What are the risks to massively forward declaration classes in header files?

Question

For every pointer p1 of class class1, are there any risks we should consider by using a forward declaration of class1 instead of including class1 header file?

I can see only the advantage: The header file will have less size.

Answer 1

I can see only the advantage: The header file will have less size.

That's not exactly the point.

Let's assume you have a class declaration in a header file like

namespace MyNamespace {
    class Baz;
}

class Foo {
public:
    void bar(const MyNamespace::Baz & x);
};

and the definition in a separate translation unit as

#include "Baz.hpp"

void Foo::bar(const MyNamespace::Baz & x) {
    // actually do something with Baz
}

and in contrast having everything included in the header file (and necessarily all dependent sources will be recompiled when Baz.hpp will be changed)

#include "Baz.hpp"

class Foo {
public:
    void bar(const MyNamespace::Baz & x);
};

with the declaration, the 1st version might help to compile the code a little bit faster.

Especially if you have your own headers and class declarations, and if any of these are likely to be changed, you only want to recompile your translation units in the codebase, and not every source file that includes your type dependent header.

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Note that forward declarations can be only used with references and pointers. Also header inlined code which dereferences to the forwarded type members cannot be used.