C++: How to use extern variables in a struct

Question

I try to implement a piece of software, following a ETSI specification. Now i have to write a struct, in which a variable is declared as extern.

file2.cpp
struct struct_one {
    extern Algo algo;
    int x;
}

---

file1.cpp
struct struct_two {
    Algo algo;
    char c;
}

So, how do i "tell" the compiler, that the two "algo" variables contain the same things? Is it enough to include file1 in file2?

Or what do i have to do?

Greetings

Answer 1

You can't do that, but you can do it this way:

file1.cpp
struct struct_two {
    Algo algo;
    char c;
}

file2.cpp
struct struct_one {
    explicit struct_one(struct_two& t) : algo(t.algo) {}

    Algo& algo;
    int x;
}

or use combination pattern('has a' relationship):

file1.cpp
struct struct_two {
    Algo algo;
    char c;
}

file2.cpp
struct struct_one {
    struct_two t;
    int x;
}