I'm testing a piece of code that performs a hash operation (sha256) of a binary file and I've got something like this:
for(i = 0; i < SHA256_DIGEST_LENGTH; i++) printf("%02x", c[i]);
This prints something like:
12b64492d18aa37d609f27cb02ce5ba381068d1ef5625193df68451c650a2b8d
I'm asking how can I do to get the string shown below into a string variable in C++.
thanks
#include <iomanip>
#include <sstream>
#include <string>
std::ostringstream oss;
for(int i = 0; i < SHA256_DIGEST_LENGTH; ++i)
{
oss << std::hex << std::setw(2) << std::setfill('0') << +c[i];
}
auto str = oss.str();
auto rather than string?For printing out hex values, you can use std::hex format; for setting width and fill character, use std::setw and std::setfill, which are part of <iomanip>.
As you do not show the data type of c, I suppose/suggest to use an unsigned integral type, e.g. unsigned char. I slightly adapted the code to make it self contained):
#define SHA256_DIGEST_LENGTH 256
#include <iostream>
#include <sstream>
#include <iomanip>
int main() {
unsigned char c[SHA256_DIGEST_LENGTH];
for (unsigned int i=0; i<SHA256_DIGEST_LENGTH; i++)
c[i]=i;
std::stringstream ss;
for(int i = 0; i < SHA256_DIGEST_LENGTH; i++) {
ss << std::hex << std::setw(2) << std::setfill('0') << (unsigned int)c[i];
}
std::cout << ss.str();
}
Output:
000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f202122232425262728292a2b2c2d2e2f303132333435363738393a3b3c3d3e3f404142434445464748494a4b4c4d4e4f505152535455565758595a5b5c5d5e5f606162636465666768696a6b6c6d6e6f707172737475767778797a7b7c7d7e7f808182838485868788898a8b8c8d8e8f909192939495969798999a9b9c9d9e9fa0a1a2a3a4a5a6a7a8a9aaabacadaeafb0b1b2b3b4b5b6b7b8b9babbbcbdbebfc0c1c2c3c4c5c6c7c8c9cacbcccdcecfd0d1d2d3d4d5d6d7d8d9dadbdcdddedfe0e1e2e3e4e5e6e7e8e9eaebecedeeeff0f1f2f3f4f5f6f7f8f9fafbfcfdfeff
std::hex, and as OP does not specify the type of c, I rewrote that part. But: adapted answer now to make a suggestion on the type of c.Just for comparison, here's the sprintf version:
#include <stdio.h>
#include <string>
std::string ss;
ss.resize(SHA256_DIGEST_LENGTH * 2 + 1); // includes space for terminating NUL
int used = 0;
for(int i = 0; i < SHA256_DIGEST_LENGTH; i++)
used += sprintf(&ss[used], "%02x", c[i]);
ss.resize(used);
Note that there's no harm in making the buffer larger than necessary initially because the final exact size is used, but if there's any possibility the buffer is too small then one must use snprintf and also pass the buffer space remaining (ss.size() - used).
char[] to receive the formatted data, then construct a string from the char[]
std::string is a smart-pointer wrapper for a char[], and even if you use a statically-sized local array, you still end up stuck with an extra copy. The way I show is better. For even more efficiency, scrap the sprintf call (which has to parse a format string) and just do two char assignments, corresponding to high- and low-nibble, directly into the std::string.resize() and thus set some known value in the new capacity before just overwriting it, or reserve() then emplace_/push_back() piece by piece, making the library check capacity, increment size, etc each time. Still, I doubt it matters usually, so the extra safety and readability of the C++ versions win in such cases.
printf, there'ssprintf. If you like iostreams, there's stringstream.