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POReturning template map value as reference
 

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  1. POReturning template map value as reference
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    <p>Well, I must say that working with C++ templates + stl made of templates + trying to do fast c++11 code is a pain. Mostly, lots of strange compiler errors... I need a help, code:</p> <pre><code>#include &lt;SFML/Graphics.hpp&gt; #include &lt;memory&gt; #include &lt;map&gt; template &lt;class T&gt; class cResourceManager { public: T&amp; get(const std::string &amp; key); bool add(const std::string &amp; key, const std::shared_ptr&lt;T&gt; &amp; ptr); private: std::map &lt;std::string, std::shared_ptr&lt;T&gt; &gt; resources; }; template &lt;class T&gt; T&amp; cResourceManager&lt;T&gt;::get(const std::string &amp; key) { class std::map&lt;std::string, std::shared_ptr&lt;T&gt;&gt;::const_iterator citr = resources.find(key); if (citr != resources.end()) return resources[key]; } template &lt;class T&gt; bool cResourceManager&lt;T&gt;::add(const std::string &amp; key, const std::shared_ptr&lt;T&gt; &amp; ptr) { if (resources.find(key) == resources.end()) { if(ptr != nullptr) { resources.insert( std::move( std::make_pair(key, ptr) ) ); return true; } } return false; } int main(int argc, char **argv) { cResourceManager&lt;sf::Texture&gt; resmgr; resmgr.add("key", std::make_shared&lt;sf::Texture&gt;() ); resmgr.get("key"); return 0; } </code></pre> <p>On the line resmgr.get("key") i get an error <strong>"main.cpp:19:51: error: invalid initialization of reference of type ‘sf::Texture&amp;’ from expression of type ‘std::map, std::shared_ptr, std::less >, std::allocator, std::shared_ptr > > >::mapped_type {aka std::shared_ptr}’"</strong> I have no idea why and trying to understand errors using templates and STL is extremely hard to me. I have no clue what's wrong.</p> <p>The second thing is a little question. On the line: <strong>resources.insert(std::move(std::make_pair(key, ptr)))</strong> do I need the std::move function to get better performance? Because I want to avoid temporary objects when working with cointainers as much as possible but I don't think that I understand everything so I'm not sure about it.</p> <p>Thanks!</p>
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    1. USildjarn
    1. USuser1873947
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    1. COWhat do you expect `get()` to return if there is *no* item already by that key? As it stands now, on there is NO return value, i.e. UB.
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      1. USWhozCraig
    2. COThere is an object with that key. look, there is add function line above.
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    3. COI'm not talking about the code that *calls* get(). I'm talking about the actual *implementation* of get().
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