Swapping values between two columns using data.tab

I have been breaking my head over translating this question to a data.table solution. (to keep it simple I'll use the same data set)
When V2 == "b I want to swap the columns between V1 <-> V3.

dt <- data.table(V1=c(1,2,4), V2=c("a","a","b"), V3=c(2,3,1))
#V1 V2 V3
#1:  1  a  2
#2:  2  a  3
#3:  4  b  1

The code below would be the working solution for data.frame, however because of the amount of frustration this has given me because I was using a data.table without realising I'm now determined to find a solution for data.table.

dt <- data.table(V1=c(1,2,4), V2=c("a","a","b"), V3=c(2,3,1))
df <- as.data.frame(dt)
df[df$V2 == "b", c("V1", "V3")] <- df[df$V2 == "b", c("V3", "V1")] 
#  V1 V2 V3
#1  1  a  2
#2  2  a  3
#3  1  b  4

I have tried writing a lapply function looping through my target swapping list, tried to narrow down the problem to only replace one value, attempted to call the column names in different ways but all without success.
This was the closest attempt I've managed to get:

> dt[dt$V2 == "b", c("V1", "V3")] <- dt[dt$V2 == "b", c(V3, V1)]
#Warning messages:
#1: In `[<-.data.table`(`*tmp*`, dt$V2 == "b", c("V1", "V3"), value = c(1,  :
#  Supplied 2 items to be assigned to 1 items of column 'V1' (1 unused)
#2: In `[<-.data.table`(`*tmp*`, dt$V2 == "b", c("V1", "V3"), value = c(1,  :
#  Supplied 2 items to be assigned to 1 items of column 'V3' (1 unused)

How can we get the data.table solution?

回答1:

We can try

dt[V2=="b", c("V3", "V1") := .(V1, V3)]

回答2:

For amusement only. @akruns' solution is clearly superior. I reasoned that I could create a temporary copy, make the conditional swap, and then delete the copy all using [.data.table operations in sequence:

 dt[, tv1 := V1][V2=="b", V1 := V3][V2=="b", V3 := tv1][ , tv1 := NULL]

> dt
   V1 V2 V3
1:  1  a  2
2:  2  a  3
3:  1  b  4