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    <p>It's exactly as you said.</p> <p>When you pass an object by value, its <em>copy constructor</em> is invoked to produce a new instance of such object that will be used inside the function. The changes done to such new object won't be reflected to the original one<sup>1</sup>.</p> <p>As with structures, the default copy constructor just does a shallow copy of the original object - i.e., its fields are copied<sup>2</sup> to the new instance; in many cases this is not desirable (e.g. if the object wraps a pointer/another resource), so there are classes which redefine the copy constructor or disable it completely. Objects of these last classes can only be passed by pointer or reference.</p> <p>Passing objects by value can be costly if they are bigger than a pointer (in size) or in general if their copy constructor isn't "cheap". On the other hand, in comparison to pointers, the pass-by-value yields the usual advantages of not having to specify the pointer ownership, letting the callee do whatever it wants with the object, etc.</p> <p>Notice that passing an object by value kills the polymorphism. This because a function receiving an object by value receives a statically typed object, with a precise size and type, so any attempt to pass an object of a derived class will result in object slicing (the copy constructor for the base class is called, that by default just copies the fields that are available in the base class).</p> <p>This is the reason why often the preferred method of passing objects is by <code>const</code> reference. This yields several advantages:</p> <ul> <li>no copies involved; the object that the callee will see will be exactly the one specified at the moment of the call;</li> <li>no changes to the original object can be made, thanks to the <code>const</code> qualifier;</li> <li>if however the callee needs to change a copy of the object, it can still construct a copy by itself from the reference;</li> <li>no awkward pointer syntax;</li> <li>polymorphism preserved, since behind the scenes we're actually passing a pointer;</li> <li>no big doubts about object ownership: the general rule about references is that they are owned by the caller.</li> </ul> <hr /> <ol> <li><p>As far as the "raw fields" of the object are concerned; naturally if the original object and the copy continue to share a pointer/handle to the same resource some modifications to one may affect the other.</p></li> <li><p>Primitive types (and in general PODs) are copied bitwise, while the copy constructor is called for non-POD types.</p></li> </ol>
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    1. POin C++, what's the difference between an object and a pointer to an object?
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    1. CO+1 for the quality of this answer - maybe the downvote was a mistake ? :)
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