Hex

References

Many examples come from cplusplus.com and stackoverflow.com.

C / C ++

Read an integer from an hex string

(See [1], [2]).

Homebrew

const char _hex[] = "0123456789ABCDEF";

// Change a 0x0A into 'A'
char itoh(int nibble)
{
    return _hex[nibble];
}

// Change a 'A' into 0x0A
int htoi(char hex)
{
    int isletter = (hex & 0x40) >> 6;
    return (isletter | (isletter << 3)) + ( hex & 0x0F );
}

// Change "A010" into 0xA010
template<class T>
T hextoint_BE(string h)
{
    T i=0;

    for(string::const_iterator it=h.begin(); it!=h.end(); ++it)
    {
        i=(i<<4)|htoi(*it);
    }

    return i;
}

// Change "A010" into "\xA0\x10"
string hextostring_BE(const string& h)
{
    string x;
    for(unsigned int i=0; (i+1)<h.length(); i+=2) {
        x += (htoi(h[i]) << 4)+ htoi(h[i+1]);
    }
    return x;
}

// Change 0xA010 into "A010"
template<class T> 
string inttohex_BE(T i, unsigned int size)
{
    string sh;
    while( i > 0 )
    {
        sh = itoh(i&0xF) + sh;
        i>>=4;
    }

    if(!size)
        return sh;
    if(size<sh.length())
        return string(sh,sh.length()-size,size);
    else
        return string(size-sh.length(),'0')+sh;
}

// Change "\xA0\x10" into "A010"
string stringtohex_BE(const string& s, unsigned int size)
{
    string h;
    for(string::const_iterator it=s.begin();it!=s.end();++it)
    {
        h += itoh((*it & 0xF0) >> 4);
        h += itoh((*it & 0x0F) >> 0);
    }

    if(!size)
        return h;
    if(size<h.length())
        return string(h,h.length()-size,size);
    else
        return string(size-h.length(),'0')+h;
}

Using atol

/* atol example */
#include <stdio.h>
#include <stdlib.h>

int main ()
{
  long int li;
  char szInput [256];
  printf ("Enter a long number: ");
  gets ( szInput );
  li = atol (szInput);
  printf ("The value entered is %d. The double is %d.\n",li,li*2);
  return 0;
}

Using strtol, strtoll

strtol (resp. strtoll) returns LONG_MIN, LONG_MAX (resp. LLONG_MIN, LLONGMAX) in case of under/overflow.

/* strtol example */
#include <stdio.h>
#include <stdlib.h>

int main ()
{
  char szNumbers[] = "2001 60c0c0 -1101110100110100100000 0x6fffff";
  char * pEnd;
  long int li1, li2, li3, li4;
  li1 = strtol (szNumbers,&pEnd,10);
  li2 = strtol (pEnd,&pEnd,16);
  li3 = strtol (pEnd,&pEnd,2);
  li4 = strtol (pEnd,NULL,0);
  printf ("The decimal equivalents are: %ld, %ld, %ld and %ld.\n", 
          li1, li2, li3, li4);
  return 0;
}

#include <cstdlib>
#include <iostream>
using namespace std;

int main() {
    string s = "abcd";
    char * p;
    long n = strtol( s.c_str(), & p, 16 );
    if ( * p != 0 ) {
        cout << "not a number" << endl;
    }
    else {
        cout << n << endl;
    }
}

Using strtoul, strtoull

Use strtoul (resp. strtoull) when value to read is greater than numeric_limits<long>::max() (resp. numeric_limits<long long>::max()).

/* strtoul example */
#include <stdio.h>
#include <stdlib.h>

int main ()
{
  char szInput [256];
  unsigned long ul;
  printf ("Enter an unsigned number: ");
  fgets (szInput,256,stdin);
  ul = strtoul (szInput,NULL,0);
  printf ("Value entered: %lu. Its double: %lu\n",ul,ul*2);
  return 0;
}

#include <cstdlib>
#include <iostream>
using namespace std;

int main() { 
    string s = "fffefffe";
    char * p;
    long n = strtoul( s.c_str(), & p, 16 ); 
    if ( * p != 0 ) {  
        cout << "not a number" << endl;
    }    else {  
        cout << n << endl;
    }
}

Using sscanf

#include <stdio.h>
main()
{
    char s[] = "fffffffe";
    int x;
    sscanf(s, "%x", &x);
    printf("%u\n", x);
}

Using stringstream

#include <sstream>
#include <iostream>

int main() {
    unsigned int x;   
    std::stringstream ss;
    ss << std::hex << "fffefffe";
    ss >> x;
    // output it as a signed type
    std::cout << static_cast<int>(x) << std::endl;
}