g++ does not link depending on optimization settings - c++

Question

So, this is eating me for the last two days:
I can't link my application, depending on which compiler I use and what optimizations levels are used:
gcc plays nicely with -O1, -O2, -O3 but fails with -O0 and
clang plays nicely with -O2 and -O3 and fails with -O1 and -O0.
There are a bunch of horrible templates in it, but I see no reason for this obscure behaviour.
Here is a minimal amount of code that reproduces the problem:
#include <map>
#include <memory>
#include <iostream>
using id_type = long;
class CB : public std::enable_shared_from_this<CB>
{
public:
CB() = default;
virtual ~CB() = default;
};
template<class F, class C> class SC : public CB
{
};
class FB
{
public:
virtual ~FB() = default;
template<class T, class B> T* as(B* v) const { return dynamic_cast<T*>(v);}
};
template<class T>
class F : public FB
{
public:
virtual std::shared_ptr<CB> create() const
{
auto n = std::make_shared<T>();
return n;
}
};
struct B
{
virtual ~B() = default;
static const id_type ID = 1;
};
class A : virtual public B, virtual public SC<A, B>
{
public:
A() = default;
};
static std::map<id_type, std::shared_ptr<FB>> crtrs {
{A::ID, std::make_shared<F<A>>()}
};
int main()
{
std::cout << crtrs.size();
}
Here is the same online https://gcc.godbolt.org/z/sb9b5E
And here are the error messages:
fld#flap ~/work/p/test1
> $ g++ -O1 main.cpp
fld#flap ~/work/p/test1
> $ g++ -O2 main.cpp
fld#flap ~/work/p/test1
> $ g++ -O3 main.cpp
fld#flap ~/work/p/test1
> $ g++ -O4 main.cpp
fld#flap ~/work/p/test1
> $ g++ -O0 main.cpp
/tmp/cc8D7sNK.o: In function `__static_initialization_and_destruction_0(int, int)':
main.cpp:(.text+0x1c0): undefined reference to `B::ID'
collect2: error: ld returned 1 exit status
fld#flap ~/work/p/test1
> $ clang++ -O0 main.cpp
/tmp/main-c49b32.o: In function `__cxx_global_var_init.1':
main.cpp:(.text.startup+0x7a): undefined reference to `B::ID'
clang-8: error: linker command failed with exit code 1 (use -v to see invocation)
fld#flap ~/work/p/test1
> $ clang++ -O1 main.cpp
/tmp/main-cf18ee.o: In function `__cxx_global_var_init.1':
main.cpp:(.text.startup+0x3c): undefined reference to `B::ID'
clang-8: error: linker command failed with exit code 1 (use -v to see invocation)
fld#flap ~/work/p/test1
> $ clang++ -O2 main.cpp
fld#flap ~/work/p/test1
> $ clang++ -O3 main.cpp
If someone has any ideas what might be the reason, any hints are more than welcome.

Answer 1

You did not provide a definition of B::ID anywhere. It just happens that with higher optimization all accesses happened to be elided by the compiler.
You need to add the definition of a static member at toplevel scope:
const id_type B::ID;
If a static const data member is only read, it does not need a separate definition because compile time constants are not considered ODR-used (One Definition Rule).
The reason that you need the definition at all is that the map constructor expects std::map::value_type in the initializer list which is std::pair<const Key, T>. The constructor that gets picked in this case is pair( const T1& x, const T2& y );
In order to call this constructor the address of A::ID which is B::ID gets taken, which constitutes an ODR-use even for a constant.
Because the pair constructor is almost trivial in this case it gets inlined at higher optimization and the only reference to &B::ID disappears because B::ID's value is known and the pair can be directly initialized.
See also: static
If you are using C++17 or newer, you can also make B:ID constexpr instead of const, then you do not need a separate definition, because constexpr is implicitly inline (inline static const should also be OK).