C++ Save/Load double (to/from files) - c++

Question

[Rewritten for clarity.]
I need to write and read doubles to and from files, in a format that will always have the same number of characters. The format doesn't need to be human-readable: it just needs to be quick to load (with as little dynamic memory and conversion stuff as possible, file space is important but doesn't matter quite as much).
Is there a standard (or at least safe and reliable) way to get the components of a double so that I can store the signicicand sign and mantissa sign as a '1' or '0' and the significand and mantissa separately in a hex format with a constant length?
Essentially, how can I grab the specific bit/number components from a double? Is it even possible to do this on separate systems (assuming the same OS family such as Windows) or is the standard for the components of doubles not enforced per OS?
I am using MinGW and of course compiling for Windows. I'd like to use the C Standard Library where possible, not the C++ Standard Library. Also, I'd like to avoid other libraries (like Boost) but if there are specific Windows functions then those would help a lot.

Answer 1

The most direct way of doing so would be to open your fstream in binary mode, and then use the write() and read() methods of fstream to read your double to/from the stream:
#include <fstream>
#include <iostream>
int main( int argc, char** argv ) {
std::fstream fp( "foo", std::fstream::in |
std::fstream::out |
std::fstream::trunc |
std::fstream::binary );
double d1, d2;
d1 = 3.14;
fp.write( (char*)&d1, sizeof( d1 ) );
fp.seekg( 0, std::fstream::beg );
fp.read( (char*)&d2, sizeof( d2 ) );
std::cout << "d1 = " << d1 << " d2 = " << d2 << std::endl;
}

Answer 2

Probably you want somthing like this:
#include <iostream>
#include <sstream>
#include <iomanip>
using namespace std;
template <typename T>
string convertToHex(const T &x)
{
char *xc = (char *)&x;
ostringstream s;
for (char *c = xc; c < xc + sizeof(x); ++c)
s << hex << setw(2) << setfill('0') << static_cast<int>(*c) << " ";
return s.str();
}
template <typename T>
void convertFromHex(string s, T &x)
{
char *xc = (char *)&x;
istringstream is(s);
for (char *c = xc; c < xc + sizeof(x); ++c)
{
int tmp;
is >> hex >> tmp;
*c = tmp;
}
}
int main()
{
double a = 10;
string as = convertToHex(a);
cout << "a: " << as << endl;
double b;
convertFromHex(as, b);
cout << "b: " << b << endl;
}
Output:
a: 00 00 00 00 00 00 24 40
b: 10

Answer 3

Here is very very simple example with boost::serializer (http://www.boost.org/doc/libs/1_54_0/libs/serialization/doc/index.html). I am using boost::archive::text_iarchive and boost::archive::text_oarchive, but you can switch it to boost::archive::binary_iarchive and boost::archive::binary_oarchive. Should work.
#include <boost/archive/text_iarchive.hpp>
#include <boost/archive/text_oarchive.hpp>
#include <sstream>
#define _USE_MATH_DEFINES
#include <cmath>
using namespace std;
int main()
{
double a = M_PI;
string text;
{
ostringstream textStream;
boost::archive::text_oarchive oa(textStream);
oa << a;
text = textStream.str();
}
cout << "a: " << text << endl;
double b;
{
istringstream textStream(text);
boost::archive::text_iarchive ia(textStream);
ia >> b;
}
cout << "b: " << b << endl;
}
Output:
a: 22 serialization::archive 9 3.1415926535897931
b: 3.14159