What might cause 'undefined symbol' for typeinfo when loading dynamic library built with C++11 enabled?

[Jonathan Wakely]

On Thu, 13 Dec 2018 at 10:46, David Aldrich wrote:
>
> Our C++ application, that runs on Linux, links to dynamic libraries built
> by us. When everything is built using gcc 7.3.0 with switch -std=c++98, the
> libraries load ok. However, when I build everything without that switch
> (i.e. with C++11)

N.B. the default is -std=gnu++14 not 11.

> dlopen() reports an undefined symbol:
>
> undefined symbol: _ZTIN8StarLibs7LinkSim2NR13StarOfdmGenNRE ( typeinfo for
> StarLibs::LinkSim::NR::StarOfdmGenNR )
>
> (The demangled symbol name is in brackets.)
>
> What could cause missing typeinfo, which is apparently for the class?

It could be any number of things. Maybe the C++98 version of the
application doesn't need that typeifo, so never notices it's not
there. Or maybe it does need it, but when built as C++11 the class
doesn't define its typeinfo. My guess is the latter. See
https://gcc.gnu.org/wiki/VerboseDiagnostics#missing_vtable (the same
logic applies to the typeinfo as to the vtable).

If your class looks like this then its key function will be different
in C++98 and C++11:

class Foo {
#if __cplusplus >= 201103:
  virtual void bar();
#endif

virtual void baz();
};

If you don't define Foo::bar() somewhere then the typeinfo will never
get emitted.


> The class has methods that return const. Also the class's header file
> #includes another class's header file and that included class contains some
> static const members.

That's completely irrelevant to typeinfo.

> I tried declaring those members in a separate .cpp
> file to allocate them storage (that recipe fixed a similar error in another
> part of the code) but it did not fix this problem.