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    <p>The key issue is whether or not you should distinguish completely populated objects from incompletely populated objects <strong>at the type level</strong>. If you decide not to make a distinction, then just use <code>boost::optional</code> or similar as you are doing: this makes it easy to get coding quickly. OTOH you can't get the compiler to enforce the requirement that a particular function requires a completely populated object; you need to perform run-time checking of fields each time.</p> <h2>Parameter-group Types</h2> <p>If you do distinguish completely populated objects from incompletely populated objects at the type level, you can enforce the requirement that a function be passed a complete object. To do this I would suggest creating a corresponding type <code>XParams</code> for each relevant type <code>X</code>. <code>XParams</code> has <code>boost::optional</code> members and setter functions for each parameter that can be set after initial construction. Then you can force <code>X</code> to have only one (non-copy) constructor, that takes an <code>XParams</code> as its sole argument and checks that each necessary parameter has been set inside that <code>XParams</code> object. (Not sure if this pattern has a name -- anybody like to edit this to fill us in?)</p> <h2>"Partial Object" Types</h2> <p>This works wonderfully if you don't really have to <em>do</em> anything with the object before it is completely populated (perhaps other than trivial stuff like get the field values back). If you do have to sometimes treat an incompletely populated <code>X</code> like a "full" <code>X</code>, you can instead make <code>X</code> derive from a type <code>XPartial</code>, which contains all the logic, plus <code>protected</code> virtual methods for performing precondition tests that test whether all necessary fields are populated. Then if <code>X</code> ensures that it can only ever be constructed in a completely-populated state, it can override those protected methods with trivial checks that always return <code>true</code>:</p> <pre><code>class XPartial { optional&lt;string&gt; name_; public: void setName(string x) { name_.reset(x); } // Can add getters and/or ctors string makeGreeting(string title) { if (checkMakeGreeting_()) { // Is it safe? return string("Hello, ") + title + " " + *name_; } else { throw domain_error("ZOINKS"); // Or similar } } bool isComplete() const { return checkMakeGreeting_(); } // All tests here protected: virtual bool checkMakeGreeting_() const { return name_; } // Populated? }; class X : public XPartial { X(); // Forbid default-construction; or, you could supply a "full" ctor public: explicit X(XPartial const&amp; x) : XPartial(x) { // Avoid implicit conversion if (!x.isComplete()) throw domain_error("ZOINKS"); } X&amp; operator=(XPartial const&amp; x) { if (!x.isComplete()) throw domain_error("ZOINKS"); return static_cast&lt;X&amp;&gt;(XPartial::operator=(x)); } protected: virtual bool checkMakeGreeting_() { return true; } // No checking needed! }; </code></pre> <p>Although it might seem the inheritance here is "back to front", doing it this way means that an <code>X</code> can safely be supplied anywhere an <code>XPartial&amp;</code> is asked for, so this approach obeys the <a href="http://en.wikipedia.org/wiki/Liskov_substitution_principle" rel="nofollow noreferrer">Liskov Substitution Principle</a>. This means that a function can use a parameter type of <code>X&amp;</code> to indicate it needs a complete <code>X</code> object, or <code>XPartial&amp;</code> to indicate it can handle partially populated objects -- in which case either an <code>XPartial</code> object or a full <code>X</code> can be passed.</p> <p>Originally I had <code>isComplete()</code> as <code>protected</code>, but found this didn't work since <code>X</code>'s copy ctor and assignment operator must call this function on their <code>XPartial&amp;</code> argument, and they don't have sufficient access. On reflection, it makes more sense to publically expose this functionality.</p>
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    1. COThanks random hacker, good answer. Have you seen this partial pattern used/implemented anywhere?
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    2. COI have used the "parameter-group" pattern several times myself to good effect, though I haven't seen it come up elsewhere. I'm sure it must be a "known pattern" of some kind though. I haven't actually used the "partial object" pattern myself, just came up with it now after thinking about it for a while, so YMMV ;)
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