C++: Hex values to String

Question

I would like to convert a hex-value ("206564697374754f") to a string (from hex to ascii). These hex-values are from gdb, so the contents are "reversed" by every two. (So the exact hex-value I need to convert is "4f75747369..."). reverse2() reverses the string appropriately, but it needs to now be converted to hex (hence the "0x", then atoi()).

The following code is what I have so far, but I run into a runtime-error. What is the issue, and is there a better way of doing this?

#include <bits/stdc++.h> 
using namespace std; 

void reverse2s(string str) 
{ 
for (int i=str.length()-2; i>=0; i-=2) {
    string hx="0x"+str[i]+str[i+1];
    cout << (char)(std::stoi( hx )); 
}
} 

// Driver code 
int main(void) 
{ 
    string s = "206564697374754f"; 
    reverse2s(s); 
    return (0); 
}

It looks like you START with the value as an ascii string, right? Start with "0x". Reverse-iterate through the string, every two characters are combined with the "0x". Push this constructed string into a `std::vector one_byte_hex;`, call it done — JohnFilleau, Mar 09 '20 at 15:44
**Recommended reading:** [Why should I not #include ?](https://stackoverflow.com/q/31816095/560648) — Asteroids With Wings, Mar 09 '20 at 15:47

1201ProgramAlarm · Answer 1 · 2020-03-09T15:52:11.783

2

The expression "0x"+str[i]+str[i+1]; does not do what you think. "0x" is a character array (not a string). Since str[i] is a character, the addition will add convert that character to an int, and perform a pointer addition. This results in Undefined Behavior.

To do the string concatenation you're expecting, you need to create a string object first:

string hx="0x"s+str[i]+str[i+1];

"0x"s will create an actual string literal to append characters to.

edited Mar 09 '20 at 15:52

answered Mar 09 '20 at 15:42

1201ProgramAlarm

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`s/string literal/std::string literal/` ;) – Asteroids With Wings Mar 09 '20 at 16:04

JohnkaS · Answer 2 · 2020-03-09T15:44:15.500

0

Well it seems like you're just trying to print it as hex,

so you could do

std::cout << std::hex << 5 << std::endl; // prints 0x5

If you don't care about performance:

std::stringstream s;
s << std::hex << num:
s.str(); // std::string containing your number as hex

If you do care about performance I have no clue

edited Mar 09 '20 at 15:44

answered Mar 09 '20 at 15:37

JohnkaS

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Then I need to convert that to a string (in ascii). – thegoodhunter-9115 Mar 09 '20 at 15:39

score 0 · Answer 3 · edited Mar 15 '20 at 01:58

0

This should work:

#include <iostream>
#include <strstream>
#include <string>

int main()
{
    std::strstream s1; // dynamic buffer
    s1 << std::hex << 12345 << std::endl;
    std::cout << "buffer: '" << s1.str() << "'\n";
    s1.freeze(false);
    return 0;
}

edited Mar 15 '20 at 01:58

Asteroids With Wings

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answered Mar 09 '20 at 15:42

lenik

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Blimey, `std::strstream` – Asteroids With Wings Mar 09 '20 at 15:48
@AsteroidsWithWings care to elaborate?? – lenik Mar 10 '20 at 00:41
Well only that I haven't seen it used in a long time and it was deprecated over 20 years ago. I'm not necessarily saying "don't use it" (I actually personally think it's underrated!), but I'm wondering why you chose it over `std::stringstream` – Asteroids With Wings Mar 10 '20 at 11:09
@AsteroidsWithWings I have never used any of those. `strstream` was shorter to type =) – lenik Mar 10 '20 at 12:30
By the way, I just discovered that [your code has a leak because you don't call `s1.freeze(false)` after `s1.str()`](https://en.cppreference.com/w/cpp/io/strstream/str). – Asteroids With Wings Mar 13 '20 at 20:22
(I decided to use `std::strstream` the other day because you inspired me and because I was tired of being forced to copy `std::string`s, but then this... lol) – Asteroids With Wings Mar 13 '20 at 20:22
@AsteroidsWithWings ok, you have completely changed my mind about `strstream`. never again =) – lenik Mar 14 '20 at 04:19
Hehe I knew there was a reason somewhere – Asteroids With Wings Mar 14 '20 at 14:34

C++: Hex values to String

3 Answers3