Roberto Bagnara bagnara@cs.unipr.it writes:

| Hi there. | | The program | | #include <gmpxx.h> | #include <iostream> | | using namespace std; | | void foo(const mpz_class& i) { | cout << "foo::i = " << i << endl; | } | | int main() { | mpq_class r(3, 4); | cout << "main::r = " << r << endl; | foo(r); | return 0; | } | | prints | | main::r = 3/4 | foo::i = 0 | | This shows that the C++ interface of GMP defines an implicit constructor | for mpz_class that takes an mpq_class object and (silently) truncates it. | It is very bad practice for such a constructor to be implicit as this can | result in bugs that are very difficult to diagnose. Of course I am not | questioning the need for such a conversion: I simply think that truncation | should only be performed when explicitly requested. | All the best,

In the version of GMP coming with the GNU/Linux distribution I'm using, there are:

typedef __gmp_expr<__gmpz_value, __gmpz_value> mpz_class; // ... typedef __gmp_expr<__gmpq_value, __gmpq_value> mpq_class;

and further (in the definition of the explicit specialization for mpz_class)

template <class T, class U> __gmp_expr(const __gmp_expr<T, U> &expr) { mpz_init(mp); __gmp_set_expr(mp, expr); }

I believe that is the constructor at fault -- it should be explicit.

I reckon the C++ interface to GMP is highly influenced by my implementation of valarray for GNU libstdc++, but I'm not sure I have such cases for the valarray implementation.

-- Gaby