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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.

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