Parameter compatibility

Parameter compatibility

The rule that decides whether a listener written against an older version of a message still matches.

The rule

A listener may drop trailing arguments. A prefix must match by type.

After this send:

SendMessage(MSGKEY("ABC"), a, b, c);   // TypeA, TypeB, TypeC

all four of these listeners are compatible:

ListenMessage(MSGKEY("ABC"), this, [](TypeA a, TypeB b, TypeC c){});
ListenMessage(MSGKEY("ABC"), this, [](TypeA a, TypeB b){});
ListenMessage(MSGKEY("ABC"), this, [](TypeA a){});
ListenMessage(MSGKEY("ABC"), this, [](){});

These are not:

ListenMessage(MSGKEY("ABC"), this, [](TypeB b){});              // wrong type at position 0
ListenMessage(MSGKEY("ABC"), this, [](TypeA a, TypeC c){});     // gap; position 1 must be TypeB
ListenMessage(MSGKEY("ABC"), this, [](TypeA, TypeB, TypeC, T4){});  // longer than the send

The semantics mirror default function arguments — the listener sees a prefix of what was sent.

Why it exists

So a message can gain a parameter without a coordinated edit. Append to the end of the send and every existing listener keeps compiling and keeps working; only the listeners that want the new value are touched.

This is the migration path for a system whose whole point is that sender and listener do not share a header. Without it, adding one argument would mean finding every listener first.

Consequences worth planning for

• Order your parameters by stability. Things you might add later go at the end. A parameter inserted in the middle is a breaking change, and nothing will tell you at C++ compile time — the mismatch surfaces as a validation failure at runtime, in the editor.
• Removing or reordering is always breaking. Only appending is safe.
• A zero-argument listener matches everything on that tag. Useful for "something happened" handlers, and a trap if you meant to receive a value and mistyped the lambda.

Where it is enforced

The recorded signature lives in UGMPMeta. Comparison happens in the hub when a listener registers and when a message is sent, under GMP_WITH_DYNAMIC_CALL_CHECK — on in Editor and Development, compiled out in Shipping. A mismatch warns and aborts that dispatch rather than calling with wrong types.

Blueprint gets the same rule earlier: node pins come from the recorded signature, so an incompatible connection fails at Blueprint compile time. See UK2Neuron.

See also

Signature inference · UGMPMeta · ListenMessage family · Class2Name