📄 25.4.htm

📁 This ECMA Standard specifies the form and establishes the interpretation of programs written in the
💻 HTM
字号:
<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /><title>Hyperlinked ECMA C# Language Specification</title><meta name="author" content="Jon Jagger" /><link rel="stylesheet" href="ecma334.css"></link></head><body><div align="right"><em><a href="http://www.jaggersoft.com">Jon Jagger</a></em></div><div align="right"><a href="mailto:jon@jaggersoft.com">jon@jaggersoft.com</a></div><form method="get" action="http://search.atomz.com/search/"><input size="30" name="sp-q"></input><input type="submit" value="Search C# Spec"></input><input type="hidden" name="sp-a" value="sp10024177"></input><input type="hidden" name="sp-f" value="ISO-8859-1"></input></form><a href="toc.htm">Table of Contents</a> <a href="1.htm">1</a> <a href="2.htm">2</a> <a href="3.htm">3</a> <a href="4.htm">4</a> <a href="5.htm">5</a> <a href="6.htm">6</a> <a href="7.htm">7</a> <a href="8.htm">8</a> <a href="9.htm">9</a> <a href="10.htm">10</a> <a href="11.htm">11</a> <a href="12.htm">12</a> <a href="13.htm">13</a> <a href="14.htm">14</a> <a href="15.htm">15</a> <a href="16.htm">16</a> <a href="17.htm">17</a> <a href="18.htm">18</a> <a href="19.htm">19</a> <a href="20.htm">20</a> <a href="21.htm">21</a> <a href="22.htm">22</a> <a href="23.htm">23</a> <a href="24.htm">24</a> <a href="25.htm">25</a> <a href="notes.htm">Notes</a> <a href="HyperlinkedCSharpECMA.zip">Download</a><span class="ruler"></span><span class="heading">ECMA-334 C# Language Specification</span><span class="navigate"><a href="25.3.htm"><img src="previous.gif" alt="previous" border="0" /></a><a href="25.5.htm"><img src="next.gif" alt="next" border="0" /></a></span><span class="clause-depth"><a href="24.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="25.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number-link"><a href="25.htm">25</a></span><span class="clause-title-previous"> Unsafe code</span></span><span class="clause-depth"><a href="25.3.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="25.5.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number">25.4</span><span class="clause-title"> Pointer conversions</span></span><span class="locator">
     Paragraph 1</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P1S1"></a>In an unsafe context, the set of available implicit conversions (<a href="13.1.htm">&#167;13.1</a>) is extended to include the following implicit pointer conversions: </span><ul><li><span class="sentence"><span class="sentence-number">2</span> <a name="P1S2"></a> From any <span class="non-terminal"><a href="25.2.htm#pointer-type">pointer-type</a></span> to the type void*.</span> </li><li><span class="sentence"><span class="sentence-number">3</span> <a name="P1S3"></a> From the null type to any <span class="non-terminal"><a href="25.2.htm#pointer-type">pointer-type</a></span>.</span> </li></ul></span><span class="locator">
     Paragraph 2</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P2S1"></a>Additionally, in an unsafe context, the set of available explicit conversions (<a href="13.2.htm">&#167;13.2</a>) is extended to include the following explicit pointer conversions: </span><ul><li><span class="sentence"><span class="sentence-number">2</span> <a name="P2S2"></a> From any <span class="non-terminal"><a href="25.2.htm#pointer-type">pointer-type</a></span> to any other <span class="non-terminal"><a href="25.2.htm#pointer-type">pointer-type</a></span>.</span> </li><li><span class="sentence"><span class="sentence-number">3</span> <a name="P2S3"></a> From <span class="keyword">sbyte</span>, <span class="keyword">byte</span>, <span class="keyword">short</span>, <span class="keyword">ushort</span>, <span class="keyword">int</span>, <span class="keyword">uint</span>, <span class="keyword">long</span>, or <span class="keyword">ulong</span> to any <span class="non-terminal"><a href="25.2.htm#pointer-type">pointer-type</a></span>.</span> </li><li><span class="sentence"><span class="sentence-number">4</span> <a name="P2S4"></a> From any <span class="non-terminal"><a href="25.2.htm#pointer-type">pointer-type</a></span> to <span class="keyword">sbyte</span>, <span class="keyword">byte</span>, <span class="keyword">short</span>, <span class="keyword">ushort</span>, <span class="keyword">int</span>, <span class="keyword">uint</span>, <span class="keyword">long</span>, or <span class="keyword">ulong</span>.</span> </li></ul></span><span class="locator">
     Paragraph 3</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P3S1"></a>Finally, in an unsafe context, the set of standard implicit conversions (<a href="13.3.1.htm">&#167;13.3.1</a>) includes the following pointer conversion: </span><ul><li><span class="sentence"><span class="sentence-number">2</span> <a name="P3S2"></a> From any <span class="non-terminal"><a href="25.2.htm#pointer-type">pointer-type</a></span> to the type void*.</span> </li></ul></span><span class="locator">
     Paragraph 4</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P4S1"></a>Conversions between two pointer types never change the actual pointer value.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P4S2"></a>In other words, a conversion from one pointer type to another has no effect on the underlying address given by the pointer.</span> </span><span class="locator">
     Paragraph 5</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P5S1"></a>When one pointer type is converted to another, if the resulting pointer is not correctly aligned for the pointed-to type, the behavior is undefined if the result is dereferenced.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P5S2"></a>In general, the concept "correctly aligned" is transitive: if a pointer to type A is correctly aligned for a pointer to type B, which, in turn, is correctly aligned for a pointer to type C, then a pointer to type A is correctly aligned for a pointer to type C.</span> </span><span class="paragraph"><span class="example">[Example: Consider the following case in which a variable having one type is accessed via a pointer to a different type: <pre class="code-example">
char c = 'A';  
char* pc = &amp;c;  
void* pv = pc;  
int* pi = (int*)pv;  
int i = *pi;    // undefined  
*pi = 123456;    // undefined  
</pre>end example]</span> </span><span class="locator">
     Paragraph 6</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P6S1"></a>When a pointer type is converted to a pointer to <span class="keyword">byte</span>, the result points to the lowest addressed <span class="keyword">byte</span> of the variable.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P6S2"></a>Successive increments of the result, up to the size of the variable, yield pointers to the remaining bytes of that variable.</span> <span class="example">[Example: For example, the following method displays each of the eight bytes in a <span class="keyword">double</span> as a hexadecimal value: <pre class="code-example">
using System;  
class Test  
{  
   static void Main() {  
      double d = 123.456e23;  
      unsafe {  
         byte* pb = (byte*)&amp;d;  
         
         for (int i = 0; i &lt; sizeof(double); ++i)  
         Console.Write(" {0,2:X}", (uint)(*pb++));  
         Console.WriteLine();  
      }  
   }  
}  
</pre></span></span><span class="paragraph"><span class="example">Of course, the output produced depends on endianness. end example]</span> </span><span class="locator">
     Paragraph 7</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P7S1"></a>Mappings between pointers and integers are implementation-defined.</span> <span class="note">[Note: However, on  32-and 64-bit CPU architectures with a linear address space, conversions of pointers to or from integral types typically behave exactly like conversions of <span class="keyword">uint</span> or <span class="keyword">ulong</span> values, respectively, to or from those integral types. end note]</span> </span><span class="ruler"></span><table><tr><td><table align="left" bgcolor="navy"><tr bgcolor="navy"><td><font face="Arial,sans-serif" size="6" color="yellow"><strong>{ JSL }</strong></font></td></tr></table></td></tr><tr><td><font face="Arial,sans-serif" size="2" color="navy"><strong>Jagger Software Ltd</strong></font></td></tr><tr><td><font face="Arial,sans-serif" size="2" color="navy"><strong>Company # 4070126</strong></font></td></tr><tr><td><font face="Arial,sans-serif" size="2" color="navy"><strong>VAT # 762 5213 42</strong></font></td></tr></table><img src="valid-html401.png" align="left" height="31" width="88" alt="Valid HTML 4.01" /><img src="vcss.gif" align="left" height="31" width="88" alt="Valid CSS" /></body></html>
⌨️ 快捷键说明

复制代码 Ctrl + C
搜索代码 Ctrl + F
全屏模式 F11
切换主题 Ctrl + Shift + D
显示快捷键 ?
增大字号 Ctrl + =
减小字号 Ctrl + -