"sign=space & pad_0s_until_width & alignment=right/internal/center" do not work together ("sign=space" is ignored)

Start by building and running the supplied C++ reproducer for boost::format, comparing its output with std::printf for space signs, zero padding, and each alignment. Trace the formatting path used by boost::format and its handling of sign and padding flags. Done means sign=space is preserved with zero padding for right, internal, and center alignment, including the Rational example.

#include<string>
#include<iostream>
#include<iomanip>
#include<boost/format.hpp>

class Rational {
public:
    Rational(int n, unsigned int d) : n_(n), d_(d) {}
    Rational(int n, int d);    // convert denominator to unsigned
    friend std::ostream& operator<<(std::ostream&, const Rational&);
private:
    int n_;               // numerator
    unsigned int d_;      // denominator
};

Rational::Rational(int n, int d) : n_(n)
{
    if (d < 0) { n_ = -n_; d = -d; } // make the denominator always non-negative.
    d_ = static_cast<unsigned int>(d);
}

std::ostream& operator<<(std::ostream& os, const Rational& r) {
    using namespace std;
    streamsize w = os.width(0); // width has to be zeroed before saving state.

    boost::io::basic_oaltstringstream<char> oss;
    oss.copyfmt(os);
    oss << r.n_ << "/" << noshowpos << r.d_;

    os.write(oss.begin(), oss.size());
    return os;
}

int main(int argc, char* argv[])
{
    using namespace std;
    using boost::format;
    using boost::io::group;
    using boost::io::str;

    std::printf("[%8s][%8s]\n", "format", "printf");

    std::printf("\nright:\n");//'+'/' ', right
    std::cerr << boost::format("[%+8d]") % 12; std::printf("[%+8d]\n", 12);
    std::cerr << boost::format("[% 8d]") % 12; std::printf("[% 8d]\n", 12);
    std::cerr << boost::format("[%+08d]") % 12; std::printf("[%+08d]\n", 12);
    std::cerr << boost::format("[% 08d]") % 12; std::printf("[% 08d] !!!\n", 12);

    std::printf("\ninternal:\n");//'+'/' ', internal('_')
    std::cerr << boost::format("[%_+8d]\n") % 12;
    std::cerr << boost::format("[%_ 8d]\n") % 12;
    std::cerr << boost::format("[%_+08d]\n") % 12;
    std::cerr << boost::format("[%_ 08d] !!!\n") % 12;

    std::printf("\ncenter:\n");//'+'/' ', center('=')
    std::cerr << boost::format("[%=+8d]\n") % 12;
    std::cerr << boost::format("[%= 8d]\n") % 12;
    std::cerr << boost::format("[%=+08d]\n") % 12;
    std::cerr << boost::format("[%= 08d] !!!\n") % 12;

    std::printf("\nleft:\n");//'+'/' ', left('-')
    std::cerr << boost::format("[%-+8d]") % 12; std::printf("[%-+8d]\n", 12);
    std::cerr << boost::format("[%- 8d]") % 12; std::printf("[%- 8d]\n", 12);
    std::cerr << boost::format("[%-+08d]") % 12; std::printf("[%-+08d]\n", 12);
    std::cerr << boost::format("[%- 08d]") % 12; std::printf("[%- 08d]\n", 12);

    Rational  r(16, 9);
    std::printf("\nright:\n");
    cerr << format("[%+8d]\n") % r;
    cerr << format("[% 8d]\n") % r;
    cerr << format("[%+08d]\n") % r;
    cerr << format("[% 08d] !!!\n") % r;

    std::printf("\ninternal:\n");
    cerr << format("[%_+8d]\n") % r;
    cerr << format("[%_ 8d]\n") % r;
    cerr << format("[%_+08d]\n") % r;
    cerr << format("[%_ 08d] !!!\n") % r;

    std::printf("\ncenter:\n");
    cerr << format("[%=+8d]\n") % r;
    cerr << format("[%= 8d]\n") % r;
    cerr << format("[%=+08d]\n") % r;
    cerr << format("[%= 08d] !!!\n") % r;

    std::printf("\nleft:\n");
    cerr << format("[%-+8d]\n") % r;
    cerr << format("[%- 8d]\n") % r;
    cerr << format("[%-+08d]\n") % r;
    cerr << format("[%- 08d]\n") % r;
}

output (boost.format on the left, std::printf on the right, the differences is marked with !!!):

[  format][  printf]

right:
[     +12][     +12]
[      12][      12]
[+0000012][+0000012]
[00000012][ 0000012] !!!

internal:
[+     12]
[      12]
[+0000012]
[00000012] !!!

center:
[   +12  ]
[    12  ]
[+0000012]
[00000012] !!!

left:
[+12     ][+12     ]
[ 12     ][ 12     ]
[+12     ][+12     ]
[ 12     ][ 12     ]

right:
[   +16/9]
[    16/9]
[000+16/9]
[000016/9] !!!

internal:
[   +16/9]
[    16/9]
[000+16/9]
[000016/9] !!!

center:
[  +16/9 ]
[   16/9 ]
[000+16/9]
[000016/9] !!!

left:
[+16/9   ]
[ 16/9   ]
[+16/9   ]
[ 16/9   ]