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POhow to specify NULL as pointer-type argument to template function
 

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    <p>This shows the gist of it:</p> <pre><code>#include &lt;utility&gt; class A { public: A() { } }; class B { public: B() { } }; typedef std::pair&lt;A*, B*&gt; ABPair; int main(int argc, char* argv[]) { B* b = 0; // no C2440 ABPair p2(new A(), b); ABPair p1(new A(), 0); // C2440 return 0; } </code></pre> <p>Is there a better way to make the p1 declaration work than just forcing a cast, e.g. <code>ABPair p1(new A(), (B*)NULL)</code>? This seems like it would be pretty common &amp; that there would be a "right" way to do it .. and that casting it is not the right way.</p> <p>On VS 2010, here's the full error:</p> <pre><code>1&gt;ClCompile: 1&gt; test.cpp 1&gt;c:\program files (x86)\microsoft visual studio 10.0\vc\include\utility(163): error C2440: 'initializing' : cannot convert from 'int' to 'B *' 1&gt; Conversion from integral type to pointer type requires reinterpret_cast, C-style cast or function-style cast 1&gt; c:\program files (x86)\microsoft visual studio 10.0\vc\include\utility(247) : see reference to function template instantiation 'std::_Pair_base&lt;_Ty1,_Ty2&gt;::_Pair_base&lt;_Ty,int&gt;(_Other1 &amp;&amp;,_Other2 &amp;&amp;)' being compiled 1&gt; with 1&gt; [ 1&gt; _Ty1=A *, 1&gt; _Ty2=B *, 1&gt; _Ty=A *, 1&gt; _Other1=A *, 1&gt; _Other2=int 1&gt; ] 1&gt; c:\users\charliearnold\documents\visual studio 2010\projects\test\test\test.cpp(20) : see reference to function template instantiation 'std::pair&lt;_Ty1,_Ty2&gt;::pair&lt;A*,int&gt;(_Other1 &amp;&amp;,_Other2 &amp;&amp;)' being compiled 1&gt; with 1&gt; [ 1&gt; _Ty1=A *, 1&gt; _Ty2=B *, 1&gt; _Other1=A *, 1&gt; _Other2=int 1&gt; ] 1&gt;c:\program files (x86)\microsoft visual studio 10.0\vc\include\utility(163): error C2439: 'std::_Pair_base&lt;_Ty1,_Ty2&gt;::second' : member could not be initialized 1&gt; with 1&gt; [ 1&gt; _Ty1=A *, 1&gt; _Ty2=B * 1&gt; ] 1&gt; c:\program files (x86)\microsoft visual studio 10.0\vc\include\utility(167) : see declaration of 'std::_Pair_base&lt;_Ty1,_Ty2&gt;::second' 1&gt; with 1&gt; [ 1&gt; _Ty1=A *, 1&gt; _Ty2=B * 1&gt; ] 1&gt; 1&gt;Build FAILED. </code></pre>
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    1. COYour constructors don't match your object constructions. :) I figure this is for simplification, but a testcase ought to compile aside from the asked-about error. Additionally, "C2440" is a vendor-specific error code so error text would be more useful.
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    2. COOK, you've added the text now. :)
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    3. COWhy do you want to use 0 instead of NULL? This may not address the main point of your question, but isn't NULL much more readable in this case?
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      1. USJonathan Wood
 

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