c++ define a group of const struct in head file

I am using a C++ library which defines a struct.

Eg.

struct demo{
    string x;
    string y;
    string z;
    int n;
};

Normally I use this struct this way

demo s;
s.x = "AAA";
s.y = "BBB";

If I need to use certain values of the struct frequently. It makes sense to define a group of const struct in a new head file. What the right way to define the head file and implementation file(is it needed)? Further more, is there any convenient way to do this by #define preprocessor directives?

const struct AAABBB;
const struct AAACCC;
...
const struct BBBCCC;

in which x=AAA, y=BBB respectively.