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POCRTP-related compiler error on pointer-to-a-member-function default value
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  1. POCRTP-related compiler error on pointer-to-a-member-function default value
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    copied!<p> Hi there,</p> <p>While making a CRTP-based generic wrapper to call arbitrary library functions, I've encountered a problem which I have trouble understanding. Here is a very simplified code to illustrate the problem:</p> <pre class="lang-c++ prettyprint-override"><code>#include &lt;iostream&gt; template&lt; typename PValue, typename PDerived &gt; class TBase { private: typedef TBase TSelf_; typedef PDerived TDerived_; protected: typedef PValue TValue_; protected: TBase( void ) { std::cout &lt;&lt; " TBase::TBase() " &lt;&lt; std::endl; } public: void Foo( void ) { std::cout &lt;&lt; " TBase::Foo() " &lt;&lt; std::endl; } template&lt; typename PType &gt; static void Call( PType /*pSomething*/, void(TDerived_::*pFunction)( void ) = &amp;TSelf_::Foo, TDerived_ pDerived = TDerived_() ) { ( pDerived.*pFunction )(); std::cout &lt;&lt; " static TBase::Call(). " &lt;&lt; std::endl; } }; template&lt; typename PValue &gt; class TDerived : public TBase&lt; PValue, TDerived&lt; PValue &gt; &gt; { friend class TBase&lt; PValue, TDerived&lt; PValue &gt; &gt; ; private: typedef TBase&lt; PValue, TDerived &gt; TBase_; typedef TDerived TSelf_; public: TDerived( void ) : TBase_() { std::cout &lt;&lt; " TDerived::TDerived() " &lt;&lt; std::endl; } void Foo( void ) { std::cout &lt;&lt; " TDerived::Foo() " &lt;&lt; std::endl; } void Bar( void ) { std::cout &lt;&lt; " TDerived::Bar() " &lt;&lt; std::endl; } }; int main( void ) { TDerived&lt; int &gt;::Call( 1 ); TDerived&lt; int &gt;::Call( 1, &amp;TDerived&lt; int &gt;::Foo ); TDerived&lt; int &gt;::Call( 1, &amp;TDerived&lt; int &gt;::Bar, TDerived&lt; int &gt; () ); return ( 0 ); } </code></pre> <p>Everything compiles and works as intended. However, if I try to use pointer to <code>TDerived::Foo()</code> as a default argument for the second parameter in <code>TBase::Call(...)</code>:</p> <pre class="lang-c++ prettyprint-override"><code>static void Call( PType /*pSomething*/, void(TDerived_::*pFunction)( void ) = &amp;TDerived_::Foo, TDerived_ pDerived = TDerived_() ) </code></pre> <p>compilers gives a syntax error... I have a feeling it is related to how compiler parses code and that it cannot figure out pointer to a function of yet to be defined (or instantiated) class. However, it has no problem calling <code>TDerived</code> constructor as a default argument for the third parameter of <code>TBase::Call(...)</code>. Can someone give me a definite answer about what's going on? Why derived class MFP is not accepted, and object of derived class is accepted as default arguments?</p> <p>Thanks.</p> <p>EDIT: compiler's error (MSVS2010 command line compiler):</p> <pre class="lang-c++ prettyprint-override"><code>FMain.cpp(224) : error C2061: syntax error : identifier 'TDerived_'; FMain.cpp(233) : see reference to class template instantiation 'TBase&lt;PValue,PDerived&gt; with [PValue=int,PDerived=TDerived&lt;int&gt;]' being compiled; FMain.cpp(323) : see reference to class template instantiation 'TDerived&lt;PValue&gt; with [PValue=int]' being compiled </code></pre> <p>It's a syntax error - it does not recognize <code>TDerived_</code> as type in default argument for MFP. There are other errors following this one, they are all syntax errors, since function definition is ill-formed now. That is how I understand it.</p> <p>EDIT: Basically, I don't understand why can I use an object of <code>TDerived_</code> as a default argument, but can not use a pointer to a member function as a default argument.</p> <p>EDIT: Ok, this is driving me crazy now. First of all, I changed to <code>typedef TBase&lt; PValue, TDerived &gt; TBase_;</code> as it was pointed out (thank you, guys!). Indeed, it only compiled under MSVC++, since this compiler does not do two-part parsing; i.e., on codepad.org (which uses g++ 4.1.2) it didn't compile. Second, after that, I tried to use <code>static void Call( PType /*pSomething*/, void(TDerived_::*pFunction)( void ) = &amp;TDerived_::Foo, TDerived_ pDerived = TDerived_() )</code> on codepad.org and... it compiled and run correctly! So I'm REALLY confused now: people explained to me why it's not correct (and I couldn't understand "why" (see my previous EDIT)) and now it turns out g++ compiles it correctly... Does it mean it just MSVC++ problem and not the code? Or code does have a problem from the Standard point of view (and I cannot see it) and g++ accept it "by mistake" (unlikely, I think)?.. Help?!</p>
 

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