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POFunctions and functors as arguments to template functions
 

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    <p>I'm looking for a way to pass function pointers, functors or lambdas to a template function <code>g</code> which uses the passed function's argument types, for example:</p> <pre><code>template&lt;class T1, class T2, class T3&gt; struct wrapper_t { boost::function&lt;void(T1,T2,T3)&gt; f; wrapper_t( boost::function&lt;void(T1,T2,T3)&gt; f ) : f(f) {} void operator( std::vector&lt;T1&gt; &amp;a, std::vector&lt;T2&gt; &amp;b, T3 c ) { assert(a.size() == b.size()); for(size_t i = 0 ; i != a.size() ; i++) f(a[i], b[i], c); } }; template&lt;class T1, class T2, class T3&gt; wrapper_t&lt;T1,T2,T3&gt; make_wrapper( boost::function&lt;void(T1,T2,T3)&gt; f ) { return wrapper_t&lt;T1,T2,T3&gt;( f ); } void f(int, double, char) {}; wrapper_t&lt;int, double, char&gt; w0(f); // need to repeat types auto w1 = make_wrapper(f); // more comfortable std::vector&lt;int&gt; a{{1, 2, 3}}; std::vector&lt;double&gt; b{{1.0, 2.0, 3.0}}; w0( a, b, 'c' ); w1( a, b, 'c' ); </code></pre> <p>The <code>make_wrapper</code> function only exists to extract the types from the argument, some syntactic sugar to avoid having to type them twice.</p> <hr> <p>A minimal example for my problem is this function:</p> <pre><code>template&lt;class T&gt; void g1( const boost::function&lt;void(T)&gt; &amp; ) {} </code></pre> <p>Using these as input</p> <pre><code>void f1(int) {} struct f2_t { void operator()(int) {} }; </code></pre> <p>it fails to infer <code>T=int</code></p> <pre><code>f2_t f2; g1( f1 ); // mismatched types ‘const std::function&lt;void(T)&gt;’ and ‘void(int)’ g1( f2 ); // ‘f2_t’ is not derived from ‘const std::function&lt;void(T)&gt;’ g1( [](int){} ); // ‘::&lt;lambda(int)&gt;’ is not derived from ‘… g1&lt;int&gt;( f1 ); // ok g1&lt;int&gt;( f2 ); // ok g1&lt;int&gt;( [](int){} ); // ok </code></pre> <p>But <code>T=int</code> can be inferred from a plain function pointer, but of this doesn't work with the functor or lambda either:</p> <pre><code>template&lt;class T&gt; void g2( void (*)(T) ) {} g2( f1 ); // ok g2( f2 ); // mismatched types … g2&lt;int&gt;( f2 ); // ok g2( [](int){} ); // mismatched types … g2&lt;int&gt;( [](int){} ); // ok </code></pre> <p><em>Is there a way to infer <code>T</code> not just for plain function pointers but for functors and lambdas, too?</em></p> <p>Or does it have to be something like this?</p> <pre><code>template&lt;class F&gt; void g( F ) { typedef first_argument_of&lt;F&gt;::type T; } </code></pre> <p>(in my real code I need to deconstruct a function with four arguments this way, but <code>std::function::…argument_type</code> only exists for one or two arguments; <code>boost::function</code> has argN_type, but I don't think I can use that anyway since <code>F</code> is not always a <code>function</code> which is my problem, see above, etc)</p>
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    1. COCurious: where are you getting your `T` from to feed this `template` deduced `std::function<void(T)>` in order to invoke it?
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      1. USYakk - Adam Nevraumont
 

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