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<h1>Clang's C++ Compatibility</h1>
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<ul>
<li><a href="#intro">Introduction</a></li>
<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="#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>
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<h2 id="intro">Introduction</h2>
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<p>Clang strives to strictly conform to the C++ standard. That means
it will reject invalid C++ code that another compiler may accept.
This page helps you decide whether a Clang error message means a
C++-conformance bug in your code and how you can fix it.</p>
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<h2 id="vla">Variable-length arrays</h2>
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<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>
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<h2 id="init_static_const">Initialization of non-integral static const data members within a class definition</h2>
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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.
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<h2 id="dep_lookup">Unqualified lookup in templates</h2>
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Some versions of GCC accept the following invalid code:
<pre>
template <typename T> struct Foo {
void Work(T x) {
func(x);
}
};
...
void func(int x);
...
template struct Foo<int>; // or anything else that instantiates Foo<int>::Work
</pre>
The standard says that unqualified names like <tt>func</tt> are looked up
when the template is defined, not when it's instantiated. Since
<tt>void func(int)</tt> was not declared yet when <tt>Foo</tt> was
defined, it's not considered. The fix is usually to
declare <tt>func</tt> before <tt>Foo</tt>.
<p>This is complicated by <i>argument-dependent lookup</i> (ADL),
which is done when unqualified names are called as functions,
like <tt>func(x)</tt> above. The standard says that ADL is performed
in both places if any of the arguments are type-dependent, like
<tt>x</tt> is in this example. However, ADL does nothing for builtin
types like <tt>int</tt>, so the example is still invalid. See
[basic.lookup.argdep] for more information.
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<h2 id="dep_lookup_bases">Unqualified lookup into dependent bases of class templates</h2>
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Some versions of GCC accept the following invalid code:
<pre>
template <typename T> struct Base {
void DoThis(T x) {}
static void DoThat(T x) {}
};
template <typename T> struct Derived : public Base<T> {
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->
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->
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<T></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-></tt>:
<pre>
void Work(T x) {
<b>this-></b>DoThis(x);
<b>this-></b>DoThat(x);
}
</pre>
Alternatively, you can tell the compiler exactly where to look:
<pre>
void Work(T x) {
<b>Base<T></b>::DoThis(x);
<b>Base<T></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!
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<h2 id="bad_templates">Templates with no valid instantiations</h2>
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The following code contains a typo: the programmer
meant <tt>init()</tt> but wrote <tt>innit()</tt> instead.
<pre>
template <class T> class Processor {
...
void init();
...
};
...
template <class T> void process() {
Processor<T> 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<T></tt>. Suppose that later on in
this file the programmer adds an explicit specialization
of <tt>Processor</tt>, like so:
<pre>
template <> class Processor<char*> {
void innit();
};
</pre>
Now the program will work — 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.
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<h2 id="default_init_const">Default initialization of const variable of a class type requires user-defined default constructor</h2>
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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>
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