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    <p>You're pointing to an object of type <code>NuclearSubmarine</code> with a pointer to type <code>Car</code>. You can't do that, because these types are unrelated.</p> <p>This error invokes undefined behaviour, which causes your program to behave in an unpredictable way.</p> <hr> <p>If you're interested in why this happens, have a read on how virtual functions are implemented internally: <a href="http://en.wikipedia.org/wiki/Virtual_method_table" rel="nofollow">Virtual method table</a>. This will make things clear for you.</p> <hr> <p>In reply to the comment:</p> <p>That's right. <code>Vehicle</code> instances all have an internal pointer that points to a vtable which looks something like this:</p> <pre><code>0: Vehicle::~Vehicle 1: Vehicle::StartEngine // this is a null pointer </code></pre> <p><code>Car</code> instances have their vptr point to a vtable which looks something like this:</p> <pre><code>0:Vehicle::~Vehicle // this one didn't get overridden 1:Car::startEngine 2:Car::openGasTrap </code></pre> <p><code>NuclearSubmarine</code>'s vtable looks like this:</p> <pre><code>0:Vehicle::~Vehicle // this one didn't get overridden 1:NuclearSubmarine::startEngine 2:NuclearEngine::fireNuclearMissile. </code></pre> <p>If you have this,</p> <pre><code>Vehicle* v = new Car(); v-&gt;startEngine(); </code></pre> <p>it gets compiled into something like this (pseudocode ahead):</p> <pre><code>Vehicle* v = new Car(); // OK, v is a Vehicle and startEngine is a Vehicle's virtual function of index 1 // let's call it! StartEngineFunc* f = v.VPTR[1]; // look up the vtable using the object's virtual pointer CallMethod(v, f); </code></pre> <p>The virtual pointer refers the function lookup into the correct vtable w/r/t the object's actual runtime type.</p> <p>This allows you to call methods of derived classes via pointers to base classes, since derived classes will have their virtual table (the first part of it) corresponding to its parent class.</p> <p>However, if two classes are not related by a parent-child relationship, their virtual tables will have different meaning. That's what happens in your case.</p> <p><em>(Note that while the vtable mechanism is an implementation detail - it's very common in compilers, but it isn't enforced in any way and compilers are free to implement it differently - so you shouldn't count on that in your programs.)</em></p>
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    1. COHi Kos my ques is diff I know I should not do that but just curious to know why this is happening. Is it happening because it is invoking through offset address mechanism? means lets say it sets offset=2 in this case since opencase is second fun and then it is adding this to vptr of nuclear where nuclearmissile fun is placed. is it so?
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    2. COThat's right. I've added some more explanations
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    3. COThank you very much Kos fr such a nice exp!!! But i was thinking how the order of lookup is being decided. is it order of declaring in the class? but when i changed the order of startengine and nuclear*** it still gave "fire". can you explain that also?
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