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POPassing an arbitrary lambda expression to a function?
 

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  1. POPassing an arbitrary lambda expression to a function?
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    <p>I have a pretty exotic problem, since I am trying to create some sort of a compiler... I want to pass a lambda expression to a templated function like this:</p> <pre><code> template&lt;class T&gt; delegate&lt;T&gt;* bind(std::function&lt;T&gt; func) { return nullptr; } </code></pre> <p>So that I can now call</p> <pre><code>bind([&amp;](int a) { // do something }); </code></pre> <p>... So normally, this wouldn't be a problem, since std::function is able to capture a lambda. But here the problem is that (let's assume) I don't know or don't want to provide the info about what "T" is exactly. It could be any function signature you could also insert into std::function&lt;>...</p> <p>I also need to pass this "inferred" signature back to the "delegate" class I want to return and that delegate class needs to be a pointer to class...</p> <p>For now I have come up with this:</p> <pre><code>template&lt;class T&gt; struct delegate : public std::function&lt;T&gt; { delegate(const std::function&lt;T&gt;&amp; func) : std::function&lt;T&gt;(func) { } delegate(const delegate&lt;T&gt;&amp; func) { } }; template&lt;class T&gt; delegate&lt;T&gt;* bind(std::function&lt;T&gt;&amp;&amp; func) { return new delegate&lt;T&gt;(std::forward&lt;std::function&lt;T&gt;&gt;(func)); } </code></pre> <p>But the above example of a call to "bind" fails with "failed template argument deduction". How can I get this to work without having to specify the "T" parameter explicitly when calling "bind" (then it works, at least)?</p> <p>Since in theory my compiler has all the info to workaround this issue, I could just insert the instance of "T", but this would make the generated code unnecessarily convoluted.</p> <p>BTW, I am using the lastest Clang compiler.</p> <hr> <p>Here is the final solution:</p> <pre><code> template&lt;typename T&gt; struct get_signature; template&lt;typename Mem, typename Ret, typename... Args&gt; struct get_signature&lt;Ret(Mem::*)(Args...) const&gt; { typedef Ret type(Args...); }; template&lt;class T&gt; delegate&lt;typename get_signature&lt;decltype(&amp;T::operator())&gt;::type&gt;* bind(T func) { return nullptr; } </code></pre> <p>Be aware that you may need to adjust the "const" modifiers to your needs.</p>
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    1. USthesaint
    1. USthesaint
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    1. COIt's not a wise idea to inherit from `std::function`. I'll bet you it doesn't have a virtual destructor.
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      1. POPassing an arbitrary lambda expression to a function?
      1. USchris
    2. COThere was something wrong with passing `std::function` as a parameter, but I don't remember what exactly.
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      1. POPassing an arbitrary lambda expression to a function?
      1. USBartek Banachewicz
    3. COLet's not worry about these details right now, I just want this template argument deduction to work! This is just a snippet demonstrating the idea. Might be that I mixed it up right now. I mean c++11 was meant to screw that passing by reference but I can remember that some things like shared_ptr and probably also std::function were an exception.
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