[Boost-users] assigning a "float128" variable with a regular "double" variable

[Jacques Mequin via Boost-users]

I am a fresh new user   ( I need to extend some EIGEN library feature by using 
your  float128 )

How to assign a "float128" variable with a regular "double" variable ?

With my source code in attachement

      double       double_pi =  3.141592653589793     ;
      float128  float128_pi =  3.141592653589793Q  ;

      produces the output that looks fine to me

      double_pi =
      3.141592653589793e+00

      float128_pi =
      3.141592653589793000000000000000000e+00

but  thestatement    float128_pi = double_pi ;    produces

      float128_pi =
      3.141592653589793115997963468544185e+00

How to get  3.141592653589793000000000000000000e+00 also from an assigment  ???

What am I doing wrong  ?

Thanks for any help

Regards,
    Jacques


//
//   g++ -o jm.cpp -I/JM/SOFT/boost_1_77_0  jm.cpp -lquadmath
//

#include <boost/multiprecision/float128.hpp>

#include <iostream>

using boost::multiprecision::float128 ;

int main( )
{
    std::cout.setf( std::ios_base::scientific ) ;
    std::cout.setf( std::ios_base::showpoint  ) ;  // include any trailing zeros

    double      double_pi =  3.141592653589793  ;

    float128  float128_pi =  3.141592653589793Q ;

    std::cout << "  double_pi = " << std::endl << std::setprecision(   
std::numeric_limits<double>::digits10 ) <<    double_pi  << std::endl << 
std::endl ;
    std::cout << "float128_pi = " << std::endl << std::setprecision( 
std::numeric_limits<float128>::digits10 ) <<  float128_pi  << std::endl << 
std::endl ;

    float128_pi = double_pi ;

    std::cout << "float128_pi = " << std::endl << std::setprecision( 
std::numeric_limits<float128>::digits10 ) <<  float128_pi  << std::endl << 
std::endl ;

    std::cout << "sizeof   double_pi = " << sizeof(   double_pi ) << std::endl  
            ;
    std::cout << "sizeof float128_pi = " << sizeof( float128_pi ) << std::endl 
<< std::endl ;
}

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