class ExtensionIdentifier

This is the type of actual extension objects. E.g. if you have:

extend Foo {

optional int32 bar = 1234;

}

then "bar" will be defined in C++ as:

ExtensionIdentifier

<Foo

, PrimitiveTypeTraits

<int32

_t>, 5, false> bar(1234);

Note that we could, in theory, supply the field number as a template

parameter, and thus make an instance of ExtensionIdentifier have no

actual contents. However, if we did that, then using an extension

identifier would not necessarily cause the compiler to output any sort

of reference to any symbol defined in the extension's .pb.o file. Some

linkers will actually drop object files that are not explicitly referenced,

but that would be bad because it would cause this extension to not be

registered at static initialization, and therefore using it would crash.