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    <p>As long as <code>value</code> is defined only in the base class, and the operation doesn't need to access any derived members, you might be able to get away with only defining the base operator and letting implicit type casting handle the rest. As for errors with different types, it might be worth a small sacrifice to use an enum-based system to track the types, and then do a simple comparison to check for invalid conditions.</p> <pre><code>enum eTypeEnum {BASE, DER1, DER2}; class Base { public: virtual ~Base(){} Base(double value) : eType(BASE),value(value) {} Base(const Base&amp; B) { value=B.value; } Base operator+ (const Base&amp; B) const { if (eType != B.eType) return -1; //error condition return Base(value+B.value); } double getVal(){return value;} protected: eTypeEnum eType; double value; }; class Derived1 : public Base { public: Derived1(double value) : Base(value) {eType = DER1;} }; class Derived2 : public Base { public: Derived2(double value) : Base(value) {eType = DER2;} }; int main() { int tmp; Derived1 a(4.0); Derived2 b(3.0); Base c(2.0); cout &lt;&lt; "aa:" &lt;&lt; (a+a).getVal(); // 8 cout &lt;&lt; "\nbb:" &lt;&lt; (b+b).getVal(); // 6 cout &lt;&lt; "\nba:" &lt;&lt; (b+a).getVal(); // 7 cout &lt;&lt; "\nab:"&lt;&lt; (a+b).getVal(); // 7 cout &lt;&lt; "\ncc:"&lt;&lt; (c+c).getVal(); // 4 cout &lt;&lt; "\nac:"&lt;&lt; (a+c).getVal(); // 6 cout &lt;&lt; "\nbc:" &lt;&lt; (b+c).getVal(); // 5 cout &lt;&lt; "\nabc:" &lt;&lt; (a+b+c).getVal();// 9 cout &lt;&lt; endl; cin &gt;&gt; tmp; return 0; } </code></pre> <p>Outputs: aa:8 bb:6 ba:-1 ab:-1 cc:4 ac:-1 bc:-1 abc:1</p> <p>The only issue I see is that when chaining multiple operations together, the casting screws up the handling. Here, <code>a+b+c</code> 432 evaluates as <code>(a+b)+c</code> so the <code>a+b</code> bit experiences the error condition (returning -1), but gets cast as a <code>Base</code> which lets <code>(-1)+c</code> return '1'.</p>
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    1. PODerive overloaded operator, but operate on same types only
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    1. COBut I want addition of unequal types to **fail**; this compiles just fine...
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    2. COOh, missed that part. I've updated my post to reflect the addition of a type-tracking enum system that allows the base operator to verify that the arguments are correct. One vulnerability is that clients can spoof their type in their constructors, and the enum needs to be updated as new classes are added, but it's probably the lightest code you can do without using RTTI.
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    3. COHmmm...the `-1` you are returning is implicitly cast to `Base`, which is no longer possible once you make `Base`'s constructor private (which I have). Moreover, it's *still* not a compile time error, plus there are the weaknesses that you already indicated. Sorry, I'm downvoting this one.
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