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copied!<p>I have always understood it this way: the purpose of the <code>iostream</code> class is to read and/or write a stream of characters, which, if you think about it, are abstract entities that are only represented by the computer using a character encoding. The C++ standard makes great pains to avoid pinning down the character encoding, saying only that "Objects declared as characters (<code>char</code>) shall be large enough to store any member of the implementation's basic character set," because it doesn't need to force the "implementation basic character set" to define the C++ language; the standard can leave the decision of <em>which</em> character encoding is used to the implementation (compiler together with an STL implementation), and just note that <code>char</code> objects represent single characters in some encoding.</p> <p>An implementation writer could choose a single-octet encoding such as <a href="http://en.wikipedia.org/wiki/ISO/IEC_8859-1" rel="noreferrer">ISO-8859-1</a> or even a double-octet encoding such as <a href="http://en.wikipedia.org/wiki/UCS-2" rel="noreferrer">UCS-2</a>. It doesn't matter. As long as a <code>char</code> object is "large enough to store any member of the implementation's basic character set" (note that this explicitly forbids <a href="http://en.wikipedia.org/wiki/Variable-width_encoding" rel="noreferrer">variable-length encodings</a>), then the implementation may even choose an encoding that represents basic Latin in a way that is incompatible with any common encoding!</p> <p>It is confusing that the <code>char</code>, <code>signed char</code>, and <code>unsigned char</code> types share "char" in their names, but it is important to keep in mind that <code>char</code> does not belong to the same family of fundamental types as <code>signed char</code> and <code>unsigned char</code>. <code>signed char</code> is in the family of signed integer types:</p> <blockquote> <p>There are four <em>signed integer types</em>: "signed char", "short int", "int", and "long int."</p> </blockquote> <p>and <code>unsigned char</code> is in the family of unsigned integer types:</p> <blockquote> <p>For each of the signed integer types, there exists a corresponding (but different) <em>unsigned integer type</em>: "unsigned char", "unsigned short int", "unsigned int", and "unsigned long int," ...</p> </blockquote> <p>The one similarity between the <code>char</code>, <code>signed char</code>, and <code>unsigned char</code> types is that "[they] occupy the same amount of storage and have the same alignment requirements". Thus, you can <code>reinterpret_cast</code> from <code>char *</code> to <code>unsigned char *</code> in order to determine the numeric value of a character in the execution character set.</p> <p>To answer your question, the reason why the STL uses <code>char</code> as the default type is because the standard streams are meant for reading and/or writing streams of characters, represented by <code>char</code> objects, not integers (<code>signed char</code> and <code>unsigned char</code>). The use of <code>char</code> versus the numeric value is a way of separating concerns.</p>

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