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POC++ : How to pass function pointer, stored in a local variable, as a template parameter
 

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  1. POC++ : How to pass function pointer, stored in a local variable, as a template parameter
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    <pre><code>using namespace std; float test1(float i){ return i * i; } int test2(int i){ return i+9; } struct Wrapper{ typedef void (*wrapper_type)(int); template&lt;class R, class A&gt; void wrap(string name,R (*fn) (A) ){ wrapper_type f_ = &amp;Wrapper::wrapper1&lt;R,A,fn&gt;; // in the real program, f_ would be passed in an array to some c library wrappers[name] = f_; } void run(int i){ map&lt;string,wrapper_type&gt;::iterator it, end = wrappers.end(); for(it=wrappers.begin();it!=end;it++){ wrapper_type wt = (*it).second; (*wt)(i); } } template&lt;class R, class A, R (*fn) (A)&gt; static void wrapper1(int mul){ //do something A arg = mul; R result = (*fn)( arg ); cout &lt;&lt; "Result: " &lt;&lt; result &lt;&lt; endl; } map&lt;string,wrapper_type&gt; wrappers; }; int main(int argc, char** argv) { Wrapper w; w.wrap("test1",&amp;test1); w.wrap("test2",&amp;test2); w.run(89); return 0; } </code></pre> <p>Here's the g++ error:</p> <pre><code>main.cpp:31: error: ‘fn’ is not a valid template argument for type ‘float (*)(float)’ because function ‘fn’ has not external linkage </code></pre> <p>From what i understand, the problem is that a local variable has no linkage; thus it can not be used as a template parameter. </p> <p>So, i wanted to know if there's a way to get around this problem or another technique to accomplish the same ? </p> <p>Thanks.</p> <p>Edit 1:</p> <p>I totally understand that I can't pass a value that can not be determined at compile time as a template paramter. What i'm asking is - is there a better way to do this? Right now the solution that works for me is :</p> <pre><code>template&lt;class R, class A,R (*fn) (A)&gt; void wrap(string name){ wrapper_type f_ = &amp;Wrapper::wrapper1&lt;R,A,fn&gt;; // in the real program, f_ would be passed in an array to sm c library wrappers[name] = f_; } </code></pre> <p>and called as :</p> <pre><code>w.wrap&lt;float, float, &amp;test1&gt;("test1"); w.wrap&lt;int, int, &amp;test2&gt;("test2"); </code></pre> <p>But here I've to pass argument-type during wrapping. Is there someway to avoid this ? </p> <p>EDIT 2:</p> <p>Just to clarify or add more info: The interface I want to present to the user has to be similar to LuaBind or Boost.Python i.e. Wrapper.wrap("test1",&amp;test1); should be sufficient. </p>
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    1. COWhen do you know which function that pointer points to - at runtime or at compile time?
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