常量32768为0x8000之间的区别类型可以有所作为？ [英] Can the type difference between constants 32768 and 0x8000 make a difference?

问题描述

本标准规定了十六进制常量像为0x8000（比有符号整数更大的拟合）的无符号的（就像八进制常量），而小数常量像32768签署长。 （确切类型假设一个16位整数，32位长）。然而，在常规的C环境都将具有相同的重presentation，在二进制 1000 0000 0000 0000 。
可能的情况下这种差别确实会产生不同的结果？换句话说，有可能的情况下这种差异事项呢？

The Standard specifies that hexadecimal constants like 0x8000 (larger than fits in a signed integer) are unsigned (just like octal constants), whereas decimal constants like 32768 are signed long. (The exact types assume a 16-bit integer and a 32-bit long.) However, in regular C environments both will have the same representation, in binary 1000 0000 0000 0000. Is a situation possible where this difference really produces a different outcome? In other words, is a situation possible where this difference matters at all?

推荐答案

是的，它能够决定的事情。如果您的处理器具有16位的 INT 和一个32位的长键入，32768的类型为长（因为32767是最大的正值一个符号的16位拟合 INT ），而为0x8000（因为它也被认为为 unsigned int类型），这仍然可以用一个16位的 unsigned int类型。

Yes, it can matter. If your processor has a 16-bit int and a 32-bit long type, 32768 has the type long (since 32767 is the largest positive value fitting in a signed 16-bit int), whereas 0x8000 (since it is also considered for unsigned int) still fits in a 16-bit unsigned int.

现在考虑下面的程序：

int main(int argc, char *argv[])
{
  volatile long long_dec = ((long)~32768);
  volatile long long_hex = ((long)~0x8000);

  return 0;
}

在32768被认为是长，否定将反转32位，
导致重新presentation 0xFFFF7FFF型长;演员阵容
多余。
当为0x8000被认为是 unsigned int类型，否定将反转
16位，造成了重presentation 0x7FFF的类型 unsigned int类型;
剧组随后将零扩展到 0x00007FFF长值。
看的H＆amp; S5，第2.7.1页24ff

When 32768 is considered long, the negation will invert 32 bits, resulting in a representation 0xFFFF7FFF with type long; the cast is superfluous. When 0x8000 is considered unsigned int, the negation will invert 16 bits, resulting in a representation 0x7FFF with type unsigned int; the cast will then zero-extend to a long value of 0x00007FFF. Look at H&S5, section 2.7.1 page 24ff.

这是最好的增加与 U ， UL 或→为宜。

It is best to augment the constants with U, UL or L as appropriate.

这篇关于常量32768为0x8000之间的区别类型可以有所作为？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！