Description

Avoid using forward declarations where possible. Just #include the headers you need:

Forward declarations can hide a dependency, allowing user code to skip necessary recompilation when headers change.
A forward declaration may be broken by subsequent changes to the library. Forward declarations of functions and templates can prevent the header owners from making otherwise-compatible changes to their APIs, such as widening a parameter type, adding a template parameter with a default value, or migrating to a new namespace.
Forward declaring symbols from namespace std:: yields undefined behavior.
It can be difficult to determine whether a forward declaration or a full #include is needed. Replacing an #include with a forward declaration can silently change the meaning of code:
```
  // b.h:
  struct B {};
  struct D : B {};

  // good_user.cc:
  #include "b.h"
  void f(B*);
  void f(void*);
  void test(D* x) { f(x); }  // calls f(B*)
```

If the #include was replaced with forward decls for B and D, test() would call f(void*).

Forward declaring multiple symbols from a header can be more verbose than simply #includeing the header.
Structuring code to enable forward declarations (e.g. using pointer members instead of object members) can make the code slower and more complex.

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