Why and where should 'void' is used in a c++ program

"void" is used to indicate that a function will not return a value. It is also used for a variable when a type is not specified. Here's a Microsoft discussion:

void (C++) (http://msdn.microsoft.com/en-us/library/fxky5d0w(VS.80).aspx)

void is used when a particular function does not return any value.. it is used to indicate that function is not returning any value.. :)