在C ++中对void指针执行算术 [英] Performing arithmetic on a void pointer in C++

问题描述

我试图对地址执行算术。我需要确保它在一个4字节的字边界。这需要我获得指针的无符号整数表示以执行一些数学。我这样做：

  // memStartAddr的类型为void *（来自外部API函数）
 data32 tempAddr = reinterpret_cast< data32>（memStartAddr）; 
 //如果需要，调整指向下一个字边界的指针
 tempAddr = wordAlign（tempAddr）; 
 // memStartAddress是类型unsigned char * 
的类变量memStartAddress = reinterpret_cast< data8 *>（tempAddr）; 
 //计算由于上述调整而丢失的字节
 data32 delta =（tempAddr-reinterpret_cast< data32>（memStartAddress））; 
 //利用整数算术来确保可用大小是4字节的多重
 //。剩余内容丢失。如有必要，收缩可用尺寸。 
 memSize =（（size-delta）/ 4）* 4; 
  

这一切都适用于我的测试，但是，使用reinterpret_cast被认为是禁止的做法。有另一种方法吗？

您正在寻找 memStartAddress = std： ：align（4，objSize，memStartAddr，objSize + 3）; 。标准函数比你需要的更灵活一点，所以你需要告诉它你最多需要一个+3的地址变化。

I am trying to perform arithmetic on an address. I need to make sure that it is on a 4 byte word boundary. This requires me to get an unsigned integer representation of the pointer to perform some math on. I have done this:

//memStartAddr is of type void* (from an external API function)
data32 tempAddr = reinterpret_cast<data32>(memStartAddr);
//adjust pointer to the next word boundary if needed
tempAddr = wordAlign(tempAddr);
//memStartAddress is a class variable of type unsigned char*
memStartAddress = reinterpret_cast<data8*>(tempAddr);
//calculate bytes lost due to above adjustment
data32 delta = (tempAddr - reinterpret_cast<data32>(memStartAddress));
//Take advantage of integer arithmetic to ensure usable size is a multiple
//of 4 bytes. Remainders are lost. Shrinks usable size if necessary.
memSize = ((size - delta) / 4) * 4;

This all works in my tests, however, using reinterpret_cast is considered a forbidden practice. Is there another way to do this? Or is an exception to the rule warranted here?

解决方案

You're looking for memStartAddress = std::align(4, objSize, memStartAddr, objSize+3);. The standard function is a bit more flexible than you need, so you need to tell it that you want at most a +3 change in address.

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