StackOverflow2013

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POoverloaded const and non-const class methods returning references in C++
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copied!<p>I have a data-structure class in C++ with an accessor to some object (may be large) and I have const and non-const methods using this accessor so I need to overload it. I am looking for a <strong>critique</strong> of the code below - maybe there is a way to accomplish the same thing that is cleaner?</p> <p>The way I understand it, there are two ways to achieve this without duplicating the code in the accessor in the following case, the method get(). I am not sure if there are serious issues with either of these two methods and <strong>I would like some guidance</strong> here.</p> <p>I like method A because:</p> <ul> <li>only one const_cast</li> <li>const-version of the method get() returns a copy</li> <li>the non-const method gets the non-const reference directly</li> </ul> <p>I don't like method A because:</p> <ul> <li>the non-const method get() is const only by contract, (not checked by compiler)</li> <li>harder to get a const-reference, though not impossible</li> </ul> <p>I like method B because:</p> <ul> <li>the const-ness of the const method get() is checked by compiler</li> <li>copy of the returned object is controlled by the user</li> </ul> <p>I don't like method B because:</p> <ul> <li>requires two const_casts which is hard to read</li> </ul> <p>here is the (minimal) example code of the two cases.</p> <pre class="lang-cpp prettyprint-override"><code>/** * summary: * Two classes with an overloaded method which is * guaranteed (by contract) not to change any * internal part of the class. However, there is a * version of this method that will return a non-const * reference to an internal object, allowing the user * to modify it. Don't worry about why I would ever * want to do this, though if you want a real-world * example, think about std::vector<>::front() * * The difference between A and B can be summarized * as follows. In both cases, the second method merely * calls the first, wrapped with the needed * const_cast's * * struct A { * int& get(); * int get() const; * }; * * struct B { * const int& get() const; * int& get(); * }; * **/ struct A { int _val; A() : _val(7) {}; // non-const reference returned here // by a non-const method int& get() { // maybe lots of calculations that you do not // wish to be duplicated in the const version // of this method... return _val; } // const version of get() this time returning // a copy of the object returned int get() const { // CONST-CAST!!?? SURE. return const_cast<A*>(this)->get(); } // example of const method calling the // overloaded get() method int deep_get() const { // gets a copy and makes // a copy when returned // probably optimized away by compiler return this->get(); } }; struct B { int _val; B() : _val(7) {}; // const reference returned here // by a const method const int& get() const { // maybe lots of calculations that you do not // wish to be duplicated in the non-const // version of this method... return _val; } // non-const version of get() this time returning // a copy of the object returned int& get() { // CONST-CAST!? TWO OF THEM!!?? WHY NOT... return const_cast<int&>(const_cast<const B*>(this)->get()); } // example of const method calling the // overloaded get() method int deep_get() const { // gets reference and makes // a copy when returned return this->get(); } }; int main() { A a; a.get() = 8; // call non-const method a.deep_get(); // indirectly call const method B b; b.get() = 8; // call non-const method b.deep_get(); // indirectly call const method } </code></pre>

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