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<title>Using the GNU Compiler Collection (GCC): C++ Concepts</title>
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<a name="C_002b_002b-Concepts"></a>
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<p>
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Next: <a href="Java-Exceptions.html#Java-Exceptions" accesskey="n" rel="next">Java Exceptions</a>, Previous: <a href="Type-Traits.html#Type-Traits" accesskey="p" rel="prev">Type Traits</a>, Up: <a href="C_002b_002b-Extensions.html#C_002b_002b-Extensions" accesskey="u" rel="up">C++ Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<a name="C_002b_002b-Concepts-1"></a>
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<h3 class="section">7.11 C++ Concepts</h3>
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<p>C++ concepts provide much-improved support for generic programming. In
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particular, they allow the specification of constraints on template arguments.
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The constraints are used to extend the usual overloading and partial
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specialization capabilities of the language, allowing generic data structures
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and algorithms to be “refined” based on their properties rather than their
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type names.
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</p>
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<p>The following keywords are reserved for concepts.
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</p>
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<dl compact="compact">
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<dt><code>assumes</code></dt>
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<dd><p>States an expression as an assumption, and if possible, verifies that the
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assumption is valid. For example, <code>assume(n > 0)</code>.
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</p>
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</dd>
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<dt><code>axiom</code></dt>
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<dd><p>Introduces an axiom definition. Axioms introduce requirements on values.
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</p>
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</dd>
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<dt><code>forall</code></dt>
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<dd><p>Introduces a universally quantified object in an axiom. For example,
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<code>forall (int n) n + 0 == n</code>).
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</p>
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</dd>
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<dt><code>concept</code></dt>
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<dd><p>Introduces a concept definition. Concepts are sets of syntactic and semantic
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requirements on types and their values.
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</p>
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</dd>
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<dt><code>requires</code></dt>
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<dd><p>Introduces constraints on template arguments or requirements for a member
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function of a class template.
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</p>
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</dd>
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</dl>
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<p>The front end also exposes a number of internal mechanism that can be used
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to simplify the writing of type traits. Note that some of these traits are
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likely to be removed in the future.
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</p>
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<dl compact="compact">
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<dt><code>__is_same (type1, type2)</code></dt>
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<dd><p>A binary type trait: true whenever the type arguments are the same.
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</p>
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</dd>
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</dl>
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