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POCan I create a predicate that will accept both functions and functors as a parameter?
 

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  1. POCan I create a predicate that will accept both functions and functors as a parameter?
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    <p>I'm working on a problem in C++ that involves lots of subset and transformation operations on a large quantity of data. To that end, I've created a map function and something like list comprehensions. I've found that a bunch of the predicates I'm writing also have inverses, so I'd need to write:</p> <pre><code>template &lt;typename type_t&gt; bool HasTenFoo(const type_t &amp;t) { return t.foo &gt;= 10.0; } </code></pre> <p>and</p> <pre><code>template &lt;typename type_t&gt; bool DoesntHaveTenFoo(const type_t &amp;t) { return t.foo &lt; 10.0; } </code></pre> <p>Neither of those is a real example, but they're representative. I'm also using a fair number of functors like:</p> <pre><code>class HasEnoughFoo { public: HasEnoughFoo (double bar) { this-&gt;bar = bar; } template&lt;typename type_t&gt; bool operator()(const type_t &amp;t) const { return t.foo &gt;= bar; } private: double bar; }; </code></pre> <p>some of which should have inverses as well. Rather than duplicate code unnecessarily, I'd like to write a functor that takes a predicate as an argument and returns the (value of the) inverse of that predicate. My fist cut at one is below:</p> <pre><code>/* -- Returns the opposite of some other predicate -------------------------- */ template&lt;typename predicate_t&gt; class Not { public: template &lt;typename predicate_t&gt; Not(predicate_t *p) { predicate = p; } template &lt;typename type_t&gt; bool operator()(const type_t &amp;t) const { return !(*predicate)(t); } private: predicate_t *predicate; }; </code></pre> <p>I would call that with something like:</p> <pre><code>new_list = old_list.subset(Not&lt;HasEnoughFoo&gt;(&amp;HasEnoughFoo(10.0)); </code></pre> <p>or</p> <pre><code>new_list = old_list.subset(Not&lt;HasTenFoo&gt;(&amp;HasTenFoo)); </code></pre> <p>That seems to work well when <code>predicate_t</code> is a functor like <code>HasEnoughFoo</code>, but fails when <code>predicate_t</code> refers to a regular function like <code>HasTenFoo</code>.</p> <p>Visual Studio complains that <code>'HasTenFoo' is not a valid template type argument for parameter 'predicate_t'</code>. Is there any way to write a Not() predicate that will work with functors and functions or am I doomed to write dozens of predicates and their inverses as well?</p>
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