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| |
| <!-- ======================================================================= --> |
| <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="#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="#c++">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="#objective-c++">Objective-C++ compatibility</a> |
| <ul> |
| <li><a href="#implicit-downcasts">Implicit downcasts</a></li> |
| </ul> |
| <ul> |
| <li><a href="#Use of class as method name">Use of class as method name</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 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, similar |
| to the manner in which both GCC and Clang disallow jumps into the scope of |
| variables which have user defined constructors (in C++).</p> |
| |
| <p>Variables marked with <tt>__block</tt> require special runtime initialization |
| before they can be used. A jump into the scope of a <tt>__block</tt> variable |
| would bypass this initialization and therefore the variable cannot safely be |
| used.</p> |
| |
| <p>For example, consider the following code fragment:</p> |
| |
| <pre> |
| int f0(int c) { |
| if (c) |
| goto error; |
| |
| __block int x; |
| x = 1; |
| return x; |
| |
| error: |
| x = 0; |
| return x; |
| } |
| </pre> |
| |
| <p>GCC accepts this code, but it will crash at runtime along the error path, |
| because the runtime setup for the storage backing the <tt>x</tt> variable will |
| not have been initialized. 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 x; |
| ^ |
| </pre> |
| |
| <p>Some instances of this construct may be safe if the variable is never used |
| after the jump target, however the protected scope checker does not check the |
| uses of the variable, only the scopes in which it is visible. You should rewrite |
| your code to put the <tt>__block</tt> variables in a scope which is only visible |
| where they are used.</p> |
| |
| <!-- ======================================================================= --> |
| <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</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>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 @class or @protocol</h3> |
| <!-- ======================================================================= --> |
| |
| <p>GCC allows declaration of C variables in a @class or @protocol, but not |
| C functions. Clang does not allow variable or C function declarations. External |
| declarations, however, is allowed. Variables may only be declared in an |
| @implementation. |
| |
| <pre> |
| @interface XX |
| int x; // not allowed in clang |
| int one=1; // not allowed in clang |
| extern int OK; |
| @end |
| |
| </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="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> |
| |
| <!-- ======================================================================= --> |
| <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="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: cannot convert from |
| superclass 'Base *' to subclass '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> |
| |
| <!-- ======================================================================= --> |
| <h3 id="Use of class as method name">Use of class as method name</h3> |
| <!-- ======================================================================= --> |
| |
| <p>Use of 'class' name to declare a method is allowed in objective-c++ mode to |
| be compatible with GCC. However, use of property dot syntax notation to call |
| this method is not allowed in clang++, as [I class] is a suitable syntax that |
| will work. So, this test will fail in clang++. |
| |
| <pre> |
| @interface I { |
| int cls; |
| } |
| + (int)class; |
| @end |
| |
| @implementation I |
| - (int) Meth { return I.class; } |
| @end |
| <pre> |
| |
| |
| </div> |
| </body> |
| </html> |