atof and stringstream produce different results - c++

Question

I have been looking into a problem whereby I am converting a float to a human readable format, and back. Namely a string. I have ran into issues using stringstream and found that atof produces "better" results.
Notice, I do not print out the data in this case, I used the debugger to retrieve the values:
const char *val = "73.31";
std::stringstream ss;
ss << val << '\0';
float floatVal = 0.0f;
ss >> floatVal; //VALUE IS 73.3100052
floatVal = atof(val); //VALUE IS 73.3099976
There is probably a reasonable explanation to this. If anybody can enlighten me I'd be greatful :).

Answer 1

Answer is based on the assumption that OP uses MSVC
atof is indeed better in reading floating point values than istream.
See this example:
#include <iostream>
#include <sstream>
#include <iomanip>
#include <cstdlib>
int main()
{
const char *val = "73.31";
std::stringstream ss;
ss << val;
float floatVal = 0.0f;
ss >> floatVal;
std::cout << "istream>>(float&) :" << std::setw(18) << std::setprecision(15) << floatVal << std::endl;
double doubleVal = atof(val);
std::cout << "double atof(const char*) :" << std::setw(18) << std::setprecision(15) << doubleVal << std::endl;
floatVal = doubleVal;
std::cout << "(float)double atof(const char*) :" << std::setw(18) << std::setprecision(15) << floatVal << std::endl;
doubleVal = floatVal;
std::cout << "(double)(float)double atof(const char*) :" << std::setw(18) << std::setprecision(15) << floatVal << std::endl;
}
Output:
istream>>(float&) : 73.3100051879883
double atof(const char*) : 73.31
(float)double atof(const char*) : 73.3099975585938
(double)(float)double atof(const char*) : 73.3099975585938
The compiler even warns about the conversion from doubleto float this:
warning C4244: '=': conversion from 'double' to 'float', possible loss of data
I also found this page: Conversions from Floating-Point Types
Update:
The value 73.3099975585938 seems to be the correct float interpretation of the double value 73.31.
Update:
istream>>(double&) works correctly as well:
#include <iostream>
#include <sstream>
#include <iomanip>
#include <cstdlib>
int main()
{
const char *val = "73.31";
std::stringstream ss;
ss << val;
double doubleVal = 0.0f;
ss >> doubleVal;
std::cout << "istream>>(double&) :" << std::setw(18) << std::setprecision(15) << doubleVal << std::endl;
}
Output:
istream>>(double&) : 73.31
For arithmetic types istream::operator>> uses num_get::get.
num_get::get should be using something like scanf("%g") for float source
BUT:
#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <string>
#include <iomanip>
#include <cstdlib>
int main()
{
std::string s = "73.31";
float f = 0.f;
sscanf(s.c_str(), "%g", &f);
std::cout << std::setw(18) << std::setprecision(15) << f << std::endl;
}
Output:
73.3099975585938
For me this looks like there might be a bug in Microsoft num_get