How to serialize a GMP rational number?

Question

Is there a way to serialize a GMP rational number in binary form? I only see functions for reading/writing to a FILE, but even there it is a text form. I could convert to numerator/denominator and use the integer functions, but again there only a FILE output is available. I need to be able to get the raw bytes or write to a C++ stream.

Answer 1

Using the mpz_export and mpz_import function (thanks to Marc for pointing that out), I came up with the below code. This is part of a number class that contains a mpz_class value.

This really shows that GMP doesn't have a proper import/export functionality. The below is more of a workaround than use of a feature.

void number::write( std::ostream & out ) const {
    int8_t neg = value.get_num() < 0;
    out.write( (char*)&neg, sizeof(neg) );

    size_t c;
    void * raw = mpz_export( nullptr, &c, 1, 1, 0, 0, value.get_num().get_mpz_t() );
    out.write( (char*)&c, sizeof(c) );
    out.write( (char*)raw, c );
    free(raw);

    raw = mpz_export( nullptr, &c, 1, 1, 0, 0, value.get_den().get_mpz_t() );
    out.write( (char*)&c, sizeof(c) );
    out.write( (char*)raw, c );
    free(raw);
}

void number::read( std::istream & in ) {
    mpz_class num, den;
    size_t s;
    std::vector<uint8_t> v;

    int8_t neg;
    in.read( (char*)&neg, sizeof(neg) );

    in.read( (char*)&s, sizeof(s) );
    v.resize(s);
    in.read( (char*)&v[0], s );
    mpz_import( num.get_mpz_t(), s, 1, 1, 0, 0, &v[0] );

    in.read( (char*)&s, sizeof(s) );
    v.resize(s);
    in.read( (char*)&v[0], s );
    mpz_import( den.get_mpz_t(), s, 1, 1, 0, 0, &v[0] );

    value = mpq_class(num) / mpq_class(den);
    if( neg ) {
        value = -value;
    }
}