UnrealEngine FName anatomy

what FName is

  • one of the 3 main string types in Unreal Engine, which is used for indicating individual object in game
  • as its purpose, FName do not have features for string manipulation such as trim, reverse…etc.
  • it would be better to consider FName is a literal index rather than a digital index


  • you may have seen the expression, “FNames are case-insensitive”, which means you cannot make more than one FName with the same alphabet arrangement
  • for example, if you already have an object its name is “Sheri”, you cannot make an object its name is “sheri”. let us see below:
  • why this happens ? find it out in the code
  • we can find the phrase at the ContentBrowserUtils::IsValidObjectPathForCrate()
    • the function validates whether new object can be created in certain directory
  • as the comment says, the phrase is shown when there is already an object with the same path
    • in other words, you can create an object with the same name in different path
  • yes, not only object, folder is also affected with the rule

FName as a hash value

  • now, you should have a question:
    • “how does Unreal Engine save the object path ?”
  • we can assume that may be not literal method, rather hash method
    • because literal method would tell us that upper case differs from lower case
  • digging one step, we can find something strange
  • as we saw, the warning pharse is shown when there is already an object with the same path
    • we can see returning some object ( that is not nullptr ) in picture above
  • ObjectPath does not have the identical indices
    • especially, ComparisonIndex and DisplayIndex
    • strangefully, DisplayIndex indicates the intended string, other does not

※ more information about the indices in #2 reference

  • digging one step more, ComparisonIndex is set by one of FName that is created earlier
    • it may be confusing, let us summary as table
String ComparisonIndex
Sheri (AssetName) 884751
/Game/ThirdPersonCPP/Blueprints/Sheri.Sheri (ObjectPath) 884754
sheri (AssetName) 886628
/Game/ThirdPersonCPP/Blueprints/sheri.sheri (ObjectPath) 886631

※ the value of index is not consistent. it can have a different value each time
※ more information about the names and pathes in #3 reference

lower-case string compare

  • digging one step more !!
    • why did later one set by earlier one ?
  • let us look at StaticFindObjectFastInternalThreadSafe()
    • the function finds and returns an certain object(or package) depending on parameters
  • at line #588, there is an == operation between two FNames
    • if they are the same, the expression must become true
  • then, we got the one last destination FName::operator==()
    • actually, we should go to StrnicmpImpl() due to the callstack below
    • FName::operator==() → FNameHelper::EqualString() → StringAndNumberEqualsString() → FPlatformString::Strnicmp() → StrnicmpImpl()
  • in this function, each character of each string is compared in lower case
  • now we understand why Sheri and sheri are identical in using FName


  • FName is not identified with case
    • we could see it with Sheri and sheri example
    • every FName is compared in lower case
  • FName is used with index format
    • for effectiveness, real string is cached and is accessed with index
    • there are two indices, ComparisonIndex and DisplayIndex
  • FName duplication test is relying on path name
    • you can have two objects such as /Game/Sheri/Umbrella and /Game/Donita/Umbrella
    • this means that the same AssetName can exist multiple times in one project