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    <p>Clarify the operations you want to perform on LogicComponent. Assuming you are trying to achieve something like this: </p> <p>Step 1: Add a new entry to the map:</p> <pre><code>LogicComponent comp; EntityID id = 99; UnitInfos info = comp.add(id); </code></pre> <p>Step 2: Initialize the info:</p> <pre><code>info.x = 10.0; info.y = 11.0 // etc </code></pre> <p>Step 3: Get the info object again:</p> <pre><code>UnitInfos info2 = comp.instances[id]; // this is uninitialized. </code></pre> <p>Then, a few code comments are in order: </p> <p>The info object returned by comp.add is a COPY of the object you added to the map. By modifying it, you are not modifying what is in the map. </p> <p>The simplest fix is to create a map of pointers to the object instead of the object itself. </p> <pre><code>typedef std::map&lt;EntityID, T*&gt; pinstances_map; T * add(EntityID id) { T* t = new T(); instances[id] = t; return t; }; // initialize as UnitInfo *info = comp.add(id); info-&gt;x = 10.0; info-&gt;y = 11.0; // retrieve as UnitInfos *info = comp.instances[id]; </code></pre> <p>Also, do use an accessor method to get the mapped value, instead of exposing the map object as public. Make the instances variable protected, and add a public get() method.</p> <p>Edit: This code works fine for me:</p> <pre><code>#include &lt;map&gt; #include &lt;iostream&gt; using namespace std; template&lt;typename T&gt; class Component { public: typedef map&lt;long, T*&gt; pinstances_map; pinstances_map instances; T * add(long id) { T *t = new T(); instances[id] = t; return t; } }; struct UnitInfo { float x, y; }; class LogicComponent: public Component&lt;UnitInfo&gt; {}; int main() { LogicComponent comp; UnitInfo *info = comp.add(99); info-&gt;x = 10.0; info-&gt;y = 11.0; UnitInfo *info2 = comp.instances[99]; cout &lt;&lt; info2-&gt;x &lt;&lt; " " &lt;&lt; info2-&gt;y; return 0; } </code></pre>
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    1. POStoring struct instances in a std::map
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    1. COWhat you described is exactly what I trying to achieve. Switching to what you have describded is somethink I tried, but when calling info->x after your last line, i get a EXC_BAD_ACCESS, with info pointing to 0x0.
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    2. COI agree, this code works. When I try to access it from another part ofthe code (the render function passed to `glutDisplayFunc` and `glutIdleFunct`, the pointers still points to 0x0.
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    3. COAre you accessing the same LogicComp object?
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