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POAccessing a method of a member object in C++
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copied!<p>I am trying to access a method of an object (<code>myEOS.calc(...)</code>) from a method of an object (<code>myPlanet</code>) of another class containing an instance of the first class (<code>static EOS myEOS</code>):</p> <pre><code>// class in class class EOS { public: static float Y; float calc(float); }; float EOS::calc(float x){ return x; // some complicated calculation } class Planet { public: static EOS myEOS; // want only one instance; cf. below static void setX(float* X); // must be static; cf. below }; void Planet::setX(float* X) { *X = myEOS.calc(*X); // will give an error } int main(){ Planet myPlanet; } </code></pre> <p>This returns the linking-time error</p> <pre><code>In function `Planet::setX(float*)': test.cpp:(.text+0x1a): undefined reference to `Planet::myEOS' collect2: ld returned 1 exit status </code></pre> <p>Compiling separately, with <code>-c</code>, the classes and the main program (with an <code>#include</code> in the main file) gives no error; this looks like the key to the solution but I do not see the lock!</p> <p>Does someone know what the problem is? I hope my intent is clear from what I have, even if there is some fundamental misconception. I thought I somewhat understood classes and reread tutorials but saw no discussion of classes within classes (not nested classes). I was not able to find a similar question on this site (from which I can usually get all the answers I need!) either.</p> <p>By the way, following <a href="https://stackoverflow.com/questions/5058014/construction-of-object-with-non-default-constructor-inside-c-class">somebody else's question</a>, adding explicit constructors (and correctly initialising the EOS in Planet's initialiser list) did not remove the compiler complaint about "undefined reference to <code>Planet::myEOS'" (this done without the</code>static` keyword).</p> <p>Finally, note that <code>Planet::setX</code> needs to be static because a pointer to this method must have a "class-less" signature as it is passed to a function that cannot handle methods/classes:</p> <pre><code> void (*fun_ptr)(float*) = & (Planet::setX); </code></pre> <p>This also forces the object <code>myEOS</code> to be static (needs to be accessed from a static function), and anyway the initialisation of <code>EOS</code> objects is expensive.</p> <p>Thanks a lot for anybody's help!</p>

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