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POUnable to pass null function-pointer as template argument
 

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    <p>I would like to pass a function as a template argument to another function so that it can be stored and called back later. In some cases, I want to pass NULL for the call back, but I'm having trouble. Here's an example of what I'd like to be able to do:</p> <pre><code>#include &lt;iostream&gt; struct Foo { int i; }; template &lt;typename T&gt; T* T_new() { return new T(); } Foo* Foo_new() { return new Foo(); } template &lt;typename T, T* (*func)()&gt; T* T_new() { if (func) return func(); else return NULL; } int main(void) { // Works Foo* f1 = T_new&lt;Foo&gt;(); std::cout &lt;&lt; f1 &lt;&lt; std::endl; // Works Foo* f2 = T_new&lt;Foo, Foo_new&gt;(); std::cout &lt;&lt; f2 &lt;&lt; std::endl; // fails to compile, "no matching function for call to ‘T_new()’" // Foo* f3 = T_new&lt;Foo, NULL&gt;(); // std::cout &lt;&lt; f3 &lt;&lt; std::endl; return 0; } </code></pre> <p>I found <a href="https://stackoverflow.com/questions/4596499/how-to-specify-null-as-pointer-type-argument-to-template-function">this</a> similar question, but that deals with passing null as an argument to the constructor, not passing null as a template argument, and the trick there (using <code>(Foo*)0</code>) doesn't work as a template argument. </p> <p>Is there a way to work around this or do some tricky template specialisation or some other clever thing to get the desired effect?</p> <p>EDIT:</p> <p>The above was a simplified example that illustrated the problem I was having, but here's the concrete problem I'm trying to solve. I have <a href="https://bitbucket.org/alexames/luawrapper/src/37d6c4ad5886/LuaWrapper.h" rel="nofollow noreferrer">this project</a> I'm working on. This is a set of functions that make mixing C++ and Lua simpler for me (For various reasons I don't want to use LuaBind or the other existing functions I've found out there). The important function to this question is <code>luaW_register&lt;T&gt;</code>, near the bottom. This is a slightly out of date version, but it works in almost all cases. It doesn't work, however, if the constructor is private, which has just come when I tried mixing this with Box2D's <code>b2Body</code> (which needs to be made from a <code>b2World</code>). <code>luaW_defaultallocator&lt;T&gt;()</code> (and <code>luaW_defaultdeallocator&lt;T&gt;()</code>) still gets created since I'm using it as the default argument in <code>luaW_register&lt;T&gt;()</code>. </p> <p>My proposed solution was to pull the <code>allocator</code> parameter out into template parameters of <code>luaW_Register</code>. Then, if I want to use some other function to get my objects for a specific type, <code>luaW_defaultallocator</code> will not even be created. In cases like <code>b2Body</code>s, where they can't create themselves at all, I would like to be able to just pass in <code>NULL</code> as a template argument (which seems perfectly reasonable, but the compiler is choking on it for reasons that are still unclear to me, it seems like if I can set a value to <code>NULL</code> anywhere else in code I ought to be able to for templates as well). A hack I initially implemented was to pass in a boolean argument to my function which would disable the ability to call <code>Foo.new</code> from my Lua code, but that doesn't stop <code>defaultallocator</code> from compiling, and if I can use the null check in and working the way I would like it has the nice side effect of letting me simply check if there's an allocator and use that to control whether or not the <code>new</code> function gets added to the lua table.</p> <p>tl;dr: my goal was to go from this:</p> <pre><code>template &lt;typename T&gt; void luaW_register(lua_State* L, const char* classname, const luaL_reg* table, const luaL_reg* metatable, const char** extends = NULL, bool disablenew = false, T* (*allocator)() = luaW_defaultallocator&lt;T&gt;, void (*deallocator)(T*) = luaW_defaultdeallocator&lt;T&gt;) </code></pre> <p>to this:</p> <pre><code>template &lt;typename T, T* (*allocator)() = luaW_defaultallocator&lt;T&gt;, void (*deallocator)(T*) = luaW_defaultdeallocator&lt;T&gt; &gt; void luaW_register(lua_State* L, const char* classname, const luaL_reg* table, const luaL_reg* metatable, const char** extends = NULL) </code></pre> <p>to avoid instantiation of luaW_defaultallocator in some cases, but it's looking like it might not be possible. </p> <p>The closest solution I've seen so far is to provide a function like <code>luaW_cannotalloc&lt;T&gt;(lua_State*)</code> which returns NULL and can be checked for in my luaW_register function instead of null. I suppose that would work, but it means more typing and needing to remember that function name, and NULL seems much cleaner.</p>
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    1. COWhy can't you just pass a function which returns null? Then you can simply remove the `if` from T_new as well.
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    2. COI don't know what "tricky template specialization" means to you. What's wrong with doing a partial specialization for the second argument being NULL excising the `if` and putting the `else` clause in the specialized version?
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    3. CO@Conspicuous: if you can code up a working example, please share. I couldn't figure it out.
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