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POWhy is 'object slice' needed in C++ ? Why it is allowed ? For more bugs?
 

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  1. POWhy is 'object slice' needed in C++ ? Why it is allowed ? For more bugs?
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    <p>Why C++ standard allow object slice ? </p> <p>Please don't explain c++ object slice concept to me as I knew that.</p> <p>I am just wondering what's the intention behind this c++ feature(object slice) design ? </p> <p>To get novice more bugs?</p> <p>Wouldn't it be more type safe for c++ to prevent object slice ?</p> <p>Below is just a standard and basic slice example:</p> <pre><code>class Base{ public: virtual void message() { MSG("Base "); } private: int m_base; }; class Derived : public Base{ public: void message() { MSG("Derived "); } private: int m_derive; }; int main (void) { Derived dObj; //dObj get the WELL KNOWN c++ slicing below //evilDerivedOjb is just a Base object that cannot access m_derive Base evilDerivedOjb = dObj; //evilDerivedObj is type Base evilDerivedOjb.message(); //print "Baes" here of course just as c++ standard says } </code></pre> <p>Thanks in advance.</p> <p>================================================================================= After reading all the answers and comments I think I should express my question better in the first place but here it comes:</p> <p>When there is a is-a relationship(public inheritnace), instead of private/protected inheritance , you can do the following:</p> <pre><code>class Base{ public: virtual void foo(){MSG("Base::foo");} }; class Derived : public Base{ public: virtual void foo(){MSG("Derived::foo");} }; int main (void) { Base b; Derived d; b = d; //1 Base * pB = new Derived(); //2 Base&amp; rB = d; //3 b.foo(); //Base::foo pB-&gt;foo(); //Derived::foo rB.foo(); //Derived::foo } </code></pre> <p>It's well known that only 2 &amp; 3 works polymorphically while one is the infamous object slicing which produce nothing but a bug !</p> <p>Note 1, 2 and 3 <strong>NEED</strong> is-a relationship to work.<br> If you are using private/protect inheritance, you will get compile error for all of them :</p> <pre><code>'type cast' : conversion from 'Derived *' to 'const Base &amp;' exists, but is inaccessible 'type cast' : conversion from 'Derived *' to 'Base *' exists, but is inaccessible 'type cast' : conversion from 'Derived *' to 'Base &amp;' exists, but is inaccessible </code></pre> <p>So my question(original intention) was to ask would it be better if c++ standard make 1 a compile error while keep allowing 2 and 3 ?</p> <p>Hope I have expressed my question better this time.</p> <p>Thanks</p>
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    1. COWhy is this off topic?
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    2. COIt is not. Closevote is wrong.
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    3. CO+1 why is this off topic ??!! I couldn't find the answer in c++ 03 standard(14882) or not even in stackoverflow !
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