Separate files for instance declarations?

Question

I'm kind of new to c++, if I have a program with a lot of instances declared in the middle of the program and I want them to be in a separate file, like a header file, if I do that I only get a lot of errors, how can I fix this?

Edit: I realized it doesn't give errors from declaring them, only when I use functions, is there any way to use functions in a header file? Like if I have a class called rectangle and I create a rectangle called 'r', then I have a function to change its color and I want to do that in the header file.

Answer 1

This way you kill the linker man!

Headers are public interface for your programs, they have their implementations in the source file.
And instances should go to the main .
There are exceptions like static linkage or extern but if you're new to the language avoid them .

Stick to a normal structure
Headers and sources define behaviour
aaaa.h => aaaa.cpp
bbb.h => bbb.cpp
...
Main(){....// behaves