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    <p>First of all, the only difference between a <code>class</code> and a <code>struct</code> in C++ is that a class' members are <code>private</code> by default and a struct's members are <code>public</code> by default. Compiler-generated ctors and dtors are visible in both cases - unless otherwise stated by the programmer (e.g. they move the default ctor into a <code>private</code> section). Otherwise construction and destruction of instances of user-defined types marked <code>class</code> wouldn't be possible without explicit, public declaration - thus defying the very purpose of compiler-generated functions.</p> <p>So basically, what you do in your example is merely <em>composition</em> of two user defined types which is perfectly legal. When you create an instance of <code>capture</code>, an instance of <code>test</code> is created as well.</p> <p>What you <em>can't</em> do is publicly access <code>AttachToInput()</code> from outside of <code>test</code> and <em>derived</em> types of <code>test</code>. You need to declare the function <code>public</code> in order for this line to compile:</p> <pre><code>h_cap.h_app.AttachToInput(); // error: member function of `test` is protected </code></pre> <p>On another, unrelated note (but I came across it so I mention it), your <code>class test</code> holds a raw pointer to <code>char</code>. Holding raw pointers is ok, if the lifetime of the entity that's being pointed is guaranteed to exceed the lifetime of the object that holds the pointer. Otherwise, it's very likely the object itself is responsible for the destruction of said entity. You need to be sure about who owns what and who's responsible for allocation and deallocation of stuff.</p> <p><strong>EDIT</strong>: It should be noted, that Alan Stokes proposed the same in the comment section while I wrote this answer. :)</p> <p><strong>EDIT2</strong>: Slight oversight, implicit default access is also assumed for base classes depending on how the <em>derived</em> class is declared. See <a href="https://stackoverflow.com/questions/92859/what-are-the-differences-between-struct-and-class-in-c">What are the differences between struct and class in C++?</a>.</p>
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    1. COHello Thokra, Thanks for your help. It clarified most of my doubts. I have updated my question now. Can you take a look at it and help me resolve this issue. Good day.
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    2. CO@RajuBabannavar: The segfault you get is not due to the call to `AttachToInput()`. I suggest you debug into the implementation of the function and make sure it's not accessing invalid addresses. With a stub implementation of `AttachToInput()`, your example works fine - as it should.
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