C++ convert hex string to signed integer

Question

I want to convert a hex string to a 32 bit signed integer in C++.

So, for example, I have the hex string \"fffefffe\". The binary representation of this is 111111

Answer 1

Working example with strtoul will be:

#include 
#include 
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;
    }
}

strtol converts string to long. On my computer numeric_limits::max() gives 0x7fffffff. Obviously that 0xfffefffe is greater than 0x7fffffff. So strtol returns MAX_LONG instead of wanted value. strtoul converts string to unsigned long that's why no overflow in this case.

Ok, strtol is considering input string not as 32-bit signed integer before convertation. Funny sample with strtol:

#include 
#include 
using namespace std;

int main() { 
    string s = "-0x10002";
    char * p;
    long n = strtol( s.c_str(), & p, 16 ); 
    if ( * p != 0 ) {  
        cout << "not a number" << endl;
    }    else {  
        cout << n << endl;
    }
}

The code above prints -65538 in console.