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copied!<h1>What the Standard says</h1> <p>The standard says (18.2)</p> <blockquote> <p>nullptr_t is defined as follows:</p> <pre><code>namespace std { typedef decltype(nullptr) nullptr_t; } </code></pre> <p>The type for which nullptr_t is a synonym has the characteristics described in 3.9.1 and 4.10.</p> </blockquote> <p>Where 3.9.1 basically says it should be of the same size as <code>void*</code> and 4.10 specifies the conversion rules for <code>nullptr</code>.</p> <p><strong>Edit:</strong> 3.9.9 furthermore explicitly states that <code>nullptr_t</code> is a scalar type, which means the expected initialization rules for built-in types from 8.5 apply:</p> <ul> <li>Default-initialization (<code>nullptr_t n;</code>), which leaves the value of <code>n</code> undefined. As Johannes Schaub pointed out correctly, this compiles fine with the newest version of Clang.</li> <li>Value-initialization (<code>nullptr_t n = nullptr_t();</code>), which initializes n to 0.</li> </ul> <p>This behavior is identical to e.g. <code>int</code>, so <code>nullptr_t</code> is definitely default-constructible. The interesting question here is: What does it mean for <code>nullptr_t</code> to have undefined value? At the end of the day, there is only one meaningful possible value for <code>nullptr_t</code>, which is <code>nullptr</code>. Furthermore the type itself is only defined through the semantics of the <code>nullptr</code>literal. Do these semantics still apply for an unitialized value?</p> <h1>Why that question doesn't matter in practice</h1> <p>You don't want to declare a new variable of type <code>nullptr_t</code>. The only meaningful semantic of that type is already expressed through the <code>nullptr</code> literal, so whenever you would use your custom variable of type <code>nullptr_t</code>, you can just as well use <code>nullptr</code>.</p> <h1>What does matter in practice</h1> <p>The only exception to this comes from the fact that you can take non-type template parameters of type <code>nullptr_t</code>. For this case, it is useful to know which values can convert to <code>nullptr_t</code>, which is described in 4.10:</p> <blockquote> <p>A null pointer constant is an integral constant expression (5.19) prvalue of integer type that evaluates to zero or a prvalue of type <code>std::nullptr_t</code>. [...] A null pointer constant of integral type can be converted to a prvalue of type <code>std::nullptr_t</code>.</p> </blockquote> <p>Which basically does just what you'd expect: You can write</p> <pre><code>nullptr_t n = 0; // correct: 0 is special </code></pre> <p>but not</p> <pre><code>nullptr_t n = 42; // WRONG can't convert int to nullptr_t </code></pre> <p>Both gcc 4.6 and Clang SVN get this right.</p>

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