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copied!<p>In both C and C++ a declaration of an object <code>a</code> of incomplete type will match the definition of object <code>a</code> where the type is complete. What you observe simply illustrates the fact that in C++ you are allowed to use incomplete types in non-defining declarations. But once you get to the definition the type must be complete.</p> <p>This behavior is not restricted to arrays. For example, you can declare</p> <pre><code>extern class X x; </code></pre> <p>for a totally unknown class <code>X</code>, and then, when <code>class X</code> is already fully defined, you can define</p> <pre><code>X x; </code></pre> <p>which will match to the above declaration.</p> <p>The same thing happens with your array. Firstly you declare an object of incomplete type</p> <pre><code>extern int a[]; </code></pre> <p>and then you define it with complete type</p> <pre><code>int a[100]; </code></pre> <p>The types here indeed are not matching. However, the C++ language never required them to match. For example, 3.9/7 explicitly states </p> <blockquote> <p>The declared type of an array object might be an array of unknown size and therefore be incomplete at one point in a translation unit and complete later on; the array types at those two points (“array of unknown bound of T” and “array of N T”) are different types.</p> </blockquote> <p>It means that <em>the same</em> array object can have incomplete type initially, but acquire a complete type later on. (See also the example in 3.9/7). This doesn't, of course, mean that you can declare <code>a</code> as an <code>int</code> and then define it as a <code>double</code>. The only type-related freedom you have here is, again, to complete an incomplete type. No more.</p>

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