vim:Function to comment a visually selected text

In your vimrc add following function

"Function for commenting a block of Visually selected text

function Comment(fl, ll)

let i=a:fl

let comment="//"

while i<=a:ll

let cl=getline(i)

let cl2=comment.cl

call setline(i, cl2)

let i=i+1

endwhile

endfunction

"Function for Un-Commenting a block of Visually selected text

function UnComment(fl, ll)

let i=a:fl

let comment="//"

while i<=a:ll

let cl=getline(i)

let cl2=substitute(cl, "//", "", "")

call setline(i, cl2)

let i=i+1

endwhile

endfunction

" map visual mode keycombo 'com' to this function

vmap com :call Comment(fl, ll)

vmap uncom :call UnComment(fl, ll)