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    <p>Because I may write <code>Foo</code> and its friend <code>Bar</code> (thus there is a trust relationship). </p> <p>But do I trust the people who write classes that are derived from <code>Bar</code>?<br> Not really. So they should not inherit friendship.</p> <p>Any change in the internal representation of a class will require a modification to anything that is dependent on that representation. Thus all members of a class and also all friends of the class will require modification. </p> <p>Therefore if the internal representation of <code>Foo</code> is modified then <code>Bar</code> must also be modified (because friendship tightly binds <code>Bar</code> to <code>Foo</code>). If friendship was inherited then all class derived from <code>Bar</code> would also be tightly bound to <code>Foo</code> and thus require modification if <code>Foo</code>'s internal representation is changed. But I have no knowledge of derived types (nor should I. They may even be developed by different companies etc). Thus I would be unable to change <code>Foo</code> as doing so would introduce breaking changes into the code base (as I could not modify all class derived from <code>Bar</code>).</p> <p>Thus if friendship was inherited you are inadvertently introducing a restriction on the ability to modify a class. This is undesirable as you basically render useless the concept of a public API.</p> <p>Note: A child of <code>Bar</code> can access <code>Foo</code> by using <code>Bar</code>, just make the method in <code>Bar</code> protected. Then the child of <code>Bar</code> can access a <code>Foo</code> by calling through its parent class.</p> <p>Is this what you want?</p> <pre><code>class A { int x; friend class B; }; class B { protected: // Now children of B can access foo void foo(A&amp; a, int n) { a.x = n; } }; class D : public B { public: foo(A&amp; a, int n) { B::foo(a, n + 5); } }; </code></pre>
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    1. POWhy does C++ not allow inherited friendship?
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    1. USMartin York
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    1. COBingo. It's all about limiting the damage you can cause by changing a class's internals.
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    2. COFrankly, the case I'm really thinking about this is the Attorney-Client pattern, where an intermediate acts as a limited interface to outsides classes by presenting wrapper methods to the underlying restricted-access class. Saying that an interface is available to all the children of other classes rather than an exact class would be much more useful than the system at present.
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    3. CO@Jeff: http://www.drdobbs.com/184402053
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