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POPure virtual functions may not have an inline definition. Why?
 

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  1. POPure virtual functions may not have an inline definition. Why?
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    <p>Pure virtual functions are those member functions that are virtual and have the <em>pure-specifier</em> ( <code>= 0;</code> )</p> <p><strong><em>Clause 10.4 paragraph 2</em></strong> of C++03 tells us what an abstract class is and, as a side note, the following:</p> <p><em>[Note: a function declaration cannot provide both a pure-specifier and a definition —end note] [Example:</em></p> <pre><code>struct C { virtual void f() = 0 { }; // ill-formed }; </code></pre> <p><em>—end example]</em></p> <p>For those who are not very familiar with the issue, please note that <strong><em>pure virtual functions can have definitions</em></strong> but the above-mentioned clause forbids such definitions to appear inline (lexically in-class). (For uses of defining pure virtual functions you may see, for example, <a href="http://www.gotw.ca/gotw/031.htm" rel="noreferrer">this GotW</a>)</p> <p>Now for all other kinds and types of functions it is allowed to provide an in-class definition, and this restriction seems at first glance absolutely artificial and inexplicable. Come to think of it, it seems such on second and subsequent glances :) But I believe the restriction wouldn't be there if there weren't a specific reason for that.</p> <p>My question is: does anybody <strong><em>know</em></strong> those specific reasons? <em>Good</em> guesses are also welcome.</p> <p><strong><em>Notes:</em></strong></p> <ul> <li>MSVC does allow PVF's to have inline definitions. So don't get surprised :)</li> <li>the word <code>inline</code> in this question does not refer to the <em>inline</em> keyword. It is supposed to mean <em>lexically in-class</em></li> </ul>
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    1. USLightness Races in Orbit
    1. USArmen Tsirunyan
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    1. COLooks a bit weird when using a function try block: `virtual void f() = 0 try { } catch(...) { }`
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      1. POPure virtual functions may not have an inline definition. Why?
      1. USJohannes Schaub - litb
    2. COHere's a guess: It was simpler to create a compiler that didn't allow it and given its very rare usage, was an easy decision. The fact that some compilers (you cited MSVC) do allow it simply means that some compiler authors weren't bothered by the extra work.
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      1. POPure virtual functions may not have an inline definition. Why?
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    3. CO@Johannes: What's weird about it? Function try blocks are themselves weird :)
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