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diff --git a/www/cxx_compatibility.html b/www/cxx_compatibility.html index fe03240..6aa0bbf 100644 --- a/www/cxx_compatibility.html +++ b/www/cxx_compatibility.html @@ -2,6 +2,7 @@ "http://www.w3.org/TR/html4/strict.dtd"> <html> <head> +<meta HTTP-EQUIV="REFRESH" content="5; url=compatibility.html#c++"> <META http-equiv="Content-Type" content="text/html; charset=ISO-8859-1" /> <title>Clang - C++ Compatibility</title> <link type="text/css" rel="stylesheet" href="menu.css" /> @@ -19,285 +20,8 @@ <h1>Clang's C++ Compatibility</h1> <!-- ======================================================================= --> -<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> - -<!-- ======================================================================= --> -<h2 id="intro">Introduction</h2> -<!-- ======================================================================= --> - -<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> - -<!-- ======================================================================= --> -<h2 id="vla">Variable-length arrays</h2> -<!-- ======================================================================= --> - -<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> - -<!-- ======================================================================= --> -<h2 id="init_static_const">Initialization of non-integral static const data members within a class definition</h2> -<!-- ======================================================================= --> - -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. - -<!-- ======================================================================= --> -<h2 id="dep_lookup">Unqualified lookup in templates</h2> -<!-- ======================================================================= --> - -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. - -<!-- ======================================================================= --> -<h2 id="dep_lookup_bases">Unqualified lookup into dependent bases of class templates</h2> -<!-- ======================================================================= --> - -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! - -<!-- ======================================================================= --> -<h2 id="bad_templates">Templates with no valid instantiations</h2> -<!-- ======================================================================= --> - -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. - -<!-- ======================================================================= --> -<h2 id="default_init_const">Default initialization of const variable of a class type requires user-defined default constructor</h2> -<!-- ======================================================================= --> - -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> - + <p>The Clang C++ compatibility page has moved. You will be directed <a href="compatibility.html#c++">to its new home</a> in 5 seconds.</p> + </div> </body> </html> |
