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| <!-- ======================================================================= --> |
| <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> |
| |
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