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  1. POCompare boost::any contents
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    copied!<p>I am using a container to hold a list of pointers to anything:</p> <pre><code>struct Example { std::vector&lt;boost::any&gt; elements; } </code></pre> <p>To insert elements in this container, I had written a couple of helper functions (members of the <code>struct Example</code>):</p> <pre><code>void add_any(boost::any&amp; a) { elements.push_back(a); } template&lt;typename T&gt; void add_to_list(T&amp; a) { boost::any bany = &amp;a; add_any(bany); } </code></pre> <p>Now, I would like to insert elements only when they are not present in this container. To do this, I thought that I would only need to call <code>search</code> over <code>elements</code> with an appropriate comparator function. However, I do not know how to compare the <code>boost::any</code> instances.</p> <p><strong>My question:</strong> Knowing that my <code>boost::any</code> instances always contain a pointer to something; is it possible to compare two <code>boost::any</code> values?</p> <hr> <p><strong>update</strong></p> <p>I thank you for your answers. I have also managed to do this in a <em>probably</em> unsafe way: using <code>boost::unsafe_any_cast</code> to obtain a <code>void**</code> and comparing the underlying pointer.</p> <p>For the moment, this is working fine. I would, however, appreciate your comments: maybe this is a big mistake!</p> <pre><code>#include &lt;boost/any.hpp&gt; #include &lt;iostream&gt; #include &lt;vector&gt; #include &lt;string&gt; using namespace std; bool any_compare(const boost::any&amp; a1, const boost::any&amp; a2) { cout &lt;&lt; "compare " &lt;&lt; *boost::unsafe_any_cast&lt;void*&gt;(&amp;a1) &lt;&lt; " with: " &lt;&lt; *boost::unsafe_any_cast&lt;void*&gt;(&amp;a2); return (*boost::unsafe_any_cast&lt;void*&gt;(&amp;a1)) == (*boost::unsafe_any_cast&lt;void*&gt;(&amp;a2)); } struct A {}; class Example { public: Example() : elements(0), m_1(3.14), m_2(42), m_3("hello"), m_4() {}; virtual ~Example() {}; void test_insert() { add_to_list(m_1); add_to_list(m_2); add_to_list(m_3); add_to_list(m_4); add_to_list(m_1); // should not insert add_to_list(m_2); // should not insert add_to_list(m_3); // should not insert add_to_list(m_4); // should not insert }; template &lt;typename T&gt; void add_to_list(T&amp; a) { boost::any bany = &amp;a; add_any(bany); } private: vector&lt;boost::any&gt; elements; double m_1; int m_2; string m_3; A m_4; void add_any(const boost::any&amp; a) { cout &lt;&lt; "Trying to insert " &lt;&lt; (*boost::unsafe_any_cast&lt;void*&gt;(&amp;a)) &lt;&lt; endl; vector&lt;boost::any&gt;::const_iterator it; for (it = elements.begin(); it != elements.end(); ++it) { if ( any_compare(a,*it) ) { cout &lt;&lt; " : not inserting, already in list" &lt;&lt; endl; return; } cout &lt;&lt; endl; } cout &lt;&lt; "Inserting " &lt;&lt; (*boost::unsafe_any_cast&lt;void*&gt;(&amp;a)) &lt;&lt; endl; elements.push_back(a); }; }; int main(int argc, char *argv[]) { Example ex; ex.test_insert(); unsigned char c; ex.add_to_list(c); ex.add_to_list(c); // should not insert return 0; } </code></pre>
 

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