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    <p>In the SO thread <a href="https://stackoverflow.com/questions/2156634/why-pure-virtual-function-is-initialized-by-0">&ldquo;Why pure virtual function is initialized by 0?&rdquo;</a> Jerry Coffin provided this quote from Bjarne Stroustrup&rsquo;s <a href="http://www2.research.att.com/~bs/dne.html" rel="nofollow noreferrer"><em>The Design &amp; Evolution of C++</em></a>, section §13.2.3, where I've added some emphasis of the part I think is relevant:</p> <blockquote> <p>The curious <code>=0</code> syntax was chosen over the obvious alternative of introducing a new keyword pure or abstract because at the time I saw no chance of getting a new keyword accepted. Had I suggested pure, Release 2.0 would have shipped without abstract classes. Given a choice between a nicer syntax and abstract classes, I chose abstract classes. Rather than risking delay and incurring the certain fights over pure, I used the tradition C and C++ convention of using 0 to represent "not there." The <code>=0</code> syntax fits with <strong>my view that a function body is the initializer for a function</strong> and also with the (simplistic, but usually adequate) view of the set of virtual functions being implemented as a vector of function pointers. [ &hellip; ]</p> </blockquote> <p>So, when choosing the syntax Bjarne was thinking of a function body as a kind of initializer part of the declarator, and <code>=0</code> as an alternate form of initializer, one that indicated &ldquo;no body&rdquo; (or in his words, &ldquo;not there&rdquo;).</p> <p>It stands to reason that one cannot both indicate &ldquo;not there&rdquo; and have a body &ndash; in that conceptual picture.</p> <p>Or, still in that conceptual picture, having two initializers.</p> <p>Now, that's as far as my telepathic powers, google-foo and soft-reasoning goes. I surmise that nobody's been Interested Enough&trade; to formulate a proposal to the committee about having this purely syntactical restriction lifted, and following up with all the work that that entails. Thus it's still that way.</p>
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    1. POPure virtual functions may not have an inline definition. Why?
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    1. USCheers and hth. - Alf
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    1. COWow... 21 votes (so far), and unless I've missed something, you've said not one word to address the question (which was actually why out-of-line definitions are possible but inline not).
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    2. COquotations are powerful :)
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    3. CO@Tony: I'm sorry that you didn't understand the relevance of my answer. I tried my best. It didn't work for you. I don't know how to say it more clearly. Possibly there's a communication gap between us.
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      1. USCheers and hth. - Alf
 

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