FSigSource

FSigSource

What a message is sent from, and what a listener filters on. Not restricted to UObject.

Declared in Plugins/GMP/Source/GMP/GMP/GMPSignals.inl

#define GMP_SIG_BASE_ALIGN 8
struct GMP_API alignas(GMP_SIG_BASE_ALIGN) ISigSource
{
    ~ISigSource();
};

struct FSigSource
{
    using AddrType = intptr_t;
    enum EAddrMask : AddrType
    {
        EObject  = 0x00,
        ESignal  = 0x01,
        External = 0x02,
        ExtKey   = 0x04,
        EAll     = GMP_SIG_BASE_ALIGN - 1
    };
    explicit FSigSource(std::nullptr_t = nullptr) {}
};

It is a tagged pointer

FSigSource is one pointer-sized value. Because ISigSource is alignas(8), the low three bits of any source address are always zero, and GMP uses them to record what kind of source it is:

Tag	Meaning
EObject	a UObject; lifetime tracked by weak-pointer staleness
ESignal	derives from ISigSource; its destructor unregisters
External	a registered external type; lifetime is the caller's problem
ExtKey	an external key form

Practical implication: a source costs one pointer, there is no wrapper allocation, and comparison is an integer compare. That is why object-level filtering is cheap enough to do inside the store.

Three ways to be a source

1 — Be a UObject. Nothing to do.

2 — Derive from ISigSource. The destructor unregisters, so lifetime is automatic. Use this for your own long-lived C++ objects.

3 — Register an external type with the GMP_EXTERNAL_SIGSOURCE macro / TExternalSigSource specialisation. After that any address of that type can be a source, including plain data structures that others "subscribe to the changes of". This is how the script backends attach a language runtime — the Puerts isolate is used directly as a source.

Lifetime, per kind

Kind	Cleanup
UObject	automatic; stale objects are skipped on dispatch
ISigSource	automatic, in the destructor
external	manual — you must call the cleanup when the object dies

Missing the manual cleanup for an external source is the failure mode to watch for: the address may be reused by a later allocation, and messages would then match an unrelated object.

See also

Dispatch layers · NotifyMessage family · StoreObjectMessage