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<html><!-- #BeginTemplate "/Templates/tmpl.dwt" --><!-- DW6 --><head><!-- #BeginEditable "doctitle" --> <title>PTypes: string: typecasts</title><!-- #EndEditable --> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"><link rel="stylesheet" href="styles.css"></head><body bgcolor="#FFFFFF" leftmargin="40" marginwidth="40"><p><a href="../index.html"><img src="title-20.png" width="253" height="39" alt="C++ Portable Types Library (PTypes) Version 2.0" border="0"></a> <hr size="1" noshade><!-- #BeginEditable "body" --> <p class="hpath"><a href="index.html">Top</a>: <a href="basic.html">Basic types</a>: <a href="string.html">string</a>: Typecasts </p><blockquote> <pre class="lang">#include <ptypes.h>class string { operator (const char*)() const;}</pre></blockquote><p>A string object can be assigned to a variable or passed as an actual parameter of type <span class="lang">const char*</span> implicitly (by default). Such assignments should be used carefully, since the library does not keep track of whether a char pointer refers to the given string buffer. To make sure that a char pointer refers to a valid string buffer, always make the scope of a char pointer variable smaller than or equal to the scope of a string object. In most cases passing a string object to a system or API call is safe (see examples below). This typecast operator does not perform any actions and simply returns a pointer to the string buffer.</p><p>The value of the char pointer is guaranteed to be non-NULL. Even if the string is empty, the char pointer will refer to a null-symbol.</p><p>A string buffer can not be modified through a constant char pointer. If you want to modify the string buffer through a char pointer, use <span class="lang">unique(string&)</span> function instead. This function always returns a reference to a unique string buffer (i.e. when the reference count is 1).</p><p><b>Compatibility note</b>: MSVC and GCC may treat type casts in different ways when passing a string object to a function that takes (...) parameters, e.g. <span class="lang">printf()</span> or <span class="lang">outstm::putf()</span>. You should explicitly instruct the compiler to cast the string object to <span class="lang">(const char*)</span> to avoid this problem. PTypes provides a shorter typedef <span class="lang">pconst</span> for this.</p><p><b>Examples</b></p><blockquote> <pre>void assignment_example(){ string s = "abcdef"; const char* p = s; <span class="comment">// do string manipulation here...</span>}void function_call_example(){ string s = "abcdef"; puts(s); printf("%s\n", pconst(s));}</pre></blockquote><p class="seealso">See also: <a href="string.constructors.html">Constructors/destructors</a>, <a href="string.operators.html">Operators</a>, <a href="string.manipulation.html">Manipulation</a>, <a href="string.conversion.html">Conversion</a></p><!-- #EndEditable --><hr size="1"><a href="../index.html" class="ns">PTypes home</a></body><!-- #EndTemplate --></html>⌨️ 快捷键说明
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