[英]Printing hex characters
I would like to understand the results of printing out a char and an unsigned char.我想了解打印出一个字符和一个无符号字符的结果。
#include <iostream>
#include <string>
using namespace std;
int main()
{
int8_t a = 0xA1;
uint8_t b = 0xA1;
printf("0x%x,", a);
printf("0x%x,", b);
std::cout << std::hex << a << ",";
std::cout << std::hex << b << std::endl;
}
Result结果
0xffffffa1,0xa1,�,�
I don't understand why the signed char turns into an uint or int, and why std::hex fails miserably.我不明白为什么有符号的 char 变成了 uint 或 int,以及为什么 std::hex 惨遭失败。
int8_t a = 0xA1;
causes signed overflow, thus the behavior is undefined.导致有符号溢出,因此行为未定义。 If you turned on correct compiler flags, you'd get something like:
如果你打开了正确的编译器标志,你会得到类似的东西:
error: overflow in conversion from 'int' to 'int8_t' {aka 'signed char'} changes value from '161' to '-95' [-Werror=overflow]
8 | int8_t a = 0xA1;
| ^~~~
Besides, %x
expects an unsigned int
.此外,
%x
需要一个unsigned int
。 This also causes undefined behavior.这也会导致未定义的行为。 You meant to do something like:
你的意思是做这样的事情:
#include <iostream>
int main()
{
int8_t a = 42; // Doesnt overflow
uint8_t b = 42;
std::printf("%#x,", static_cast<unsigned int>(a));
std::printf("%#x,", static_cast<unsigned int>(b));
std::cout << std::hex << static_cast<unsigned int>(a) << ",";
std::cout << std::hex << static_cast<unsigned int>(b) << std::endl;
}
Output: 0x2a,0x2a,2a,2a
输出:
0x2a,0x2a,2a,2a
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