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diff --git a/contrib/llvm/tools/clang/www/cxx_compatibility.html b/contrib/llvm/tools/clang/www/cxx_compatibility.html new file mode 100644 index 0000000..fe03240 --- /dev/null +++ b/contrib/llvm/tools/clang/www/cxx_compatibility.html @@ -0,0 +1,303 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" + "http://www.w3.org/TR/html4/strict.dtd"> +<html> +<head> + <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" /> + <link type="text/css" rel="stylesheet" href="content.css" /> + <style type="text/css"> +</style> +</head> +<body> + +<!--#include virtual="menu.html.incl"--> + +<div id="content"> + +<!-- ======================================================================= --> +<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> + +</div> +</body> +</html> |
