Converting two's complement output to signed decimal

Question

I apologize if this question has been answered already but I have not been able to find what I am looking for.

I am working in c++ with an SPI device. The SPI device outputs data in 16 bit words in 2's complement form. I am trying to convert this data into decimal for use with a filter.

I've attached some sample code that asks the user to input a number in twos complement and then outputs the signed decimal version.

#include <iostream>     
#include <stdlib.h>
#include <cstdint>
#include <cmath>
#include <bitset>
using std::cout;
using std::endl;
using std::cin;
using std::hex;
using std::dec;
using std::bitset;
int main () {
    uint16_t x2=0;
    cout<<"Please enter the number you would like to convert from 2's complement.  "<<endl;
    cin>>x2;
    int diff=0x0000-x2;
    cout<<"The number you have entered is: "<<dec<<diff<<endl;
return 0;
}

When I run this program and input something like 0x3B4A it always outputs 0. I'm not entirely sure what is going on and I'm very new to c++ so please excuse me if this is a stupid question. Also, please ignore anything extra in the header. This is partof a large project and I couldn't remember what parts of the header go with this specific section of code.

Thanks!

Edit: This is mostly for Ben. After reading your most recent comment I made the following changes but am still simply getting the decimal equivalent of the hexadecimal number I entered

#include <iostream>     
#include <stdlib.h>
#include <cstdint>
#include <cmath>
#include <bitset>
using std::cout;
using std::endl;
using std::cin;
using std::hex;
using std::dec;
using std::bitset;
int main () {
    int16_t x2=0;
    cout<<"Please enter the number you would like to convert from 2's complement.  "<<endl;
    cin>>hex>>x2;
    int flags= (x2>>14) & 3;
    int16_t value=(x2 << 2) >> 2;
    cout<<"The number you have entered is: "<<dec<<value<<endl;
return 0;
}

Answer 1

The standard library function strtol converts string input to a number, and supports the 0x prefix as long as you pass 0 as the radix argument.

Since int16_t is almost certainly 16-bit two's complement signed, you can just use that.

Answer 2

I'm not sure it is necessary for the OP's question, but for anybody who is just looking for the formula for converting a 16bit 2's complement unsigned integer to a signed integer, I think a variant of it looks like this (for input val):

(0x8000&val ? (int)(0x7FFF&val)-0x8000 : val)

This amounts to:

• if first bit is 1, it is a negative number with all other bits in 2's complement
• extract negative part by subtracting off the 0x8000
• otherwise the lower bits are just the positive integer value

Probably a good idea to wrap this in a function and do some basic error checking (can also enforce that the input is actually an unsigned 16 bit integer).

Answer 3

You asked cin to read as decimal (by making no format changes) so as soon as it reads the x, which is not a 0-9 digit, it stops, leaving you with zero.

Just add hex to your cin line: cin >> hex >> x2;