Strange behaviour for data.frames without column names

Question

There is an unexpected behaviour for data.frames without column names. The following works as expected:

df <- data.frame(a = 1:5, b = 5:9)
df + 1
##   a  b
## 1 2  6
## 2 3  7
## 3 4  8

but if we remove the column names then the behaviour is strange:

names(df) <- NULL
df + 1
## data frame with 0 columns and 0 rows

The same is happening if the names are removed with unname, setNames. Any ideas of why this happens and is it (for some reason) expected behaviour?

Edit: So it is documented that nameless data.frames have unsupported results (thanks @neilfws, @Suren) but I am also interested to the why this happens. I try to find the actual c (?) code that makes this simple example to brake.

Answer 1

In the documentation for data.frame, it says:

The column names should be non-empty, and attempts to use empty names will have unsupported results.

So, it is expected that outcome may not be the desired ones if the column names are empty.