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POMove semantics and virtual methods
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copied!<p>In C++11 we are guided in some cases to pass objects by value and in others by const-reference. However, this guideline depends on the <strong>implementation</strong> of the method, not just on its interface and intended usage by its clients. </p> <p>When I write an interface, I do not know how it will be implemented. Is there a good rule of thumb for writing method signatures? For example - in the following code fragment, should I use <code>Bar1</code> or <code>Bar2</code>?</p> <pre><code>class IFoo { public: virtual void Bar1(std::string s) = 0; virtual void Bar2(const std::string& s) = 0; }; </code></pre> <p>You can stop reading here if you agree that the correct signature depends on the implementation. Here is an example that shows why I believe so.</p> <p>In the following example, we should pass the string by value:</p> <pre><code>class Foo { std::string bar; Foo(std::string byValue) : bar(std::move(byValue)) { } }; </code></pre> <p>Now we can instantiate Foo in an efficient manner in all cases:</p> <pre><code>Foo foo1("Hello world"); // create once, move once Foo foo2(s); // the programmer wants to copy s. One copy and one move Foo foo3(std::move(t)); // the programmer does not need t anymore. No copy at all </code></pre> <p>In other cases we prefer to pass objects by const reference. For example, in the following case we never want to copy/store the argument, just use its methods:</p> <pre><code>void DoStuff(const std::string& byRef) { std::cout << byRef.length() << std::endl; } </code></pre> <p>All possible usages of the above method are already as efficient as possible.</p> <p><strong>Update</strong></p> <p>I believe I forgot to show the issues with the const-reference alternative. If the above class <code>Foo</code> was implemented this way:</p> <pre><code>class Foo { std::string bar; Foo(const std::string& byRef) : bar(byRef) { } }; </code></pre> <p>Then we would have the following results:</p> <pre><code>Foo foo1("Hello world"); // Here we would have one more copy of the string. It is less efficient. Foo foo2(s); // One copy, like before Foo foo3(std::move(t)); // Irrelevant here. </code></pre> <p>Alex.</p>

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