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POPassing unique pointer to template function
 

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    <p>I'm trying to figure out how exactly the <code>unique_ptr</code> type plays with templates in C++11, and am not having much luck: Specifically, I'm trying to make a template function which inserts a <code>unique_ptr</code> value for a given unique key in a given map if there is not already a value for the key in the map, and moves ownership of the value pointed to by the <code>unique_ptr</code> to the value in the map; If the key is already present, a runtime error is thrown.</p> <p>Now, moving the value of a <code>unique_ptr</code> is quite straightforward if you don't have to pass it to a function:</p> <pre><code>#include &lt;iostream&gt; #include &lt;memory&gt; #include &lt;unordered_map&gt; using namespace std; int main(int argc, char *argv[]) { unordered_map&lt;char, unique_ptr&lt;int&gt; &gt; testMap; char key1 = 'A'; unique_ptr&lt;int&gt; val1(new int(1)); testMap[key1] = move(val1); // Print the results cout &lt;&lt; "testMap[" &lt;&lt; key1 &lt;&lt; "] = " &lt;&lt; *testMap[key1] &lt;&lt; endl; return 0; } </code></pre> <h1>Non-template function</h1> <p>Passing the <code>unique_ptr</code> to a function is a bit more complicated:</p> <pre><code>#include &lt;iostream&gt; #include &lt;memory&gt; #include &lt;unordered_map&gt; using namespace std; void moveValueForUniqueKey(unordered_map&lt;char, unique_ptr&lt;int&gt; &gt;&amp; targetMap, char key, unique_ptr&lt;int&gt; value) throw(char) { // Check if the key is already in the map auto it = targetMap.find(key); if (it != targetMap.end()) { throw key; } else { targetMap[key] = move(value); } } int main(int argc, char *argv[]) { unordered_map&lt;char, unique_ptr&lt;int&gt; &gt; testMap; char key1 = 'A'; unique_ptr&lt;int&gt; val1(new int(1)); // Try inserting the first key-value pair try { moveValueForUniqueKey(testMap, key1, move(val1)); } catch (char&amp; duplicateKey) { cerr &lt;&lt; "Key '" &lt;&lt; duplicateKey &lt;&lt; "' already in map." &lt;&lt; endl; } // Print the key-value pairs for (pair&lt;const char, unique_ptr&lt;int&gt; &gt;&amp; entry : testMap) { cout &lt;&lt; "testMap['" &lt;&lt; entry.first &lt;&lt; "'] = " &lt;&lt; *entry.second &lt;&lt; endl; } unique_ptr&lt;int&gt; val2(new int(2)); // Try inserting the key again try { moveValueForUniqueKey(testMap, key1, move(val2)); } catch (char&amp; duplicateKey) { cerr &lt;&lt; "Key '" &lt;&lt; duplicateKey &lt;&lt; "' already in map." &lt;&lt; endl; } // Print the key-value pairs again for (pair&lt;const char, unique_ptr&lt;int&gt; &gt;&amp; entry : testMap) { cout &lt;&lt; "testMap['" &lt;&lt; entry.first &lt;&lt; "'] = " &lt;&lt; *entry.second &lt;&lt; endl; } return 0; } </code></pre> <p>This code outputs:</p> <pre><code>testMap['A'] = 1 Key 'A' already in map. testMap['A'] = 1 </code></pre> <h1>Function template</h1> <p>Now, when I try to make a template of this function, replacing the previous declaration/implementation of <code>moveValueForUniqueKey(...)</code> with:</p> <pre><code>template&lt;typename K, typename V, typename M&gt; void moveValueForUniqueKey(M targetMap, K key, unique_ptr&lt;V&gt; value) throw(char) { // Check if the key is already in the map auto it = targetMap.find(key); if (it != targetMap.end()) { throw key; } else { targetMap[key] = move(value); } } // Instantiate template function template void moveValueForUniqueKey(unordered_map&lt;char, unique_ptr&lt;int&gt; &gt;&amp; targetMap, char key, unique_ptr&lt;int&gt; value) throw(char); </code></pre> <p>I simply get the compiler error <code>use of deleted function ‘constexpr std::pair&lt;_T1, _T2&gt;::pair(const std::pair&lt;_T1, _T2&gt;&amp;) [with _T1 = const char, _T2 = std::unique_ptr&lt;int&gt;, std::pair&lt;_T1, _T2&gt; = std::pair&lt;const char, std::unique_ptr&lt;int&gt; &gt;]’</code>.</p> <p>What exactly is going on here, and how can you accomplish what I'm trying here (pass <code>unique_ptr</code> objects into template functions using move semantics)? </p>
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    1. CO`remove_reference<unique_ptr<int>&>::type` is redundant...
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    2. COI don't understand: Why `remove_reference<unique_ptr<int>&>::type` and not just `unique_ptr<int>`?
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    3. COOkay, that's strange: while playing around with it I thought I needed that, but it seems to compile now with just `unique_ptr<int>`...
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