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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="#vector_builtins">"missing" vector __builtin functions</a></li> + <li><a href="#lvalue-cast">Lvalue casts</a></li> + <li><a href="#blocks-in-protected-scope">Jumps to within <tt>__block</tt> variable scope</a></li> + <li><a href="#block-variable-initialization">Non-initialization of <tt>__block</tt> variables</a></li> + <li><a href="#inline-asm">Inline assembly</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> + <li><a href="#objc_objs-cast">Internal Objective-C types</a></li> + <li><a href="#c_variables-class">C variables in @class or @protocol</a></li> + </ul> + </li> + <li><a href="#cxx">C++ compatibility</a> + <ul> + <li><a href="#vla">Variable-length arrays</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> + <li><a href="#param_name_lookup">Parameter name lookup</a></li> + </ul> + </li> + <li><a href="#cxx11">C++11 compatibility</a> + <ul> + <li><a href="#deleted-special-func">Deleted special member + functions</a></li> + </ul> + </li> + <li><a href="#objective-cxx">Objective-C++ compatibility</a> + <ul> + <li><a href="#implicit-downcasts">Implicit downcasts</a></li> + </ul> + <ul> + <li><a href="#class-as-property-name">Using <code>class</code> as a property name</a></li> + </ul> + </li> +</ul> + +<!-- ======================================================================= --> +<h2 id="c">C compatibility</h2> +<!-- ======================================================================= --> + +<!-- ======================================================================= --> +<h3 id="inline">C99 inline functions</h3> +<!-- ======================================================================= --> +<p>By default, Clang builds C code according to the C99 standard, +which provides different semantics for the <code>inline</code> keyword +than GCC's default behavior. For example, consider the following +code:</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, <code>inline</code> means that a function's definition is +provided only for inlining, and that there is another definition +(without <code>inline</code>) somewhere else in the program. That +means that this program is incomplete, because if <code>add</code> +isn't inlined (for example, when compiling without optimization), then +<code>main</code> will have an unresolved reference to that other +definition. Therefore we'll get a (correct) link-time error like this:</p> + +<pre> +Undefined symbols: + "_add", referenced from: + _main in cc-y1jXIr.o +</pre> + +<p>By contrast, GCC's default behavior follows the GNU89 dialect, +which is the C89 standard plus a lot of extensions. C89 doesn't have +an <code>inline</code> keyword, but GCC recognizes it as an extension +and just treats it as a hint to the optimizer.</p> + +<p>There are several ways to fix this problem:</p> + +<ul> + <li>Change <code>add</code> to a <code>static inline</code> + function. This is usually the right solution if only one + translation unit needs to use the function. <code>static + inline</code> functions are always resolved within the translation + unit, so you won't have to add a non-<code>inline</code> definition + of the function elsewhere in your program.</li> + + <li>Remove the <code>inline</code> keyword from this definition of + <code>add</code>. The <code>inline</code> keyword is not required + for a function to be inlined, nor does it guarantee that it will be. + Some compilers ignore it completely. Clang treats it as a mild + suggestion from the programmer.</li> + + <li>Provide an external (non-<code>inline</code>) definition + of <code>add</code> somewhere else in your program. The two + definitions must be equivalent!</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> + +<p>All of this only applies to C code; the meaning of <code>inline</code> +in C++ is very different from its meaning in either GNU89 or C99.</p> + +<!-- ======================================================================= --> +<h3 id="vector_builtins">"missing" vector __builtin functions</h3> +<!-- ======================================================================= --> + +<p>The Intel and AMD manuals document a number "<tt><*mmintrin.h></tt>" +header files, which define a standardized API for accessing vector operations +on X86 CPUs. These functions have names like <tt>_mm_xor_ps</tt> and +<tt>_mm256_addsub_pd</tt>. Compilers have leeway to implement these functions +however they want. Since Clang supports an excellent set of <a +href="../docs/LanguageExtensions.html#vectors">native vector operations</a>, +the Clang headers implement these interfaces in terms of the native vector +operations. +</p> + +<p>In contrast, GCC implements these functions mostly as a 1-to-1 mapping to +builtin function calls, like <tt>__builtin_ia32_paddw128</tt>. These builtin +functions are an internal implementation detail of GCC, and are not portable to +the Intel compiler, the Microsoft compiler, or Clang. If you get build errors +mentioning these, the fix is simple: switch to the *mmintrin.h functions.</p> + +<p>The same issue occurs for NEON and Altivec for the ARM and PowerPC +architectures respectively. For these, make sure to use the <arm_neon.h> +and <altivec.h> headers.</p> + +<p>For x86 architectures this <a href="builtins.py">script</a> should help with +the manual migration process. It will rewrite your source files in place to +use the APIs instead of builtin function calls. Just call it like this:</p> + +<pre> + builtins.py *.c *.h +</pre> + +<p>and it will rewrite all of the .c and .h files in the current directory to +use the API calls instead of calls like <tt>__builtin_ia32_paddw128</tt>.</p> + +<!-- ======================================================================= --> +<h3 id="lvalue-cast">Lvalue casts</h3> +<!-- ======================================================================= --> + +<p>Old versions of GCC permit 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> + +<!-- ======================================================================= --> +<h3 id="blocks-in-protected-scope">Jumps to within <tt>__block</tt> variable scope</h3> +<!-- ======================================================================= --> + +<p>Clang disallows jumps into the scope of a <tt>__block</tt> +variable. Variables marked with <tt>__block</tt> require special +runtime initialization. A jump into the scope of a <tt>__block</tt> +variable bypasses this initialization, leaving the variable's metadata +in an invalid state. Consider the following code fragment:</p> + +<pre> +int fetch_object_state(struct MyObject *c) { + if (!c->active) goto error; + + __block int result; + run_specially_somehow(^{ result = c->state; }); + return result; + + error: + fprintf(stderr, "error while fetching object state"); + return -1; +} +</pre> + +<p>GCC accepts this code, but it produces code that will usually crash +when <code>result</code> goes out of scope if the jump is taken. (It's +possible for this bug to go undetected because it often won't crash if +the stack is fresh, i.e. still zeroed.) Therefore, Clang rejects this +code with a hard error:</p> + +<pre> +t.c:3:5: error: goto into protected scope + goto error; + ^ +t.c:5:15: note: jump bypasses setup of __block variable + __block int result; + ^ +</pre> + +<p>The fix is to rewrite the code to not require jumping into a +<tt>__block</tt> variable's scope, e.g. by limiting that scope:</p> + +<pre> + { + __block int result; + run_specially_somehow(^{ result = c->state; }); + return result; + } +</pre> + +<!-- ======================================================================= --> +<h3 id="block-variable-initialization">Non-initialization of <tt>__block</tt> +variables</h3> +<!-- ======================================================================= --> + +<p>In the following example code, the <tt>x</tt> variable is used before it is +defined:</p> +<pre> +int f0() { + __block int x; + return ^(){ return x; }(); +} +</pre> + +<p>By an accident of implementation, GCC and llvm-gcc unintentionally always +zero initialized <tt>__block</tt> variables. However, any program which depends +on this behavior is relying on unspecified compiler behavior. Programs must +explicitly initialize all local block variables before they are used, as with +other local variables.</p> + +<p>Clang does not zero initialize local block variables, and programs which rely +on such behavior will most likely break when built with Clang.</p> + + +<!-- ======================================================================= --> +<h3 id="inline-asm">Inline assembly</h3> +<!-- ======================================================================= --> + +<p>In general, Clang is highly compatible with the GCC inline assembly +extensions, allowing the same set of constraints, modifiers and operands as GCC +inline assembly.</p> + +<p>On targets that use the integrated assembler (such as most X86 targets), +inline assembly is run through the integrated assembler instead of your system +assembler (which is most commonly "gas", the GNU assembler). The LLVM +integrated assembler is extremely compatible with GAS, but there are a couple of +minor places where it is more picky, particularly due to outright GAS bugs.</p> + +<p>One specific example is that the assembler rejects ambiguous X86 instructions +that don't have suffixes. For example:</p> + +<pre> + asm("add %al, (%rax)"); + asm("addw $4, (%rax)"); + asm("add $4, (%rax)"); +</pre> + +<p>Both clang and GAS accept the first instruction: because the first +instruction uses the 8-bit <tt>%al</tt> register as an operand, it is clear that +it is an 8-bit add. The second instruction is accepted by both because the "w" +suffix indicates that it is a 16-bit add. The last instruction is accepted by +GAS even though there is nothing that specifies the size of the instruction (and +the assembler randomly picks a 32-bit add). Because it is ambiguous, Clang +rejects the instruction with this error message: +</p> + +<pre> +<inline asm>:3:1: error: ambiguous instructions require an explicit suffix (could be 'addb', 'addw', 'addl', or 'addq') +add $4, (%rax) +^ +1 error generated. +</pre> + +<p>To fix this compatibility issue, add an explicit suffix to the instruction: +this makes your code more clear and is compatible with both GCC and Clang.</p> + +<!-- ======================================================================= --> +<h2 id="objective-c">Objective-C compatibility</h2> +<!-- ======================================================================= --> + +<!-- ======================================================================= --> +<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>class_getInstanceSize()</code>:</p> + +<pre> + class_getInstanceSize([NSArray class]) +</pre> + +<!-- ======================================================================= --> +<h3 id="objc_objs-cast">Internal Objective-C types</h3> +<!-- ======================================================================= --> + +<p>GCC allows using pointers to internal Objective-C objects, <tt>struct objc_object*</tt>, +<tt>struct objc_selector*</tt>, and <tt>struct objc_class*</tt> in place of the types +<tt>id</tt>, <tt>SEL</tt>, and <tt>Class</tt> respectively. Clang treats the +internal Objective-C structures as implementation detail and won't do implicit conversions: + +<pre> +t.mm:11:2: error: no matching function for call to 'f' + f((struct objc_object *)p); + ^ +t.mm:5:6: note: candidate function not viable: no known conversion from 'struct objc_object *' to 'id' for 1st argument +void f(id x); + ^ +</pre> + +<p>Code should use types <tt>id</tt>, <tt>SEL</tt>, and <tt>Class</tt> +instead of the internal types.</p> + +<!-- ======================================================================= --> +<h3 id="c_variables-class">C variables in @interface or @protocol</h3> +<!-- ======================================================================= --> + +<p>GCC allows the declaration of C variables in +an <code>@interface</code> or <code>@protocol</code> +declaration. Clang does not allow variable declarations to appear +within these declarations unless they are marked <code>extern</code>.</p> + +<p>Variables may still be declared in an @implementation.</p> + +<pre> +@interface XX +int a; // not allowed in clang +int b = 1; // not allowed in clang +extern int c; // allowed +@end + +</pre> + +<!-- ======================================================================= --> +<h2 id="cxx">C++ compatibility</h2> +<!-- ======================================================================= --> + +<!-- ======================================================================= --> +<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, any base classes, or any + members of 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 initializer is a compile-time constant);</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="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> call to function 'Multiply' that is neither visible in the template definition nor found by argument-dependent lookup</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> + <b>my_file.cpp:5:5: <span class="note">note:</span> 'Multiply' should be declared prior to the call site</b> + int Multiply(int x, int y) { + <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:</p> + +<pre> <b>my_file2.cpp:5:13: <span class="error">error:</span> call to function 'operator<<' that is neither visible in the template definition nor found by argument-dependent lookup</b> + std::cout << value << "\n"; + <span class="caret"> ^</span> + <b>my_file2.cpp:17:3: <span class="error">note:</span> in instantiation of function template specialization 'Dump<ns::Data>' requested here</b> + Dump(ns::Data()); + <span class="caret"> ^</span> + <b>my_file2.cpp:12:15: <span class="error">note:</span> 'operator<<' should be declared prior to the call site or in namespace 'ns'</b> + std::ostream& operator<<(std::ostream& out, ns::Data 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> + +<!-- ======================================================================= --> +<h3 id="param_name_lookup">Parameter name lookup</h3> +<!-- ======================================================================= --> + +<p>Due to a bug in its implementation, GCC allows the redeclaration of function parameter names within a function prototype in C++ code, e.g.</p> +<blockquote> +<pre> +void f(int a, int a); +</pre> +</blockquote> +<p>Clang diagnoses this error (where the parameter name has been redeclared). To fix this problem, rename one of the parameters.</p> + +<!-- ======================================================================= --> +<h2 id="cxx11">C++11 compatibility</h2> +<!-- ======================================================================= --> + +<!-- ======================================================================= --> +<h3 id="deleted-special-func">Deleted special member functions</h3> +<!-- ======================================================================= --> + +<p>In C++11, the explicit declaration of a move constructor or a move +assignment operator within a class deletes the implicit declaration +of the copy constructor and copy assignment operator. This change came +fairly late in the C++11 standardization process, so early +implementations of C++11 (including Clang before 3.0, GCC before 4.7, +and Visual Studio 2010) do not implement this rule, leading them to +accept this ill-formed code:</p> + +<pre> +struct X { + X(X&&); <i>// deletes implicit copy constructor:</i> + <i>// X(const X&) = delete;</i> +}; + +void f(X x); +void g(X x) { + f(x); <i>// error: X has a deleted copy constructor</i> +} +</pre> + +<p>This affects some early C++11 code, including Boost's popular <a +href="http://www.boost.org/doc/libs/release/libs/smart_ptr/shared_ptr.htm"><tt>shared_ptr</tt></a> +up to version 1.47.0. The fix for Boost's <tt>shared_ptr</tt> is +<a href="https://svn.boost.org/trac/boost/changeset/73202">available here</a>.</p> + +<!-- ======================================================================= --> +<h2 id="objective-cxx">Objective-C++ compatibility</h2> +<!-- ======================================================================= --> + +<!-- ======================================================================= --> +<h3 id="implicit-downcasts">Implicit downcasts</h3> +<!-- ======================================================================= --> + +<p>Due to a bug in its implementation, GCC allows implicit downcasts +of Objective-C pointers (from a 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 *p); +void g(Base *p) { + f(p); +} +</pre> + +<p>Clang produces the following error:</p> + +<pre> +downcast.mm:6:3: error: no matching function for call to 'f' + f(p); + ^ +downcast.mm:4:6: note: candidate function not viable: cannot convert from + superclass 'Base *' to subclass 'Derived *' for 1st argument +void f(Derived *p); + ^ +</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> + +<!-- ======================================================================= --> +<h3 id="class-as-property-name">Using <code>class</code> as a property name</h3> +<!-- ======================================================================= --> + +<p>In C and Objective-C, <code>class</code> is a normal identifier and +can be used to name fields, ivars, methods, and so on. In +C++, <code>class</code> is a keyword. For compatibility with existing +code, Clang permits <code>class</code> to be used as part of a method +selector in Objective-C++, but this does not extend to any other part +of the language. In particular, it is impossible to use property dot +syntax in Objective-C++ with the property name <code>class</code>, so +the following code will fail to parse:</p> + +<pre> +@interface I { +int cls; +} ++ (int)class; +@end + +@implementation I +- (int) Meth { return I.class; } +@end +</pre> + +<p>Use explicit message-send syntax instead, i.e. <code>[I class]</code>.</p> + +</div> +</body> +</html> |