If I understood well, you need to declare a variable to store some text and would like that variable to be public, don't you?
Well, that in C/C++ is called a global variable and you have to define it outside any function. The syntax is the same as if it was a local variable:
If you put that outside any function, you'll get a variable called topic accessible from all the functions that you have declared in the same file.
I hope I've answered your question