class InternalMetadataOffset

A sentinel `InternalMetadataOffset`, which does not point to any metadata.

Constructs an `InternalMetadataOffset` which can recover the

`InternalMetadata` from a containing type `T` given the starting address of

the field at offset `FieldOffset` within `T`.

This method expects to find a field with name `_internal_metadata_` in `T`,

and the type of that field should be `InternalMetadata`.

Builds an `InternalMetadataOffset` from a dynamic offset from the start of

`T`. This is used by `DynamicMessage` to build an `InternalMetadataOffset`

for a field at a given runtime-derived offset from the start of the

message.

This function performs runtime checks to ensure that the offset from

`_internal_metadata_` to the field is within the range of an int32_t. This

is necessary to prevent integer overflow when calculating the offset.

Translates an offset relative to some class `T` to an offset relative to

the member at offset `kMemberOffset` within `T`. This is used when passing

`InternalMetadataOffset`s to members of a class where the offset was

constructed relative to the start of `T`.

For example, here is how you would pass an `InternalMetadataOffset` to a

member `Baz` of a class `Bar`, which itself is a member of `Foo`.

```cc

struct Baz {

int some_value;

InternalMetadataResolver resolver;

Baz(int value, InternalMetadataOffset offset)

: some_value(value), resolver(offset) {}

};

struct Bar {

int some_value;

Baz baz;

Bar(int value, InternalMetadataOffset offset)

: some_value(value),

baz(2 * value, offset.TranslateForMember

<offsetof

(Bar, baz)>()) {}

};

struct Foo {

InternalMetadata _internal_metadata_;

Bar field1;

explicit Foo(Arena* arena)

: _internal_metadata_(arena),

field1(123,

InternalMetadataOffset::Build

<Foo

, PROTOBUF_FIELD_OFFSET(

Foo, field1)>()) {}

};

```

If true, this `InternalMetadataOffset` does not point to any metadata.

The offset from the start of the field to the internal metadata of the

containing type (either a `MessageLite` or some other internal class, like

`RepeatedPtrFieldWithArena`).