Converting a byte array to a decimal number in C++

Question

I'm trying to convert latitude and longitude to a byte arrayusing Java and then convert the byte array to double using C++. SO far, here's what I'm doing:

I have a byte array of {0x40,0x4a, 0x62, 0x65, 0x27, 0xa2, 0x05, 0x79} which gives 52.768712 in JAVA.

I'm trying to do the same thing in C++ using:

unsigned char data[8] = { 0x40,0x4a, 0x62, 0x65, 0x27, 0xa2, 0x05, 0x79};

 double sample;
 double conversion;

 copy(data, data+ sizeof(double), reinterpret_cast<char*>(&sample));
 memcpy(&conversion, data, sizeof(double));

 cout <<fixed << sample << endl;
 cout <<fixed << conversion << endl;

But this outputs

93624845218206212768416858147698568034806357864636725490778543828533000856049725637297881892477906693728822997596617150540736669122257619339758101303551253860040255473731364930515446918886205365571056786975140751691636070476504921373224351498841838203471739594278994672877568.000000

What am I doing wrong here?

Fred Larson · Accepted Answer · 2018-04-12 16:07:59Z

1

Endianness. Java is big-endian, while C++ is platform dependent.

This works for me on an Intel-based Windows PC (little-endian):

#include <iostream>
#include <cstring>
#include <algorithm>

int main()
{
    unsigned char data[8] = { 0x40,0x4a, 0x62, 0x65, 0x27, 0xa2, 0x05, 0x79};

     double sample;
     double conversion;

     std::reverse(std::begin(data), std::end(data)); // Reverse byte order here
     std::copy(data, data+ sizeof(double), reinterpret_cast<char*>(&sample));
     std::memcpy(&conversion, data, sizeof(double));

     std::cout << std::fixed << sample << std::endl;
     std::cout << std::fixed << conversion << std::endl;
}

Output:

52.768712
52.768712

Note that this is inherently non-portable, since endianness, size of double, and floating point representation are not defined by the C++ language standard.

edited Apr 12, 2018 at 16:07

answered Apr 12, 2018 at 16:01

Fred Larson

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