Language Compatibility

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.

C compatibility

C99 inline functions

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:

inline int add(int i, int j) { return i + j; }

int main() {
  int i = add(4, 5);
  return i;
}

In C99, this is an incomplete (incorrect) program because there is no external definition of the add 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.:

Undefined symbols:
  "_add", referenced from:
      _main in cc-y1jXIr.o

There are several ways to fix this problem:

Lvalue casts

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.,

lvalue.c:2:3: error: assignment to cast is illegal, lvalue casts are not
      supported
  (int*)addr = val;
  ^~~~~~~~~~ ~

To fix this problem, move the cast to the right-hand side. In this example, one could use:

  addr = (float *)val;

Objective-C compatibility

Cast of super

GCC treats the super identifier as an expression that can, among other things, be cast to a different type. Clang treats super as a context-sensitive keyword, and will reject a type-cast of super:

super.m:11:12: error: cannot cast 'super' (it isn't an expression)
  [(Super*)super add:4];
   ~~~~~~~~^

To fix this problem, remove the type cast, e.g.

  [super add:4];

Size of interfaces

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 sizeof operator to an Objective-C class when using this ABI:

sizeof.m:4:14: error: invalid application of 'sizeof' to interface 'NSArray' in
      non-fragile ABI
  int size = sizeof(NSArray);
             ^     ~~~~~~~~~

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 class_getInstanceSize():

  class_getInstanceSize([NSArray class])

C++ compatibility

Variable-length arrays

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:

If your code uses variable length arrays in a manner that Clang doesn't support, there are several ways to fix your code:

  1. 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 int size = ...; to const int size = ...; (if the definition of size is a compile-time integral constant);
  2. use an std::string instead of a char [];
  3. use std::vector or some other suitable container type; or
  4. allocate the array on the heap instead using new Type[] - just remember to delete[] it.

Initialization of non-integral static const data members within a class definition

The following code is ill-formed in C++'03: 
class SomeClass {
 public:
  static const double SomeConstant = 0.5;
};

const double SomeClass::SomeConstant;
Clang errors with something similar to: 
.../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;
                      ^              ~~~
Only integral 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: 
class SomeClass {
 public:
  static const double SomeConstant;
};

const double SomeClass::SomeConstant = 0.5;
Note that the forthcoming C++0x standard will allow this.

Unqualified lookup in templates

Some versions of GCC accept the following invalid code: 

template <typename T> T Squared(T x) {
  return Multiply(x, x);
}

int Multiply(int x, int y) {
  return x * y;
}

int main() {
  Squared(5);
}

Clang complains: 

  my_file.cpp:2:10: error: use of undeclared identifier 'Multiply'
    return Multiply(x, x);
           ^

  my_file.cpp:10:3: note: in instantiation of function template specialization 'Squared<int>' requested here
    Squared(5);
    ^

The C++ standard says that unqualified names like Multiply are looked up in two ways.

First, the compiler does unqualified lookup 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 Multiply hasn't been declared yet at this point, unqualified lookup won't find it.

Second, if the name is called like a function, then the compiler also does argument-dependent lookup (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.

In our example, Multiply is called with dependent arguments, so ADL isn't done until the template is instantiated. At that point, the arguments both have type int, 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 Multiply, the code doesn't compile.

Here's another example, this time using overloaded operators, which obey very similar rules. 

#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());
}

Again, Clang complains about not finding a matching function:

my_file.cpp:5:13: error: invalid operands to binary expression ('ostream' (aka 'basic_ostream<char>') and 'ns::Data const')
  std::cout << value << "\n";
  ~~~~~~~~~ ^  ~~~~~
my_file.cpp:17:3: note: in instantiation of function template specialization 'Dump<ns::Data>' requested here
  Dump(ns::Data());
  ^

Just like before, unqualified lookup didn't find any declarations with the name operator<<. Unlike before, the argument types both contain class types: one of them is an instance of the class template type std::basic_ostream, and the other is the type ns::Data that we declared above. Therefore, ADL will look in the namespaces std and ns for an operator<<. Since one of the argument types was still dependent during the template definition, ADL isn't done until the template is instantiated during Use, which means that the operator<< 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!

There are two ways to fix this problem:

  1. 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.
  2. Move the function into the same namespace as one of its arguments so that ADL applies.

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].

Unqualified lookup into dependent bases of class templates

Some versions of GCC accept the following invalid code: 
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!
  }
};
Clang correctly rejects it with the following errors (when Derived is eventually instantiated): 
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) {}
Like we said above, unqualified names like DoThis and DoThat are looked up when the template Derived 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 Base<T> because its type depends on the template argument T, so the standard says we should just ignore it. See [temp.dep]p3 for details.

The fix, as Clang tells you, is to tell the compiler that we want a class member by prefixing the calls with this->: 

  void Work(T x) {
    this->DoThis(x);
    this->DoThat(x);
  }
Alternatively, you can tell the compiler exactly where to look: 
  void Work(T x) {
    Base<T>::DoThis(x);
    Base<T>::DoThat(x);
  }
This works whether the methods are static or not, but be careful: if DoThis is virtual, calling it this way will bypass virtual dispatch!

Incomplete types in templates

The following code is invalid, but compilers are allowed to accept it: 
  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 &);
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 opts.ForceReads 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.

We've explained the rule here in very imprecise terms; see [temp.res]p8 for details.

Templates with no valid instantiations

The following code contains a typo: the programmer meant init() but wrote innit() instead. 
  template <class T> class Processor {
    ...
    void init();
    ...
  };
  ...
  template <class T> void process() {
    Processor<T> processor;
    processor.innit();       // <-- should be 'init()'
    ...
  }
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 Processor<T>. Suppose that later on in this file the programmer adds an explicit specialization of Processor, like so: 
  template <> class Processor<char*> {
    void innit();
  };
Now the program will work — as long as the programmer only ever instantiates process() with T = char*! This is why it's hard, and sometimes impossible, to diagnose mistakes in a template definition before it's instantiated.

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.

Default initialization of const variable of a class type requires user-defined default constructor

If a class or struct has no user-defined default constructor, C++ doesn't allow you to default construct a const instance of it like this ([dcl.init], p9): 
class Foo {
 public:
  // The compiler-supplied default constructor works fine, so we
  // don't bother with defining one.
  ...
};

void Bar() {
  const Foo foo;  // Error!
  ...
}
To fix this, you can define a default constructor for the class: 
class Foo {
 public:
  Foo() {}
  ...
};

void Bar() {
  const Foo foo;  // Now the compiler is happy.
  ...
}

Objective-C++ compatibility

Implicit downcasts

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:

@interface Base @end
@interface Derived : Base @end

void f(Derived *);
void g(Base *base) {
  f(base);
}

Clang produces the following error:

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 *);
     ^

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:

  f((Derived *)base);