A forward declaration is not really a compile time dependency.
If you are forward declaring a function, you are just promising that it is present during linking. So, during compilation it is not an edge in the graph.
If you are forward declaring a data type such as a class, a full definition needs to be visible at its first use or you are using the type as a pointer:
class A;
class B {
A *d_a;
[...]
};
In this case it is not really a dependency either, since the compiler does not need to know the size of
A, since
d_a is a pointer. When you start to dereference
d_a, its definition needs to be fully visible, which is done via headers, which are a DAG through guards.
Could you give an example where C++ dependencies are not a DAG during compilation?
