便携和安全的方式来添加字节偏移到任何指针 [英] Portable and safe way to add byte offset to any pointer

问题描述

我在使用C ++时很新，并且没有掌握语言的所有复杂和微妙之处。

最常见的是什么>便携，正确和安全方式向C ++ 11中任何类型的指针添加任意字节偏移量

  someType * ptr; 
 int offset = 12345 / * bytes * /; 
 ptr = ptr + offset; //<  -  
  

---

在Stack Overflow和谷歌，但他们都提出了不同的东西。我遇到的一些变体：

1. 投放到 char * ：

     ptr = SomeType *）（（（char *）ptr）+ offset）; 
     




2. 投放至 unsigned int ：

     ptr =（SomeType *）（（unsigned int）ptr）+ offset）; 
     




3. 投射到 size_t ：

     ptr = SomeType *）（（size_t）ptr）+ offset）; 
     




4. size_t 和 ptrdiff_t 总是与指针的大小一致。因此，这些类型应该用作大数组的索引，用于存储指针和指针运算。 - 关于CodeProject的关于size_t和ptrdiff_t

     ptr =（SomeType *）（（size_t）ptr +（ptrdiff_t）offset）; 
     




5. 或类似于上述，但使用> > ：

     ptr =（SomeType *）（（intptr_t）ptr +（ptrdiff_t）offset）; 
     




6. 只转换到 intptr_t 由于 offset 已是有符号整数，因此 intptr_t 不是 size_t ：

     ptr =（SomeType *）（（intptr_t）ptr）+ offset）; 
     




在所有这些情况下，使用旧的C风格的cast，或者它是更安全还是更便携的使用 static_cast 或 reinterpret_cast / p>



我应该假设指针值本身是无符号或有符号？

解决方案

我会使用类似：

  unsigned char * bytePtr = reinterpret_cast< unsigned char *>（ptr）; 
 bytePtr + = offset; 
  




I'm quite new at working with C++ and haven't grasped all the intricacies and subtleties of the language.

What is the most portable, correct and safe way to add an arbitrary byte offset to a pointer of any type in C++11?

SomeType* ptr;
int offset = 12345 /* bytes */;
ptr = ptr + offset;             // <--

---

I found many answers on Stack Overflow and Google, but they all propose different things. Some variants I have encountered:

1. Cast to char *:

   ptr = (SomeType*)(((char*)ptr) + offset);
   

2. Cast to unsigned int:

   ptr = (SomeType*)((unsigned int)ptr) + offset);
   

3. Cast to size_t:

   ptr = (SomeType*)((size_t)ptr) + offset);
   

4. "The size of size_t and ptrdiff_t always coincide with the pointer's size. Because of this, it is these types that should be used as indexes for large arrays, for storage of pointers and pointer arithmetic." - About size_t and ptrdiff_t on CodeProject

   ptr = (SomeType*)((size_t)ptr + (ptrdiff_t)offset);
   

5. Or like the previous, but with intptr_t instead of size_t, which is signed instead of unsigned:

   ptr = (SomeType*)((intptr_t)ptr + (ptrdiff_t)offset);
   

6. Only cast to intptr_t, since offset is already a signed integer and intptr_t is not size_t:

   ptr = (SomeType*)((intptr_t)ptr) + offset);
   

And in all these cases, is it safe to use old C-style casts, or is it safer or more portable to use static_cast or reinterpret_cast for this?

Should I assume the pointer value itself is unsigned or signed?

解决方案

I would use something like:

unsigned char* bytePtr = reinterpret_cast<unsigned char*>(ptr);
bytePtr += offset;

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