Why would a struct need a friend function?

Question

I\'m trying to build a program whose source I downloaded from the internet. When I try to compile it, I get the error message

friend declaration specifying          


        

Answer 1

As many have mentioned before, a struct is the same as a class, except that its members are public by default. It can have private members.

I'm guessing here, but I think the idea behind making this a friend factory function is that it the designer of the library is reserving the right to initialize any private members added in the future.

To answer your questions:

• Moving the default parameter to the inline function seems to work.
• Moving the mkLit before the struct is a problem because it returns a Lit, and Lit hasn't been defined yet (since it now appears below mkLit).
• Removing the friend declaration inside the struct will work because the struct has no private members -- for now. If you ever merge in a newer version of the original library that changes adds private members (or makes members private by default by changing the text struct to class), then it will stop working.

(note: I pasted your code into http://godbolt.org and tried the various permutations, using different compilers)