C++ exceptions are disabled by default for all C and C++ code. If you need them, you can white-list your ebuild.
C++ exceptions are not allowed by the Google C++ Style Guide and only a few projects use them. This supports adds a some overhead to the binary file on the .eh_frame and .eh_frame_hdr sections, even if you don't use try/catch/throw, even if your program is in C. Nevertheless, they are enabled by default by the compiler, since some exception support is required even if you don't use them. For example, if function f() calls g() calls h(), and h() throws an exception that f() is supposed to catch, then g() also needs to be compiled with exception support, even if g() is a C function.
The overhead is in the range of 5% to 10% of the binary size, depending of the average size of your functions, so we disable it by default.
What to white-list
If your application uses try, catch or throw, it needs to be withe-listed because otherwise it won't compile.
If your application calls a library that might throw an exception, but you don't catch it, the application doesn't need to be compiled with exceptions support. If the library throws an exception, the application will be terminated in either case.
If your application catches an exception that is thrown by a function called by an intermediate library, the library also needs to be compiled with exceptions support. For example, if you pass a callback to a library that might throw an exception, then the library should be also compiled with exception support. That's why glib and glibc are white-listed.
To disable the exception support, we pass "-fno-exceptions -fno-unwind-tables -fno-asynchronous-unwind-tables" in CFLAGS and CXXFLAGS by default. To white-list them, we filter these flags from CFLAGS and CXXFLAGS. We also define the environment variable CXXEXCEPTIONS to 0 or 1 to determine whether the exceptions are enabled.
The flags are added on the chromiumos-overlay/chromeos/make.conf.*-target files. To re-enable this support, you can call the bash function cros_enable_cxx_exceptions defined in chromiumos-overlay/profiles/base/profile.bashrc. That function will filter out the flags from CXXFLAGS and CFLAGS.
This can be done on a per ebuild basis. For example, the dev-libs/dbus-c++ ebuild has the following snippet on chromiumos-overlay/chromeos/config/env/dev-libs/dbus-c++.