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copied!<p>I'm not sure what answer the questioner was expecting, but here's some possible solutions:</p> <p><strong>Make 'A' a pointer:</strong></p> <pre><code>//Only takes 4 or 8 bytes (32 bit vs 64 bit code) for 'A', regardless of 'A's actual size, but must point to an 'A' located elsewhere in memory with the full size. class B { A *a; //Only 4 bytes. int b; }; </code></pre> <p><strong>Make 'A' static:</strong></p> <pre><code>//Doesn't assume any of 'A's size, but all instances of 'B' shares a single instance of 'A'. class B { static A a; int b; }; </code></pre> <p><strong>Pass 'A' into the functions of 'B':</strong></p> <pre><code>//Pass in the reference to 'a' when needed, so multiple 'B's can share the same 'a' explicitly when desired. class B { void q(A &a) { this->b + a.a; return a.g(); } }; </code></pre> <p><strong>Make 'A' and 'B' not have virtual tables</strong> (probably the interviewer's point)</p> <pre><code>//By not having virtual functions, you save the (really small) cost of 4 bytes (8 bytes on 64 bit machines) class A { public: int f() { }; //Not virtual int a; } class B : public A { public: int g() { }; //Not virtual int b; } </code></pre> <p>It still costs you the size of A::a, and, unless you re-use 'a' in B instead of having B::b, you can't avoid those 4 bytes. And re-using one variable to mean something else entirely is a possible sign of really bad programming habits.</p> <p><strong>Unionize A'a and B's variables and put the functions in a single class</strong></p> <pre><code>class AB //Only 4 bytes total { public: union { int a; int b; }; void f(); void g(); }; </code></pre> <p>The bad idea about this is, you'll have to keep track of whether you should access 'a' or 'b', because both of them occupy the same 4 bytes of memory, and they can't both be using it at the same time.</p> <p>Another bad thing about this is that it is a sign that the class has too much responsibility. Is it an A or a B? If it's both, the all important question should be, "Why is it both?". It should have a <a href="http://en.wikipedia.org/wiki/Single_responsibility_principle" rel="nofollow noreferrer">single-responsibility</a>, not be a <a href="https://stackoverflow.com/questions/3179396/what-does-monolithic-mean">monolith</a> of mixed purposes.</p> <p><strong>Make 'A' a template, and inherit from <code>A<B></code>:</strong></p> <pre><code>template<typename TypeB> class A { int a; }; //Saves the cost of the virtual table... not that that really matters. class B : public A<B> { int b; }; </code></pre> <p>This last one is called the '<a href="http://en.wikipedia.org/wiki/Curiously_recurring_template_pattern" rel="nofollow noreferrer">curiously recurring template pattern</a>' (CRTP) The idea is that the inherited '<code>A<B></code>' can call access variables and functions from 'B' (if you pass B's 'this' pointer into A's constructor), and 'B' can access variables and functions directly from '<code>A<B></code>'.</p> <p>You're inheriting from a compile-time generated version of the template 'A', that is generated for 'B'.</p>

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