创建临时字符*并进行打印的正确方法是什么？

Question

I have a helper function that takes an unsigned char array of a fixed length, and returns it as a formatted char *. However, I'm having some problems.

I tried

char* byteArrayToString(unsigned char byte[6]) {
    char t[18] = {""};
    char* str = t;
    sprintf(str, "%02X:%02X:%02X:%02X:%02X:%02X", byte[0], byte[1], byte[2], byte[3], byte[4], byte[5]);
    return str;
}

and

char* byteArrayToString(unsigned char byte[6]) {
    std::string t = "";
    char* str = t;
    sprintf(str, "%02X:%02X:%02X:%02X:%02X:%02X", byte[0], byte[1], byte[2], byte[3], byte[4], byte[5]);
    return str;
}

and

char* byteArrayToString(unsigned char byte[6]) {
    char* str = new char();
    sprintf(str, "%02X:%02X:%02X:%02X:%02X:%02X", byte[0], byte[1], byte[2], byte[3], byte[4], byte[5]);
    return str;
}

The second one results in some side effects of the value of that string being changed. The first one ends up giving me junk values and the last seg faults (but I can't figure out why).

Answer 1

Proper way is to return std::string as:

#include <sstream>   //for std::ostringstream
#include <string>    //for std::string
#include <iomanip>   //for std::setw, std::setfill

std::string byteArrayToString(unsigned char byte[6]) 
{
    std::ostringstream ss;
    for(size_t i = 0 ; i < 5 ; ++i)
         ss << "0X" << std::hex << std::setw(2) << std::setfill('0') << (int) byte[i] << ":";
    ss << "0X" << std::hex << std::setw(2) << std::setfill('0') << (int) byte[5];
    return ss.str();
}

Online demo

On the callsite you can get const char* as:

std::string s = byteArrayToString(bytes);
const char *str = s.c_str();

Answer 2

The problem with your first one is not in the printing, but in the returning. You're returning a pointer to an array which has been reclaimed (because it is an automatic variable, its lifetime ends when the function returns).

Instead try:

string byteArrayToString(const unsigned char* const byte)
{
    char t[18] = {""};
    sprintf(t, "%02X:%02X:%02X:%02X:%02X:%02X", byte[0], byte[1], byte[2], byte[3], byte[4], byte[5]);
    return t;
}