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| author | Douglas Gregor <dgregor@apple.com> | 2010-06-30 22:01:08 +0000 |
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| committer | Douglas Gregor <dgregor@apple.com> | 2010-06-30 22:01:08 +0000 |
| commit | c41b6ff51bb14f59055a378c0c971a4b9cd92353 (patch) | |
| tree | 1a7df8e3f326b7233d064b66569b4bbe045b60dd /www/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/compatibility.html')
| -rw-r--r-- | www/compatibility.html | 609 |
1 files changed, 609 insertions, 0 deletions
diff --git a/www/compatibility.html b/www/compatibility.html new file mode 100644 index 0000000000..09072b6a7c --- /dev/null +++ b/www/compatibility.html @@ -0,0 +1,609 @@ +<!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>Language 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>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 <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]. + +<!-- ======================================================================= --> +<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 <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! + +<!-- ======================================================================= --> +<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 <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. + +<!-- ======================================================================= --> +<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 <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. + +<!-- ======================================================================= --> +<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> +</body> +</html> |