MSGKEY
The macro that turns a string literal into a compile-time key type.
Declared in Plugins/GMP/Source/GMP/GMP/GMPMessageKey.h
#define MSGKEY(str) GMP::TMSGKEYTyped<C_STRING_TYPE(str)>{ ... }There are three expansions in the header, selected by build configuration; they differ in how much
source-location and name data is carried. Line 96 is the baked form, 142 carries
UE_LOG_SOURCE_FILE(__FILE__) and __LINE__, 144 carries the name only.
C_STRING_TYPE("Common.Action") converts the characters into a type. Three things depend on that:
| Consequence | Why it needs a literal |
|---|---|
| The editor can collect every call site | a literal is findable in source; a computed string is not |
| The key can be baked to a static store | only a compile-time type can index a template static — see Key baking |
| The panel can jump back to the line | the macro captures __FILE__ / __LINE__ at that site |
Passing a runtime FString or FName does not fail to compile everywhere — but it silently opts out of
all three. If a tag is missing from the tag tree, this is the first thing to check.
| Form | Use |
|---|---|
MSGKEY("Tag") |
normal send and listen |
MSGKEY_SLOT("Tag") |
resolves to the static store slot, for the baked direct-send path |
MSGKEY_TYPE |
the underlying compile-time type, when you need to name it |
FMSGKEYFind |
look up an existing key without registering a new one — used by UnListenMessage
|
FMSGKEYFind is the one to reach for in teardown paths, where registering a tag as a side effect of
unregistering a listener would be wrong.
Dotted names (Common.Action, Player.Hurt) group the tag tree in the editor. The dots are presentational
grouping, not a hierarchy with lookup semantics: Player and Player.Hurt are unrelated keys, and
listening to one does not hear the other.
GMP_WITH_STATIC_MSGKEY defaults to !WITH_EDITOR, so the key string itself is dropped from the runtime
in a packaged build. Code that expects to read a tag's name back at runtime should not assume it is there.