C++ Interface General (GNU MP 6.3.0)
Description: How to install and use the GNU multiple precision arithmetic library, version 6.3.0.

### 12.1 C++ Interface General ¶

All the C++ classes and functions are available with

    #include <gmpxx.h>

Programs should be linked with the `libgmpxx` and `libgmp` libraries. For example,

    g++ mycxxprog.cc -lgmpxx -lgmp

The classes defined are

Class: mpz_class ¶
Class: mpq_class ¶
Class: mpf_class ¶

The standard operators and various standard functions are overloaded to allow arithmetic with these classes. For example,

    int
    main (void)
    {
      mpz_class a, b, c;

      a = 1234;
      b = "-5678";
      c = a+b;
      cout << "sum is " << c << "\n";
      cout << "absolute value is " << abs(c) << "\n";

      return 0;
    }

An important feature of the implementation is that an expression like `a=b+c` results in a single call to the corresponding `mpz_add`, without using a temporary for the `b+c` part. Expressions which by their nature imply intermediate values, like `a=b*c+d*e`, still use temporaries though.

The classes can be freely intermixed in expressions, as can the classes and the standard types `long`, `unsigned long` and `double`. Smaller types like `int` or `float` can also be intermixed, since C++ will promote them.

Note that `bool` is not accepted directly, but must be explicitly cast to an `int` first. This is because C++ will automatically convert any pointer to a `bool`, so if GMP accepted `bool` it would make all sorts of invalid class and pointer combinations compile but almost certainly not do anything sensible.

Conversions back from the classes to standard C++ types aren’t done automatically, instead member functions like `get_si` are provided (see the following sections for details).

Also there are no automatic conversions from the classes to the corresponding GMP C types, instead a reference to the underlying C object can be obtained with the following functions,

Function: `mpz_t` mpz_class::get_mpz_t `()` ¶
Function: `mpq_t` mpq_class::get_mpq_t `()` ¶
Function: `mpf_t` mpf_class::get_mpf_t `()` ¶

These can be used to call a C function which doesn’t have a C++ class interface. For example to set `a` to the GCD of `b` and `c`,

    mpz_class a, b, c;
    ...
    mpz_gcd (a.get_mpz_t(), b.get_mpz_t(), c.get_mpz_t());

In the other direction, a class can be initialized from the corresponding GMP C type, or assigned to if an explicit constructor is used. In both cases this makes a copy of the value, it doesn’t create any sort of association. For example,

    mpz_t z;
    // ... init and calculate z ...
    mpz_class x(z);
    mpz_class y;
    y = mpz_class (z);

There are no namespace setups in `gmpxx.h`, all types and functions are simply put into the global namespace. This is what `gmp.h` has done in the past, and continues to do for compatibility. The extras provided by `gmpxx.h` follow GMP naming conventions and are unlikely to clash with anything.