Things to know about FName

FName includes an optional instance number

Most FName in UE don’t contain numbers, but an optional number IS supported as a part of the FName.

Significantly, notice that the internal Number is always 1 greater than the number represented in the string. This allows for FName("Foo") to be distinctly different to FName("Foo_0").

The operator== considers both the PlainString part of the FName and the Number part of the name, such that FName("Foo_1") != FName("Foo_2")

If you want to compare only the PlainString part of the name, such that Foo_1 will be considered equivalent to Foo_2, the most efficient way to do so is to use GetComparisonIndex(), like:

if (FName("Foo_1").ComparisonIndex() == FName("Foo_2").ComparisonIndex())
{
    // This is true.
    // "Foo" == "Foo"; the "_1" and "_2" are ignored by ComparisonIndex()
}

Sample code

Run this code to help understand how FName handles the optional Number data:

FName Foo  ("Foo");     // same as FName("Foo", 0)
FName Foo1 ("Foo", 1);  // same as FName("Foo_0")
FName Foo2 ("Foo_1");   // same as FName("Foo", 2)

UE_LOG(LogTemp, Log, TEXT("Foo  PlainString=\"%s\", Number=%d, ToString=\"%s\""), *Foo.GetPlainNameString(), Foo.GetNumber(), *Foo.ToString());
UE_LOG(LogTemp, Log, TEXT("Foo1 PlainString=\"%s\", Number=%d, ToString=\"%s\""), *Foo1.GetPlainNameString(), Foo1.GetNumber(), *Foo1.ToString());
UE_LOG(LogTemp, Log, TEXT("Foo2 PlainString=\"%s\", Number=%d, ToString=\"%s\""), *Foo2.GetPlainNameString(), Foo2.GetNumber(), *Foo2.ToString());

// All these comparison methods include both the PlainString AND the Number
UE_LOG(LogTemp, Log, TEXT("Foo == Foo1 ? %s"), Foo == Foo1 ? TEXT("true") : TEXT("false"));

// Optimal way to compare JUST the PlainString part, ignoring the Number part
UE_LOG(LogTemp, Log, TEXT("Foo.GetComparisonIndex() == Foo1.GetComparisonIndex() ? %s"), Foo.GetComparisonIndex() == Foo1.GetComparisonIndex() ? TEXT("true") : TEXT("false"));

Result

LogTemp: Foo  PlainString="Foo", Number=0, ToString="Foo"
LogTemp: Foo1 PlainString="Foo", Number=1, ToString="Foo_0"
LogTemp: Foo2 PlainString="Foo", Number=2, ToString="Foo_1"

LogTemp: Foo == Foo1 ? false

LogTemp: Foo.GetComparisonIndex() == Foo1.GetComparisonIndex() ? true

Thanks!

Thanks to Dirtsleeper on the Unreal Source Discord for bringing this non-obvious behavior to my attention.