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POCheck at Compile-Time if Template Argument is void
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  1. POCheck at Compile-Time if Template Argument is void
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    copied!<p>I'm trying to wrap the Windows API functions to check errors when I so choose. As I found out in a previous SO question, I could use a template function to call the API function, and then call <code>GetLastError()</code> to retrieve any error it might have set. I could then pass this error to my <code>Error</code> class to let me know about it.</p> <p>Here's the code for the template function:</p> <pre><code>template&lt;typename TRet, typename... TArgs&gt; TRet Wrap(TRet(WINAPI *api)(TArgs...), TArgs... args) { TRet ret = api(args...); //check for errors return ret; } </code></pre> <p>Using this I can have code as follows</p> <pre><code>int WINAPI someFunc (int param1, BOOL param2); //body not accessible int main() { int ret = someFunc (5, true); //works normally int ret2 = Wrap (someFunc, 5, true); //same as above, but I'll get a message if there's an error } </code></pre> <p>This works wonderfully. However, there is one possible problem. Take the function</p> <pre><code>void WINAPI someFunc(); </code></pre> <p>When subbing this into the template function, it looks as follows:</p> <pre><code>void Wrap(void(WINAPI *api)()) { void ret = api(); //&lt;-- ahem! Can't declare a variable of type void... //check for errors return ret; //&lt;-- Can't return a value for void either } </code></pre> <p>To get around this, I tried creating a version of the template where I replaced <code>TRet</code> with <code>void</code>. Unfortunately, this actually just causes an ambiguity of which one to use.</p> <p>That aside, I tried using </p> <pre><code>if (strcmp (typeid (TRet).name(), "v") != 0) //typeid(void).name() == "v" { //do stuff with variable to return } else { //do stuff without returning anything } </code></pre> <p>However, <code>typeid</code> is a runtime comparison, so the code still doesn't compile due to trying to declare a void variable, even if it never will.</p> <p>Next, I tried using <code>std::is_same &lt;TRet, void&gt;::value</code> instead of <code>typeid</code>, but found out that it was a runtime comparison as well. </p> <p>At this point, I don't know what to try next. Is there any possibility of getting the compiler to believe that I know what I'm doing will run fine? I don't mind attaching an extra argument to <code>Wrap</code>, but I couldn't really get anything out of that either.</p> <p>I use Code::Blocks with GNU G++ 4.6.1, and Windows XP, as well as Windows 7. Thanks for any help, even if it is telling me that I'll have to end up just not using <code>Wrap</code> for functions that return void.</p>
 

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