### 12.4 C++ Interface Floats

When an expression requires the use of temporary intermediate `mpf_class` values, like `f=g*h+x*y`, those temporaries will have the same precision as the destination `f`. Explicit constructors can be used if this doesn’t suit.

Function: mpf_class::mpf_class (type op)
Function: mpf_class::mpf_class (type op, mp_bitcnt_t prec)

  Construct an `mpf_class`. Any standard C++ type can be used, except `long long` and `long double`, and any of the GMP C++ classes can be used.

  If prec is given, the initial precision is that value, in bits. If prec is not given, then the initial precision is determined by the type of op given. An `mpz_class`, `mpq_class`, or C++ builtin type will give the default `mpf` precision (see Initializing Floats). An `mpf_class` or expression will give the precision of that value. The precision of a binary expression is the higher of the two operands.

      mpf_class f(1.5);        // default precision
      mpf_class f(1.5, 500);   // 500 bits (at least)
      mpf_class f(x);          // precision of x
      mpf_class f(abs(x));     // precision of x
      mpf_class f(-g, 1000);   // 1000 bits (at least)
      mpf_class f(x+y);        // greater of precisions of x and y

Function: explicit mpf_class::mpf_class (const mpf_t f)
Function: mpf_class::mpf_class (const mpf_t f, mp_bitcnt_t prec)

  Construct an `mpf_class` from an `mpf_t`. The value in f is copied into the new `mpf_class`, there won’t be any permanent association between it and f.

  If prec is given, the initial precision is that value, in bits. If prec is not given, then the initial precision is that of f.

Function: explicit mpf_class::mpf_class (const char *s)
Function: mpf_class::mpf_class (const char *s, mp_bitcnt_t prec, int base = 0)
Function: explicit mpf_class::mpf_class (const string& s)
Function: mpf_class::mpf_class (const string& s, mp_bitcnt_t prec, int base = 0)

  Construct an `mpf_class` converted from a string using `mpf_set_str` (see Assigning Floats). If prec is given, the initial precision is that value, in bits. If not, the default `mpf` precision (see Initializing Floats) is used.

  If the string is not a valid float, an `std::invalid_argument` exception is thrown. The same applies to `operator=`.

Function: mpf_class operator"" _mpf (const char *str)

  With C++11 compilers, floats can be constructed with the syntax `1.23e-1_mpf` which is equivalent to `mpf_class("1.23e-1")`.

Function: mpf_class& mpf_class::operator= (type op)

  Convert and store the given op value to an `mpf_class` object. The same types are accepted as for the constructors above.

  Note that `operator=` only stores a new value, it doesn’t copy or change the precision of the destination, instead the value is truncated if necessary. This is the same as `mpf_set` etc. Note in particular this means for `mpf_class` a copy constructor is not the same as a default constructor plus assignment.

      mpf_class x (y);   // x created with precision of y

      mpf_class x;       // x created with default precision
      x = y;             // value truncated to that precision

  Applications using templated code may need to be careful about the assumptions the code makes in this area, when working with `mpf_class` values of various different or non-default precisions. For instance implementations of the standard `complex` template have been seen in both styles above, though of course `complex` is normally only actually specified for use with the builtin float types.

Function: mpf_class abs (mpf_class op)
Function: mpf_class ceil (mpf_class op)
Function: int cmp (mpf_class op1, type op2)
Function: int cmp (type op1, mpf_class op2)
Function: bool mpf_class::fits_sint_p (void)
Function: bool mpf_class::fits_slong_p (void)
Function: bool mpf_class::fits_sshort_p (void)
Function: bool mpf_class::fits_uint_p (void)
Function: bool mpf_class::fits_ulong_p (void)
Function: bool mpf_class::fits_ushort_p (void)
Function: mpf_class floor (mpf_class op)
Function: mpf_class hypot (mpf_class op1, mpf_class op2)
Function: double mpf_class::get_d (void)
Function: long mpf_class::get_si (void)
Function: string mpf_class::get_str (mp_exp_t& exp, int base = 10, size_t digits = 0)
Function: unsigned long mpf_class::get_ui (void)
Function: int mpf_class::set_str (const char *str, int base)
Function: int mpf_class::set_str (const string& str, int base)
Function: int sgn (mpf_class op)
Function: mpf_class sqrt (mpf_class op)
Function: void mpf_class::swap (mpf_class& op)
Function: void swap (mpf_class& op1, mpf_class& op2)
Function: mpf_class trunc (mpf_class op)

  These functions provide a C++ class interface to the corresponding GMP C routines.

  `cmp` can be used with any of the classes or the standard C++ types, except `long long` and `long double`.

  The accuracy provided by `hypot` is not currently guaranteed.

Function: mp_bitcnt_t mpf_class::get_prec ()
Function: void mpf_class::set_prec (mp_bitcnt_t prec)
Function: void mpf_class::set_prec_raw (mp_bitcnt_t prec)

  Get or set the current precision of an `mpf_class`.

  The restrictions described for `mpf_set_prec_raw` (see Initializing Floats) apply to `mpf_class::set_prec_raw`. Note in particular that the `mpf_class` must be restored to it’s allocated precision before being destroyed. This must be done by application code, there’s no automatic mechanism for it.