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    <p>I think there are two issues at play here. One is that the code smells bad, as you say, and the second is that it doesn't compile on your system. Regarding the smell, yes, excessive use of the <code>friend</code> keyword often indicates bad OO design. It is generally better to give an object what it needs to do its job rather than pull something out of it, which is essentially what <code>friend</code> lets you do.</p> <p>Large numbers of inner classes, IMO, are not that bad, as they merely serve to keep classes within the scope of another. This helps keep the namespace less cluttered and allows you to use common names across different functional areas.</p> <p>Regarding the compilation issue, I tried to reproduce this on my system (Ubuntu 10.04, gcc 4.4.3, two years old at this point), and created this sample program:</p> <pre><code>class A { friend class B; int x; }; class B { class Inner { public: void foo(A&amp; a) { a.x++; } }; public: void bbb() { Inner i; A a; i.foo(a); } }; int main() { B b; b.bbb(); } </code></pre> <p>This compiled with no errors or warnings. I initially had nothing marked public, but I needed to add <code>bbb()</code> to create an instance of class <code>Inner</code>, and I had to add a public method of <code>Inner</code> so I could invoke it to do something with an <code>A</code>.</p> <p>The upshot is that I think your development system is out of date. The main developers clearly have no problem compiling their code, so the difference has to be with your system, or else they have configured their systems in a way that they are not telling you about. That being said, I think they could do more to make their code more portable and easier to build. In an open source project, that is to be expected if you want others to contribute.</p>
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    1. POGiven a class and its friend class, are the latter's inner classes automatically friends to the former class?
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    1. USRandall Cook
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    1. POGiven a class and its friend class, are the latter's inner classes automatically friends to the former class?
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    1. COIs the code always supposed to be legal, or was it added in C++11 or a defect report? (I said the latter because I doubt there's any system compilers out now that require C++11. And earlier ones don't have a C++11 mode.) I think they use GCC for a boot-strapping compiler, maybe they (and you) are exploiting a GCC-specific quirk/extension. The rules have changed before: (based on another SO query) C++11 now makes inner classes friends of their surrounding class; they weren't before.
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    2. CO@CTMacUser: The code was illegal, and I did not recall it being changed, but the fact is that it has been changed in the new standard. There is an explicit example in the c++11 standard showing this.
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      1. USDavid Rodríguez - dribeas
    3. COI don't think this is a C++11 feature. As I said, my compiler is gcc 4.4.3, which is [from](http://gcc.gnu.org/releases.html) January 2010. Your compiler, gcc 4.0.1, is from July 2005, so is quite old and could easily have a problem with this code. You make a good point that gcc changes from time to time, but I would favor a newish version over a much older one, especially one so close to a major release (4.0.1 is close to 4.0, and might likely have many issues not known at the time of its release).
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