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    <p>The type expected by <code>multiset</code> second parameter is a <em>Predicate</em>. That is the type of an object <code>c</code> such that <code>c(lhs, rhs)</code> returns something <em>convertible to <code>bool</code></em>.</p> <p>The type of <code>compare</code> and <code>decltype(compare)</code> is <code>bool(shared_ptr&lt;int&gt;, shared_ptr&lt;int&gt;)</code>, a <em>function type</em>. There can't be values of <em>function type</em>, but there can be values of <em>pointers</em> and <em>references</em> to <em>function types</em>. That's why you need the <code>*</code> there. (Actually, there are values of <em>function types</em> and are those things that you declare as functions, but those cannot be created, copied, nor pretty much anything besides calling them and getting their address.) Summing up:</p> <pre><code>decltype(compare)* ==&gt; bool (*)(shared_ptr&lt;int&gt;, shared_ptr&lt;int&gt;) </code></pre> <p>which is a pointer to a function taking two <code>shared_ptr&lt;int&gt;</code>s by value and returning a <code>bool</code>. Then, in the <code>multiset</code> constructor, you give it a value of that type:</p> <pre><code>items{ compare }; // value </code></pre> <p>or more specifically, a value of a <em>function type</em> that will <em>decay</em> to a function pointer type. So let's make that explicit:</p> <pre><code>items{ &amp;compare }; // pointer to function compare </code></pre> <p>If you didn't give it anything, then it would be a <em>null pointer</em>, and when trying to call it to do the comparison your application would crash.</p> <p>In summary, you chose a hell of a complicated case to go through (and I have skipped a few things I should mention). This is a simpler approach:</p> <pre><code>struct shared_ptr_compare { bool operator()(std::shared_ptr&lt;int&gt; const&amp; lhs, std::shared_ptr&lt;int&gt; const&amp; rhs) const { return /*...*/; } }; multiset&lt;shared_ptr&lt;int&gt;, shared_ptr_compare&gt; item; // no need to initialize // shared_ptr_compare with anything, a default value will do just fine </code></pre> <p><strong>Note</strong>: I dropped the top-level <code>const</code> modifier as it has no effect on a function parameter, it only signals the function body that the values cannot be changed. So it does have meaning, but complicates explaining types.</p> <p><strong>Note 2</strong>: You should take those parameters as <code>shared_ptr&lt;int&gt; const&amp;</code> (same thing as <code>const shared_ptr&lt;int&gt;&amp;</code>), since you do not actually need a copy and those copies have to keep a reference counter which is an operation that has to be synchronized among threads.</p>
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    1. COwhat if the declaration was like this.. multiset<shared_ptr<int> > item ; wont that also insert elements orderly?
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    2. CO@TamimAdDari it will insert elements based on a different ordering: pointer values, rather than the values of what the pointers point to.
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    3. COOwh.. I get it now.. :D
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