How can I improve an ugly, dumb hack

Hi Folks:

Here's the situation:

m <- cbind(x=letters[1:3], y = letters[4:6])
m

     x   y
[1,] "a" "d"
[2,] "b" "e"
[3,] "c" "f"

## m is a 2 column character matrix

d <- data.frame(a=1:3,b=4:6)
d$c <- m
d

  a b c.x c.y
1 1 4   a   d
2 2 5   b   e
3 3 6   c   f

## But please note (as was remarked in a thread here a couple of months ago)

ncol(d)

[1] 3

## d is a ** 3 ** column data frame

Now what I wish to do is programmatically convert d to a 4 column
frame with names c("a","b","x","y"). Of course:

1. The column classes/modes must be preserved (character going to
factor and numeric remaining numeric).

2. I assume that I do not know a priori which of d's
components/columns are matrices and which are vectors.

3. There may be many more columns which are vectors or matrix than
just the three in this little example.

I can easily and sensibly accomplish these 3 tasks, but the problem is
that I run afoul of data frame column naming procedures in doing so,
about which the data.frame Help page says rather enigmatically:

"How the names of the data frame are created is complex, and the rest
of this paragraph is only the basic story." Indeed!
(This, of course, is shorthand for "Go look at the source if you want
to know!" )

Anyway, AFAICT from the Help, any "simple" approach to conversion
using data.frame results in "c.x" and "c.y" for the names of the last
two columns. I **can** get what I want by explicitly constructing the
vector of names via the following ugly hack; my question is, can it be
improved?

dd <- do.call(data.frame,d)

dd

  a b c.x c.y
1 1 4   a   d
2 2 5   b   e
3 3 6   c   f

ncol(dd)

[1] 4

cnames <- sapply(d,colnames)
cnames

$a
NULL

$b
NULL

$c
[1] "x" "y"

 names(dd) <-  unlist(ifelse(sapply(cnames,is.null),names(d),cnames))

  ##Yuck!

dd

  a b x y
1 1 4 a d
2 2 5 b e
3 3 6 c f

Cheers to all,
Bert