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(gmp.info)C++ Interface Limitations

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C++ Interface Limitations
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`mpq_class' and Templated Reading
     A generic piece of template code probably won't know that
     `mpq_class' requires a `canonicalize' call if inputs read with
     `operator>>' might be non-canonical.  This can lead to incorrect
     results.

     `operator>>' behaves as it does for reasons of efficiency.  A
     canonicalize can be quite time consuming on large operands, and is
     best avoided if it's not necessary.

     But this potential difficulty reduces the usefulness of
     `mpq_class'.  Perhaps a mechanism to tell `operator>>' what to do
     will be adopted in the future, maybe a preprocessor define, a
     global flag, or an `ios' flag pressed into service.  Or maybe, at
     the risk of inconsistency, the `mpq_class' `operator>>' could
     canonicalize and leave `mpq_t' `operator>>' not doing so, for use
     on those occasions when that's acceptable.  Send feedback or
     alternate ideas to <bug-gmp@gnu.org>.

Subclassing
     Subclassing the GMP C++ classes works, but is not currently
     recommended.

     Expressions involving subclasses resolve correctly (or seem to),
     but in normal C++ fashion the subclass doesn't inherit
     constructors and assignments.  There's many of those in the GMP
     classes, and a good way to reestablish them in a subclass is not
     yet provided.

Templated Expressions
     A subtle difficulty exists when using expressions together with
     application-defined template functions.  Consider the following,
     with `T' intended to be some numeric type,

          template <class T>
          T fun (const T &, const T &);

     When used with, say, plain `mpz_class' variables, it works fine:
     `T' is resolved as `mpz_class'.

          mpz_class f(1), g(2);
          fun (f, g);    // Good

     But when one of the arguments is an expression, it doesn't work.

          mpz_class f(1), g(2), h(3);
          fun (f, g+h);  // Bad

     This is because `g+h' ends up being a certain expression template
     type internal to `gmpxx.h', which the C++ template resolution
     rules are unable to automatically convert to `mpz_class'.  The
     workaround is simply to add an explicit cast.

          mpz_class f(1), g(2), h(3);
          fun (f, mpz_class(g+h));  // Good

     Similarly, within `fun' it may be necessary to cast an expression
     to type `T' when calling a templated `fun2'.

          template <class T>
          void fun (T f, T g)
          {
            fun2 (f, f+g);     // Bad
          }
          
          template <class T>
          void fun (T f, T g)
          {
            fun2 (f, T(f+g));  // Good
          }