 id="bad_templates">Templates with no valid instantiations</h3>
<!-- ======================================================================= -->
The following code contains a typo: the programmer
meant <tt>init()</tt> but wrote <tt>innit()</tt> instead.
<pre>
template &lt;class T&gt; class Processor {
...
void init();
...
};
...
template &lt;class T&gt; void process() {
Processor&lt;T&gt; processor;
processor.innit(); // <-- should be 'init()'
...
}
</pre>
Unfortunately, we can't flag this mistake as soon as we see it: inside
a template, we're not allowed to make assumptions about "dependent
types" like <tt>Processor&lt;T&gt;</tt>. Suppose that later on in
this file the programmer adds an explicit specialization
of <tt>Processor</tt>, like so:
<pre>
template &lt;&gt; class Processor&lt;char*&gt; {
void innit();
};
</pre>
Now the program will work &mdash; as long as the programmer only ever
instantiates <tt>process()</tt> with <tt>T = char*</tt>! This is why
it's hard, and sometimes impossible, to diagnose mistakes in a
template definition before it's instantiated.
<p>The standard says that a template with no valid instantiations is
ill-formed. Clang tries to do as much checking as possible at
definition-time instead of instantiation-time: not only does this
produce clearer diagnostics, but it also substantially improves
compile times when using pre-compiled headers. The downside to this
philosophy is that Clang sometimes fails to process files because they
contain broken templates that are no longer used. The solution is
simple: since the code is unused, just remove it.
<!-- ======================================================================= -->
<h3 id="default_init_const">Default initialization of const variable of a class type requires user-defined default constructor</h3>
<!-- ======================================================================= -->
If a <tt>class</tt> or <tt>struct</tt> has no user-defined default
constructor, C++ doesn't allow you to default construct a <tt>const</tt>
instance of it like this ([dcl.init], p9):
<pre>
class Foo {
public:
// The compiler-supplied default constructor works fine, so we
// don't bother with defining one.
...
};
void Bar() {
const Foo foo; // Error!
...
}
</pre>
To fix this, you can define a default constructor for the class:
<pre>
class Foo {
public:
Foo() {}
...
};
void Bar() {
const Foo foo; // Now the compiler is happy.
...
}
</pre>
<!-- ======================================================================= -->
<h2 id="objective-c++">Objective-C++ compatibility</h3>
<!-- ======================================================================= -->
<!-- ======================================================================= -->
<h3 id="implicit-downcasts">Implicit downcasts</h3>
<!-- ======================================================================= -->
<p>Due to a bug in its implementation, GCC allows implicit downcasts
(from base class to a derived class) when calling functions. Such code is
inherently unsafe, since the object might not actually be an instance
of the derived class, and is rejected by Clang. For example, given
this code:</p>
<pre>
@interface Base @end
@interface Derived : Base @end
void f(Derived *);
void g(Base *base) {
f(base);
}
</pre>
<p>Clang produces the following error:</p>
<pre>
downcast.mm:6:3: error: no matching function for call to 'f'
f(base);
^
downcast.mm:4:6: note: candidate function not viable: no known conversion from
'Base *' to 'Derived *' for 1st argument
void f(Derived *);
^
</pre>
<p>If the downcast is actually correct (e.g., because the code has
already checked that the object has the appropriate type), add an
explicit cast:</p>
<pre>
f((Derived *)base);
</pre>
</div>
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