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    copied!<p>The primary time to use <code>size_type</code> is in a template. Although <code>std::vector&lt;T&gt;::size_type</code> is usually <code>size_t</code>, <code>some_other_container&lt;T&gt;::size_type</code> might be some other type instead<sup>1</sup>. One of the few things a user is allowed to add to the <code>std</code> namespace is a specialization of an existing template for some user defined type. Therefore, <code>std::vector&lt;T&gt;::size_type</code> for some oddball <code>T</code> <em>could</em> actually be some type other than <code>size_t</code>, even though the base template defined in the standard library probably always uses <code>size_t</code>.</p> <p>Therefore, if you want to use the correct type for a specific container inside a template that works with that container, you want to use <code>container::size_type</code> instead of just assuming <code>size_t</code>.</p> <p>Note, however, that generic code should rarely work directly with a container. Instead, it should typically work with iterators, so instead of <code>container&lt;T&gt;::size_type</code>, it would typically use something like <code>std::iterator_traits&lt;WhateverIterator&gt;::difference_type</code> instead.</p> <hr> <ol> <li>And for some specific <code>T</code>, <code>vector&lt;T&gt;::size_type</code> might be a different type as well--one of the few things you're allowed to put into the <code>std</code> namespace is a specialization of an existing class for a user-defined type, so for some <code>T</code>, <code>vector&lt;T&gt;</code> could use a completely different container than for most other types. This is typical for <code>vector&lt;bool&gt;</code>, but possible for other types as well.</li> </ol>
 

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