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POSpecializing a template on a lambda in C++0x
 

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  1. POSpecializing a template on a lambda in C++0x
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    <p>I've written a traits class that lets me extract information about the arguments and type of a function or function object in C++0x (tested with gcc 4.5.0). The general case handles function objects:</p> <pre><code>template &lt;typename F&gt; struct function_traits { template &lt;typename R, typename... A&gt; struct _internal { }; template &lt;typename R, typename... A&gt; struct _internal&lt;R (F::*)(A...)&gt; { // ... }; typedef typename _internal&lt;decltype(&amp;F::operator())&gt;::&lt;&lt;nested types go here&gt;&gt;; }; </code></pre> <p>Then I have a specialization for plain functions at global scope:</p> <pre><code>template &lt;typename R, typename... A&gt; struct function_traits&lt;R (*)(A...)&gt; { // ... }; </code></pre> <p>This works fine, I can pass a function into the template or a function object and it works properly:</p> <pre><code>template &lt;typename F&gt; void foo(F f) { typename function_traits&lt;F&gt;::whatever ...; } int f(int x) { ... } foo(f); </code></pre> <p>What if, instead of passing a function or function object into <code>foo</code>, I want to pass a lambda expression?</p> <pre><code>foo([](int x) { ... }); </code></pre> <p>The problem here is that neither specialization of <code>function_traits&lt;&gt;</code> applies. The C++0x draft says that the type of the expression is a "unique, unnamed, non-union class type". Demangling the result of calling <code>typeid(...).name()</code> on the expression gives me what appears to be gcc's internal naming convention for the lambda, <code>main::{lambda(int)#1}</code>, not something that syntactically represents a C++ typename.</p> <p>In short, is there anything I can put into the template here:</p> <pre><code>template &lt;typename R, typename... A&gt; struct function_traits&lt;????&gt; { ... } </code></pre> <p>that will allow this traits class to accept a lambda expression?</p>
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    1. USGeorg Fritzsche
    1. USTony Allevato
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    1. CONo. Why do you think you need something like this?
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    2. COI thought my example gave a decent use case: If I have a generic algorithm that takes in a function or function object, I can use this traits class to determine not only the return type (which could also be done with decltype nowadays), but also the types of the arguments. (I left out the bulk of the code to keep the post from being too long.) Since I can pass in a function or function object, for orthogonality purposes I'd like to be able to pass in a lambda as well. This is all basically an academic exercise that arose from reading "Elements of Programming".
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      1. USTony Allevato
    3. CO@Tony: The answer is yes, I've done it. I'll be able to get back to this question a bit later, though. What traits are you trying to get?
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      1. USGManNickG
 

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