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<title>Using the GNU Compiler Collection (GCC): Type Traits</title>
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<a name="Type-Traits"></a>
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Next: <a href="C_002b_002b-Concepts.html#C_002b_002b-Concepts" accesskey="n" rel="next">C++ Concepts</a>, Previous: <a href="Namespace-Association.html#Namespace-Association" accesskey="p" rel="prev">Namespace Association</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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<hr>
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<a name="Type-Traits-1"></a>
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<h3 class="section">7.10 Type Traits</h3>
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<p>The C++ front end implements syntactic extensions that allow
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compile-time determination of
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various characteristics of a type (or of a
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pair of types).
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</p>
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<dl compact="compact">
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<dt><code>__has_nothrow_assign (type)</code></dt>
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<dd><p>If <code>type</code> is const qualified or is a reference type then the trait is
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false. Otherwise if <code>__has_trivial_assign (type)</code> is true then the trait
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is true, else if <code>type</code> is a cv class or union type with copy assignment
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operators that are known not to throw an exception then the trait is true,
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else it is false. Requires: <code>type</code> shall be a complete type,
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(possibly cv-qualified) <code>void</code>, or an array of unknown bound.
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</p>
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</dd>
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<dt><code>__has_nothrow_copy (type)</code></dt>
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<dd><p>If <code>__has_trivial_copy (type)</code> is true then the trait is true, else if
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<code>type</code> is a cv class or union type with copy constructors that
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are known not to throw an exception then the trait is true, else it is false.
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Requires: <code>type</code> shall be a complete type, (possibly cv-qualified)
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<code>void</code>, or an array of unknown bound.
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</p>
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</dd>
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<dt><code>__has_nothrow_constructor (type)</code></dt>
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<dd><p>If <code>__has_trivial_constructor (type)</code> is true then the trait is
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true, else if <code>type</code> is a cv class or union type (or array
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thereof) with a default constructor that is known not to throw an
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exception then the trait is true, else it is false. Requires:
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<code>type</code> shall be a complete type, (possibly cv-qualified)
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<code>void</code>, or an array of unknown bound.
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</p>
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</dd>
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<dt><code>__has_trivial_assign (type)</code></dt>
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<dd><p>If <code>type</code> is const qualified or is a reference type then the trait is
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false. Otherwise if <code>__is_pod (type)</code> is true then the trait is
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true, else if <code>type</code> is a cv class or union type with a trivial
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copy assignment ([class.copy]) then the trait is true, else it is
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false. Requires: <code>type</code> shall be a complete type, (possibly
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cv-qualified) <code>void</code>, or an array of unknown bound.
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</p>
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</dd>
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<dt><code>__has_trivial_copy (type)</code></dt>
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<dd><p>If <code>__is_pod (type)</code> is true or <code>type</code> is a reference type
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then the trait is true, else if <code>type</code> is a cv class or union type
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with a trivial copy constructor ([class.copy]) then the trait
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is true, else it is false. Requires: <code>type</code> shall be a complete
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type, (possibly cv-qualified) <code>void</code>, or an array of unknown bound.
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</p>
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</dd>
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<dt><code>__has_trivial_constructor (type)</code></dt>
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<dd><p>If <code>__is_pod (type)</code> is true then the trait is true, else if
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<code>type</code> is a cv class or union type (or array thereof) with a
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trivial default constructor ([class.ctor]) then the trait is true,
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else it is false. Requires: <code>type</code> shall be a complete
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type, (possibly cv-qualified) <code>void</code>, or an array of unknown bound.
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</p>
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</dd>
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<dt><code>__has_trivial_destructor (type)</code></dt>
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<dd><p>If <code>__is_pod (type)</code> is true or <code>type</code> is a reference type then
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the trait is true, else if <code>type</code> is a cv class or union type (or
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array thereof) with a trivial destructor ([class.dtor]) then the trait
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is true, else it is false. Requires: <code>type</code> shall be a complete
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type, (possibly cv-qualified) <code>void</code>, or an array of unknown bound.
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</p>
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</dd>
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<dt><code>__has_virtual_destructor (type)</code></dt>
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<dd><p>If <code>type</code> is a class type with a virtual destructor
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([class.dtor]) then the trait is true, else it is false. Requires:
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<code>type</code> shall be a complete type, (possibly cv-qualified)
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<code>void</code>, or an array of unknown bound.
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</p>
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</dd>
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<dt><code>__is_abstract (type)</code></dt>
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<dd><p>If <code>type</code> is an abstract class ([class.abstract]) then the trait
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is true, else it is false. Requires: <code>type</code> shall be a complete
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type, (possibly cv-qualified) <code>void</code>, or an array of unknown bound.
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</p>
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</dd>
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<dt><code>__is_base_of (base_type, derived_type)</code></dt>
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<dd><p>If <code>base_type</code> is a base class of <code>derived_type</code>
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([class.derived]) then the trait is true, otherwise it is false.
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Top-level cv qualifications of <code>base_type</code> and
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<code>derived_type</code> are ignored. For the purposes of this trait, a
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class type is considered is own base. Requires: if <code>__is_class
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(base_type)</code> and <code>__is_class (derived_type)</code> are true and
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<code>base_type</code> and <code>derived_type</code> are not the same type
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(disregarding cv-qualifiers), <code>derived_type</code> shall be a complete
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type. A diagnostic is produced if this requirement is not met.
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</p>
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</dd>
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<dt><code>__is_class (type)</code></dt>
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<dd><p>If <code>type</code> is a cv class type, and not a union type
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([basic.compound]) the trait is true, else it is false.
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</p>
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</dd>
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<dt><code>__is_empty (type)</code></dt>
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<dd><p>If <code>__is_class (type)</code> is false then the trait is false.
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Otherwise <code>type</code> is considered empty if and only if: <code>type</code>
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has no non-static data members, or all non-static data members, if
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any, are bit-fields of length 0, and <code>type</code> has no virtual
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members, and <code>type</code> has no virtual base classes, and <code>type</code>
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has no base classes <code>base_type</code> for which
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<code>__is_empty (base_type)</code> is false. Requires: <code>type</code> shall
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be a complete type, (possibly cv-qualified) <code>void</code>, or an array
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of unknown bound.
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</p>
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</dd>
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<dt><code>__is_enum (type)</code></dt>
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<dd><p>If <code>type</code> is a cv enumeration type ([basic.compound]) the trait is
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true, else it is false.
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</p>
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</dd>
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<dt><code>__is_literal_type (type)</code></dt>
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<dd><p>If <code>type</code> is a literal type ([basic.types]) the trait is
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true, else it is false. Requires: <code>type</code> shall be a complete type,
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(possibly cv-qualified) <code>void</code>, or an array of unknown bound.
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</p>
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</dd>
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<dt><code>__is_pod (type)</code></dt>
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<dd><p>If <code>type</code> is a cv POD type ([basic.types]) then the trait is true,
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else it is false. Requires: <code>type</code> shall be a complete type,
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(possibly cv-qualified) <code>void</code>, or an array of unknown bound.
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</p>
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</dd>
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<dt><code>__is_polymorphic (type)</code></dt>
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<dd><p>If <code>type</code> is a polymorphic class ([class.virtual]) then the trait
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is true, else it is false. Requires: <code>type</code> shall be a complete
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type, (possibly cv-qualified) <code>void</code>, or an array of unknown bound.
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</p>
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</dd>
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<dt><code>__is_standard_layout (type)</code></dt>
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<dd><p>If <code>type</code> is a standard-layout type ([basic.types]) the trait is
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true, else it is false. Requires: <code>type</code> shall be a complete
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type, (possibly cv-qualified) <code>void</code>, or an array of unknown bound.
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</p>
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</dd>
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<dt><code>__is_trivial (type)</code></dt>
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<dd><p>If <code>type</code> is a trivial type ([basic.types]) the trait is
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true, else it is false. Requires: <code>type</code> shall be a complete
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type, (possibly cv-qualified) <code>void</code>, or an array of unknown bound.
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</p>
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</dd>
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<dt><code>__is_union (type)</code></dt>
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<dd><p>If <code>type</code> is a cv union type ([basic.compound]) the trait is
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true, else it is false.
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</p>
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</dd>
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<dt><code>__underlying_type (type)</code></dt>
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<dd><p>The underlying type of <code>type</code>. Requires: <code>type</code> shall be
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an enumeration type ([dcl.enum]).
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</p>
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</dd>
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</dl>
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<hr>
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<div class="header">
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<p>
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Next: <a href="C_002b_002b-Concepts.html#C_002b_002b-Concepts" accesskey="n" rel="next">C++ Concepts</a>, Previous: <a href="Namespace-Association.html#Namespace-Association" accesskey="p" rel="prev">Namespace Association</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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