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POChaining operator= for write-only objects - OK to return rhs instead of *this?
 

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  1. POChaining operator= for write-only objects - OK to return rhs instead of *this?
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    <p>It's well known that <code>operator=</code> should return a <strike>const</strike> reference to <code>*this</code> to support chaining, but this only works if <code>*this</code> <strike>can be used as an rvalue</strike> is value-like.</p> <p><strong>Edit:</strong> Fine, <code>operator=</code> should return a non-const reference (do as the <code>int</code>s), and I meant that <code>*this</code> needs to be a meaningful <code>rhs</code> in an assignment.</p> <p>I'm wrapping a C API of name=value setter functions through a C++ class <code>ApiWrapper</code> with <code>operator[]</code> returning a temporary write-only <code>Proxy</code> with overloaded <code>operator=</code>, but the API has no getter functions so <code>Proxy</code> is effectively write-only.</p> <pre class="lang-c++ prettyprint-override"><code>ApiWrapper x; x["a"] = x["b"] = 42; // x["b"] = 42; fine: consumes 42, returns *this // x["a"] = x["b"]; error: x["b"] does not have the value </code></pre> <p>It seems to me that if I return a const reference to <code>rhs</code> instead of <code>*this</code> from <code>operator=</code>, chaining would work fine. Conceptually (proxy boilerplate code left out):</p> <pre class="lang-c++ prettyprint-override"><code>struct Proxy { template &lt;typename T&gt; T const&amp; operator=(T const&amp; rhs) const { ... // pass rhs to the API but don't store it return rhs; // return rhs, not *this } }; ApiWrapper x; x["a"] = x["b"] = 42; // x["b"] = 42; fine: consumes and returns 42 // x["a"] = 42; fine: consumes and returns 42 </code></pre> <p>This makes me suspicious though. Are there any weird side effects from returning a const reference to <code>rhs</code> instead of <code>*this</code>? The only thing I can think of is that I won't be able to use it in expressions like <code>(x["a"] = 42).doSomething()</code> but my <code>Proxy</code> cannot support anything like that anyway, since it is write-only. Or would it be better to just disallow chaining (e.g. by returning <code>void</code>)?</p> <p><strong>Edit:</strong> Even if <code>Proxy</code> is not value-like, I think supporting assignment makes sense, it allows syntactic sugar like:</p> <pre class="lang-c++ prettyprint-override"><code>// this: // rather than: ApiWrapper w; API * ptr = make_api_instance(); w["name"] = "Batman"; api_set_str(ptr, "name", "Batman"); w["age"] = 42; api_set_int(ptr, "age", 42); w["pi"] = 3.14; api_set_double(ptr, "pi", 3.14); </code></pre>
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    1. USAnders Johansson
    1. USAnders Johansson
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    1. COThe copy assignment operator should return a non-const reference.
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      1. POChaining operator= for write-only objects - OK to return rhs instead of *this?
      1. USCaptain Obvlious
    2. COHow can `operator =` be `const`?
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      1. POChaining operator= for write-only objects - OK to return rhs instead of *this?
      1. USAndy Prowl
    3. COYour opening statement doesn't really make sense. First off, assignment can't be `const` and neither should the return value be. Second, `*this` is *always* an lvalue (since it's dereferencing a pointer), and there's always the standard lvalue-to-rvalue conversion.
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