Question

I have a IEEE754 Double precision 64-bit binary string representation of a double number.
example : double value = 0.999;
Its binary representation is “0011111111101111111101111100111011011001000101101000011100101011”

I want to convert this string back to a double number in c++.
I dont want to use any external libraries or .dll’s as my program would operate in any platform.

Answer 1

C string solution:

#include <cstring>   // needed for all three solutions because of memcpy

double bitstring_to_double(const char* p)
{
    unsigned long long x = 0;
    for (; *p; ++p)
    {
        x = (x << 1) + (*p - '0');
    }
    double d;
    memcpy(&d, &x, 8);
    return d;
}

std::string solution:

#include <string>

double bitstring_to_double(const std::string& s)
{
    unsigned long long x = 0;
    for (std::string::const_iterator it = s.begin(); it != s.end(); ++it)
    {
        x = (x << 1) + (*it - '0');
    }
    double d;
    memcpy(&d, &x, 8);
    return d;
}

generic solution:

template<typename InputIterator>
double bitstring_to_double(InputIterator begin, InputIterator end)
{
    unsigned long long x = 0;
    for (; begin != end; ++begin)
    {
        x = (x << 1) + (*begin - '0');
    }
    double d;
    memcpy(&d, &x, 8);
    return d;
}

example calls:

#include <iostream>

int main()
{
    const char * p = "0011111111101111111101111100111011011001000101101000011100101011";
    std::cout << bitstring_to_double(p) << '\n';

    std::string s(p);
    std::cout << bitstring_to_double(s) << '\n';

    std::cout << bitstring_to_double(s.begin(), s.end()) << '\n';
    std::cout << bitstring_to_double(p + 0, p + 64) << '\n';
}

Note: I assume unsigned long long has 64 bits. A cleaner solution would be to include <cstdint> and use uint64_t instead, assuming your compiler is up to date and provides that C++11 header.