C ++：如何将数组中的2个字节转换为无符号短整型 [英] C++: how to cast 2 bytes in an array to an unsigned short

问题描述

我一直在使用一个遗留的C ++应用程序，并且绝对在我的舒适区之外（一个好东西）。我想知道是否有人会有这样的亲切给我一些指针（双关意）。

我需要在unsigned char数组中转换2个字节无符号短路。字节是连续的。

我想要做的一个例子：

我从套接字和地方收到一个字符串它在一个unsigned char数组中。我可以忽略第一个字节，然后下2个字节应转换为unsigned char。这将是在windows上只有所以没有大/小endian问题（我知道）。

这是我现在（不明显工作）：

  // packetBuffer是一个无符号字符数组，包含用于测试的字符串123456789
 //我需要转换字节2和3到短，2是最重要的字节
 //所以我希望得到515（2 * 256 + 3），而我所有的代码我试过给我
 //错误或2（只转换一个字节
 unsigned short myShort; 
 myShort = static_cast< unsigned_short>（packetBuffer [1]）$ ​​b $ b   
解决方案
好吧，你正在将char扩展成一个短的值，你想要的是把两个字节解释为短。 static_cast 无法从 unsigned char * 转换为无符号short * 到 void * ，然后到 unsigned short * ：
  unsigned short * p = static_cast< unsigned short *>（static_cast< void *>（& packetBuffer [1] 
  
 
 
现在，您可以取消引用p并获取短值。但是这种方法的问题是，你从unsigned char *转换为void *，然后转换为一些不同的类型。该标准不保证地址保持不变（此外，取消引用该指针将是未定义的行为）。一个更好的方法是使用位移位，它总是工作：
  unsigned short p =（packetBuffer [1]< < 8）| packetBuffer [2]; 
  
 
I have been working on a legacy C++ application and am definitely outside of my comfort-zone (a good thing). I was wondering if anyone out there would be so kind as to give me a few pointers (pun intended).

I need to cast 2 bytes in an unsigned char array to an unsigned short. The bytes are consecutive. 

For an example of what I am trying to do:

I receive a string from a socket and place it in an unsigned char array. I can ignore the first byte and then the next 2 bytes should be converted to an unsigned char. This will be on windows only so there are no Big/Little Endian issues (that I am aware of).

Here is what I have now (not working obviously):
//packetBuffer is an unsigned char array containing the string "123456789" for testing
//I need to convert bytes 2 and 3 into the short, 2 being the most significant byte
//so I would expect to get 515 (2*256 + 3) instead all the code I have tried gives me
//either errors or 2 (only converting one byte
unsigned short myShort;
myShort = static_cast<unsigned_short>(packetBuffer[1])

解决方案
Well, you are widening the char into a short value. What you want is to interpret two bytes as an short. static_cast cannot cast from unsigned char* to unsigned short*. You have to cast to void*, then to unsigned short*:
unsigned short *p = static_cast<unsigned short*>(static_cast<void*>(&packetBuffer[1]));
Now, you can dereference p and get the short value. But the problem with this approach is that you cast from unsigned char*, to void* and then to some different type. The Standard doesn't guarantee the address remains the same (and in addition, dereferencing that pointer would be undefined behavior). A better approach is to use bit-shifting, which will always work:
unsigned short p = (packetBuffer[1] << 8) | packetBuffer[2];


                        
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