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POpolymorphic C++ references
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7192069
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7
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2011-08-25T14:24:54.007
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3
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2014-10-29T19:41:25.440
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2011-08-25T14:52:40.807
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430766
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1
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18
ViewCount
19079
LastEditorDisplayName
text
Body
<p>I was wondering <strong>how</strong> you can do polymorphism with references, as opposed to pointers.</p> <p>To clarify, see the following minimal example:</p> <pre><code>class A; class B { public: A& a; ///////////////// <- #1 B(); void doStuff(); }; class A { public: virtual void doSmth() = 0; }; void B::doStuff() { a.doSmth(); } class A1 : public A { public: void doSmth() { } }; B::B() : a( * ////////////// <- #2 (new A1) /////////// <- #3 ) { } </code></pre> <p>This compiles and works, but as the most important point here is that <code>a</code> in line <code>#1</code> is a reference, so in order to be able to use it polymorphically (is that an actual word?), as shown in line <code>#3</code> I have to "convert a pointer to a reference" by dereferencing it.</p> <p>This strikes me as a bit odd, and I was wondering if there is a better (in the sense of <em>cleaner</em>) way. Is it just me?</p> <h3>Rationale</h3> <p>It would be great if I didn't need a <code>new</code> at all, but when declaring (!) <code>B</code> I have no clue how to create an instance of <code>A1</code> (!) as <code>A</code> is a forward declaration -- <code>A1</code> is implemented in the same compilation unit as <code>B</code>. Still, is there a real need for dynamic memory allocation in this case? How would you do this?</p> <p>Sorry for the slightly twofold question.</p> <h2>Edit</h2> <p>Note: <code>B</code> is huge (and I cannot make a template class of it), and will go out of scope precisely when the program terminates -- <code>a</code> is small and makes two big modules talk to each other, it will be needed as long as the instance of <code>B</code> lives (there is only one).</p> <h2>Edit 2</h2> <p>I just realised, that since both <code>A</code> and <code>B</code> are effectively singletons, I can simply create a <code>static</code> instance of <code>A1</code> in the compilation unit of <code>B</code>, avoiding dynamic memory allocation (even if there were two <code>B</code>s they could easily use the same instance of <code>A</code>). To be fair, I did not post this as answer, but will accept the answer that prompted me to come up with this <em>solution</em>.</p>
Tags
<c++><polymorphism><reference>
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polymorphic C++ references
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