Why a string key

Why a string key

The design decision everything else follows from.

What a static signature costs

A native delegate verifies signatures at compile time, so the signature must live in a header both sides include:

DECLARE_MULTICAST_DELEGATE_TwoParams(FOnSomething, Type1, Type2);

That header is an edge in the build graph. Three consequences:

1. Every translation unit that includes it rebuilds when it changes.
2. Plugin-shaped projects accumulate these edges faster than anyone tracks them.
3. Blueprint needs a parallel mechanism — Interface plus Dispatcher — wired case by case.

The one that bites is the first. You can delete a module's logic and the build still needs its header.

What GMP substitutes

A string:

FGMPHelper::NotifyMessage(MSGKEY("Common.Action"), P1, P2);
FGMPHelper::ListenMessage(MSGKEY("Common.Action"), this, [this](Type1& P1, Type2& P2){ ... });

Nothing shared is included. Either side can be deleted and the other still compiles.

What that costs, and where it is paid back

The compiler no longer checks the call. Four mechanisms take over, all of them earlier or later than C++ compile time rather than at it:

Mechanism	When it catches you	Page
Signature recorded on first use, compared afterwards	editor, and at runtime under Editor/Development	UGMPMeta, Parameter compatibility
Blueprint pins generated from the signature	Blueprint compile	UK2Neuron
Codegen'd declarations per script language	as you type, in that language's tooling	Transparent rewrite
Source location recorded per call site	when you need to find who sent it	Jump tracing

The honest summary: you trade a compile error for an editor-time error, and you get module removability in exchange. If your project never removes or reorders modules, the trade is worth less to you.

What is deliberately not solved

• Nothing prevents a typo in a tag that was never used before — it registers as a new tag. The tag tree in the editor is the mitigation.
• Nothing checks a Shipping-only code path that no editor run ever exercised, because the check is compiled out there.

See also

MSGKEY · Parameter compatibility · Signature inference · Dispatch layers