diff options
| author | Douglas Gregor <dgregor@apple.com> | 2010-06-30 22:01:08 +0000 |
|---|---|---|
| committer | Douglas Gregor <dgregor@apple.com> | 2010-06-30 22:01:08 +0000 |
| commit | c41b6ff51bb14f59055a378c0c971a4b9cd92353 (patch) | |
| tree | 1a7df8e3f326b7233d064b66569b4bbe045b60dd /www/cxx_compatibility.html | |
| parent | 3d9c6e1c97bdae6a9daedc6969bc839e93074b0f (diff) | |
New language-compatibility page, including language-compatibility information for the various language dialects Clang supports in a single, easy-to-find page
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@107325 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'www/cxx_compatibility.html')
| -rw-r--r-- | www/cxx_compatibility.html | 398 |
1 files changed, 3 insertions, 395 deletions
diff --git a/www/cxx_compatibility.html b/www/cxx_compatibility.html index 1273ed3a8c..6aa0bbf4be 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" /> @@ -19,401 +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="#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> - -<!-- ======================================================================= --> -<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> -<!-- ======================================================================= --> - -<p>Some versions of GCC accept the following invalid code: - -<pre> -template <typename T> 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<int>' 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 <iostream> - -template<typename T> -void Dump(const T& value) { - std::cout << value << "\n"; -} - -namespace ns { - struct Data {}; -} - -std::ostream& operator<<(std::ostream& out, ns::Data data) { - return out << "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<char>') and 'ns::Data const')</b> - std::cout << value << "\n"; - <span class="caret">~~~~~~~~~ ^ ~~~~~</span> -<b>my_file.cpp:17:3: <span class="note">note:</span> in instantiation of function template specialization 'Dump<ns::Data>' 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<<</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<<</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<<</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]. - -<!-- ======================================================================= --> -<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="undep_incomplete">Incomplete types in templates</h2> -<!-- ======================================================================= --> - -The following code is invalid, but compilers are allowed to accept it: - -<pre> - class IOOptions; - template <class T> bool read(T &value) { - IOOptions opts; - return read(opts, value); - } - - class IOOptions { bool ForceReads; }; - bool read(const IOOptions &opts, int &x); - template bool read<>(int &); -</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. - -<!-- ======================================================================= --> -<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> |