How Can I Convert a Hex String to a Signed Integer in C ?

Convert Hex String to Signed Integer in C

Many situations require converting a hexadecimal string to a signed integer in C . For instance, you may have a hex string like "fffefffe" representing a binary value of 11111111111111101111111111111110, which corresponds to the signed integer -65538.

Solution Using std::stringstream

One approach to achieve this conversion involves using std::stringstream:

unsigned int x;   
std::stringstream ss;
ss << std::hex << "fffefffe";
ss >> x;

Non-Negative Numbers

To handle non-negative numbers, you can use the following example:

#include <sstream>
#include <iostream>

int main() {
    unsigned int x;   
    std::stringstream ss;
    ss << std::hex << "0000000A";
    ss >> x;
    std::cout << static_cast<int>(x) << std::endl;
}

This will produce 10 as the result, which is correct for the hex string "0000000A".

C 11 String-to-Number Functions

In C 11, you can utilize string-to-number functions such as std::stoul:

std::string s = "0xfffefffe";
unsigned int x = std::stoul(s, nullptr, 16);

Other Approaches

Alternatively, you can leverage libraries like Boost, which offers functions like lexical_cast:

unsigned int x = boost::lexical_cast<int>("0x0badc0de");

Rolling Your Own lexical_cast

If you prefer a simpler approach without dependencies, you can use a custom implementation of lexical_cast:

template<typename T2, typename T1>
inline T2 lexical_cast(const T1 &amp;in) {
    T2 out;
    std::stringstream ss;
    ss << in;
    ss >> out;
    return out;
}

unsigned int x = lexical_cast<unsigned int>("0xdeadbeef"); 

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