how userdefind function work in c++?

Given that it does nothing, it works pretty well. Apart from not compiling, of course.

Try this:

void a()
   {
   }

Since it returns nothing, it should have a void type - C++ does not have a keyword for functions, you just declare the return type.

Then just call it:

a();

Mind you, given that it doesn't actually do anything, you could leave the call out and have pretty much the same affect...