ios_base::fixed & truncating trailing zeroes - c++

Question

When printing certain decimal numbers, rounding may occur (my present platform is the IBM iSeries). If I set ios_base::fixed, it does better but prints annoying trailing (and insignificant) zeroes. Anyone have a better way? For the platform I'm using, the output is this:
myval=100000
myval=99999.990000
myval as string =99999.990000
#include <iostream>
#include <string>
#include <sstream>
#include <iomanip>
#include <unistd.h>
#include <algorithm>
using namespace std;
// returns t
template<typename T> string toString(const T& val)
{
stringstream os;
os.setf(std::ios_base::fixed);
os << val;
return os.str();
}
class MyApp {
private :
public :
MyApp();
virtual ~MyApp();
void run();
operator bool() const { return true; }
};
MyApp::MyApp()
{
}
MyApp::~MyApp()
{
}
void MyApp::run()
{
double myval = 99999.99;
string myval_as_string = toString<double>(myval);
cout << "myval=" << myval << endl;
cout.setf(std::ios_base::fixed);
cout << "myval=" << myval << endl;
cout << "myval as string =" << myval_as_string << endl;
}
int main(int argc, char* argv[])
{
MyApp myApp;
myApp.run();
}

Answer 1

You're looking for precision and not fixed. precision will specify the maximum precision used but it will not automatically print more zeros so increase it as much as you need. fixed will force it out to as many zeros as precision is set to. If you have set fixed previously, you can reset it with ios_base::defaultfloat.
template<typename T> string toString(const T& val)
{
stringstream os;
os.precision(15);
os << val;
return os.str();
}
....
void MyApp::run()
{
double myval = 99999.99;
string myval_as_string = toString<double>(myval);
cout << "myval=" << myval << endl;
cout.precision(15);
cout << "myval=" << myval << endl;
cout << "myval as string =" << myval_as_string << endl;
}
For more info, you can check out the standard here: http://www.cplusplus.com/reference/ios/ios_base/precision/. Note the following line:
Using the default floating-point notation, the precision field
specifies the maximum number of meaningful digits to display in total
counting both those before and those after the decimal point. Notice
that it is not a minimum, and therefore it does not pad the displayed
number with trailing zeros if the number can be displayed with less
digits than the precision.