https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111737
Bug ID: 111737
Summary: Object holding a pointer to an uninitialized c-array
not usable in a constant expression
Product: gcc
Version: 13.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: benoit.labrique at endress dot com
Target Milestone: ---
Following code does not compile (see details below) as soon as there is an
indirection, in this case the function MakeBuffer().
This kind of construct is very common in the embedded world, where dynamic
memory allocation is not permitted. As in the code example, buffers of
different size can be passed to more generic functionality via a reference to
the base class "Collection", which is buffer-size agnostic.
// Compiled with -O2 -std=c++17
class Collection
{
protected:
constexpr Collection(void * iBuffer)
: m_buffer(iBuffer)
{
}
protected:
void * m_buffer = nullptr;
};
class Buffer final : public Collection
{
public:
explicit constexpr Buffer()
: Collection(m_buffer), m_buffer{0} // gcc doesn't like that m_buffer
is used before it is initialized. Using std::array instead won't work at all.
{
}
private:
int m_buffer[1];
};
namespace
{
inline constexpr auto MakeBuffer()
{
return Buffer(); // C++17: shall definitively perform copy elision
}
constexpr Buffer VIA_MAKE_BUFFER = MakeBuffer(); // Only gcc has a
problem with, neither clang nor msvc
constexpr Buffer VIA_BUFFER_CONSTRUCTOR = Buffer(); // Why is it now OK
for gcc here?
}
int main()
{
constexpr auto a = VIA_MAKE_BUFFER;
constexpr auto b = VIA_BUFFER_CONSTRUCTOR;
return 0;
}
https://godbolt.org/z/erPsooP4b
Compiling this code issues (ARM GCC 13.2 (or earlier)):
<source>: In function 'int main()':
<source>:38:24: error: the value of '{anonymous}::VIA_MAKE_BUFFER' is not
usable in a constant expression
38 | constexpr auto a = VIA_MAKE_BUFFER;
| ^~~~~~~~~~~~~~~
<source>:32:22: note: '{anonymous}::VIA_MAKE_BUFFER' used in its own
initializer
32 | constexpr Buffer VIA_MAKE_BUFFER = MakeBuffer(); // Only gcc
has a problem with, neither clang nor msvc
| ^~~~~~~~~~~~~~~
Alternatively: on x86-64 GCC 13.2 (or earlier):
<source>:32:51: error:
'Buffer{Collection{((void*)(&<anonymous>.Buffer::m_buffer))}, int [1]{0}}' is
not a constant expression
32 | constexpr Buffer VIA_MAKE_BUFFER = MakeBuffer(); // Only gcc
has a problem with, neither clang nor msvc
|
