Is this c++ template code valid? g++ compiles it but clang won't
I am trying to compile a small c++ program using clang with the default C++ standard library(4.6.2) on Fedora. Clang itself compiles okay and a test program using only compiles and runs fine.
My other program uses ropes which clang complains about.
/usr/lib/gcc/x86_64-redhat-linux/4.6.2/../../../../include/c++/4.6.2/ext/ropeimpl.h:433:2: error: use of undeclared identifier '_Data_allocate' _Data_allocate(_S_rounded_up_size(__old_len + __len));
A bug was filed against clang for this error message and the resolution was clang is correct, the library code is invalid.
Clang is correct here. There are no type-dependent arguments in the call to _Data_allocate, so name lookup fails at template definition time.
The context for the failing code:
// Concatenate a C string onto a leaf rope by copying the rope data.
// Used for short ropes.
template <class _CharT, class _Alloc>
typename rope<_CharT, _Alloc>::_RopeLeaf*
rope<_CharT, _Alloc>::
_S_leaf_concat_char_iter(_RopeLeaf* __r, const _CharT* __iter, size_t __len)
{
size_t __old_len = __r->_M_size;
_CharT* __new_data = (_CharT*)
_Data_allocate(_S_rounded_up_size(__old_len + __len));
_RopeLeaf* __result;
uninitialized_copy_n(__r->_M_data, __old_len, __new_data);
uninitialized_copy_n(__iter, __len, __new_data + __old_len);
_S_cond_store_eos(__new_data[__old_len + __len]);
__try
{
__result = _S_new_RopeLeaf(__new_data, __old_len + __len,
__r->_M_get_allocator());
}
__catch(...)
{
_RopeRep::__STL_FREE_STRING(__new_data, __old_len + __len,
__r->_M_get_allocator());
__throw_exception_again;
}
return __result;
}
My question is, if this code is not valid, is there an easy workaround? g++ compiles this okay.
Digging through the libstdc++ source, it appears that the definition of member function _Data_allocate results from an expansion of the __ROPE_DEFINE_ALLOCS macro in the definition of template _Rope_base (note that template instantiation rope<_CharT, _Alloc> publicly extends _Rope_base<_CharT, _Alloc> ).
You could try qualifying the call to _Data_allocate further. Instead of:
_Data_allocate(_S_rounded_up_size(__old_len + __len));
Try:
_Rope_base<_CharT, _Alloc>::_Data_allocate(_S_rounded_up_size(__old_len + __len));
Or simply:
_Base::_Data_allocate(_S_rounded_up_size(__old_len + __len));
because of the protected typedef _Rope_base<_CharT, _Alloc> _Base; in the definition of rope<_CharT, _Alloc> .
EDIT: I don't have Clang installed locally, but I tested this out with the online Clang 3.0 compiler demo.
This highly pared-down version fails to compile with Clang 3.0 (error: use of undeclared identifier '_Data_allocate'):
#include <cstddef>
#include <memory>
template <typename _CharT, class _Alloc>
class _Rope_base : public _Alloc
{
public:
typedef typename _Alloc::template rebind<_CharT>::other _DataAlloc;
static _CharT * _Data_allocate(std::size_t __n) {
return _DataAlloc().allocate(__n);
}
};
template <typename _CharT, class _Alloc = std::allocator<_CharT> >
class rope : public _Rope_base<_CharT, _Alloc>
{
protected:
typedef _Rope_base<_CharT, _Alloc> _Base;
public:
rope()
{
_Data_allocate(0);
}
};
int main()
{
rope<char> r;
}
By qualifying the call to _Data_allocate in either way suggested above, Clang 3.0 succeeds in compiling it.