将格式化的printf（c）等效于C ++中的格式化cerr

Question

I want an equivalent for printf("%2.2x", var); to cerr<< in C++.
code:

typedef unsigned char byte;  
static byte var[10];  
for(i=1; i<10; i++)  
   printf("%2.2x", var[i]);

The idea is to redirect the debugging to a file like this: ./myprog 2>out.txt .
If I don't ask too much I would like to receive explanations too.
Thanks!

Answer 1

使用fprintf(stderr, ...) ，例如：

fprintf(stderr, "%2.2x", var[i]);

Answer 2

You can do this with the stream manipulators in C++, for example:

#include <iostream>
#include <iomanip>
...
std::cerr << std::hex << std::setw(2) << std::setprecision(2) << (int)var[i];

I think setw is correct here, but have a play around, and some more are listed here: http://www.cplusplus.com/reference/iomanip/

Answer 3

另一种方法是使用boost::format ：

std::cerr << boost::format("%2.2x") % static_cast<int>(var[i]);

Answer 4

#include <iostream>
#include <iomanip>

void show_a_byte(unsigned char val) {
    std::ostream out(std::cerr.rdbuf());
    out << std::hex << std::setw(2) << std::setprecision(2)
        << static_cast<unsigned int>(val);
}

I used a temporary ostream sharing cerr 's buffer to make sure none of the manipulators leave undesired side effects on cerr . The static_cast is needed because when ostream gets a ( signed or unsigned or plain) char , it thinks it can just copy it as a raw byte.