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POCost of virtuality and inheritance for non virtual members?
 

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  1. POCost of virtuality and inheritance for non virtual members?
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    <p>The virtuality can have a double overhead :</p> <ul> <li>memory (because of vptr and vtable)</li> <li>runtime speed</li> </ul> <p>Because of the memory overhead, I use some CRTP techniques to have a kind of static virtuality, when I need very high memory optimization.</p> <p>But I wonder about the cost of virtuality for the runtime speed for non-virtual members :</p> <pre><code>#include &lt;iostream&gt; class Base { public: Base() {;} virtual ~Base() {;} public: virtual void f1() {std::cout&lt;&lt;"f1 : Base"&lt;&lt;std::endl; /* FUNCTION BODY */} void f2() {std::cout&lt;&lt;"f2 : Base"&lt;&lt;std::endl; /* FUNCTION BODY */} void f3() {f1();} }; class Derived : public Base { public: Derived() {;} virtual ~Derived() {;} public: virtual void f1() {std::cout&lt;&lt;"f1 : Derived"&lt;&lt;std::endl; /* FUNCTION BODY */} }; </code></pre> <p>And the main :</p> <pre><code>int main() { Base b; Derived d; Base* ptr = new Derived(); std::cout&lt;&lt;std::endl; b.f1(); // 1a b.f2(); // 1b b.f3(); // 1c std::cout&lt;&lt;std::endl; d.f1(); // 2a d.f2(); // 2b d.f3(); // 2c std::cout&lt;&lt;std::endl; ptr-&gt;f1(); // 3a ptr-&gt;f2(); // 3b ptr-&gt;f3(); // 3c std::cout&lt;&lt;std::endl; return 0; } </code></pre> <p>For each case : 1a, 1b ... 3c, where do I have a runtime overhead (increase of execution time) due to inheritance+virtuality compared to the case where Base and Derived are two completely independant classes with no inheritance ?</p> <p>And particularly, is there any runtime overhead for the f2 function ?</p> <p>Note : <code>std::cout</code> is just an example. <code>/* FUNCTION BODY */</code> can be 1k lines of code...</p>
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    1. USVincent
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    1. CO"For each case...where do I have...more CPU instructions?" Your compiler will produce an assembly listing if you [ask it politely](http://stackoverflow.com/questions/840321/how-can-i-see-the-assembly-code-for-a-c-program).
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      1. POCost of virtuality and inheritance for non virtual members?
      1. USJames McNellis
    2. COJust counting the number of CPU instructions will provide a false metric. Loop-unrolling by its very nature increases the number of instructions, yet 'paradoxically' it increases run-time speed. The overhead of using cout rather than printf is orders of magnitude slower than any examination of the vtable and the subsequent re-direction. (cout used as example - I understand it not to be the essence of the question)
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      1. USenhzflep
    3. CO@enhzflep : I edited my question taking in account your remarks.
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      1. USVincent
 

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