Change of file extension in jam script

How can i change extension from an input files in jam script. On linux box i have following working code -

for local var in $(objFiles)
{
    local objName = [ SHELL "var1=$(var); echo ${var1%.cpp}.obj" ] ;
    obj $(objName) : $(var) : $(properties) ;
}

1st line within for loop picks cpp file name and change its extention from <File>.cpp to <File>.obj. I need to covert these changes compatible to windows platform. I think some module in boost build process would have provided some way to do this but google search in this didnt helped much.

What changes should i do in above code so that objName would contain obj file extension and these changes remains compatible across platform (specially win/ linux).