Possible regression from gcc-3.0.x to gcc-3.2.x with std::setprecision

[Andrew Pollard <andrewp at andypo dot net>]

Hi All,

 I'm not sure whether this a standard mandated behaviour, but there has
been a change in how std::setprecision works with doubles/floats and
output (bits/locale_facets.tcc:do_put)

Basically, gcc-3.0 limits the range of the setprecision to

    std::numeric_limits<float,double>+3   (9, 18)

and gcc-3.2 and above limit it to

    std::numeric_limits<float,double>+1   (7, 16)

This means that certain numbers cannot be streamed into a stream, and then
streamed back out... getting the exact same number out. I see the reasoning
behing using +1, since the precision of a double is between 15 and 16
decimal digits, thus 16 should 'always' work... but it appears that some
numbers require up to 18 digits.

I used the following program, and tried gcc-3.0.4, gcc-3.2.1 and
gcc-3.3 (20021201)

double.cpp:
=============================================================================
#include <limits>
#include <iostream>
#include <sstream>
#include <iomanip>

int
main()
{
    int    prec = std::numeric_limits<double>::digits10+3;
    double val1 = std::numeric_limits<double>::min();
    double val2 = 0.0;

    std::stringstream ss1;
    ss1 << std::setprecision(prec) << val1 << std::ends;

    std::string st(ss1.str());

    std::stringstream ss2(st);
    ss2 >> val2;

    std::cout << std::setprecision(prec) << val1 << "\n" << val2 << std::endl;
    std::cout << std::boolalpha << (val1 == val2) << std::endl;

    return (val1 == val2);
}
=============================================================================

% g++30 double.cxx ; a.out ; echo $?
2.22507385850720138e-308
2.22507385850720138e-308
true
1

% g++32 double.cxx ; a.out ; echo $?
2.225073858507201e-308
2.225073858507201e-308
false
0

% g++33 double.cxx ; a.out ; echo $?
2.225073858507201e-308
2.225073858507201e-308
false
0

Has anyone got any comments?

Andrew.
--
 Andrew Pollard, Brooks-PRI Automation  | home: andrew@andypo.net
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