[PPL-devel] Re: Even more questionable feature of the C++ interface (GMP 4.0.1)

[Kevin Ryde]

I'm not sure why there's a constructor but not an assignment for an
mpz_t.  There's probably a good reason.  Gerardo?

But yes, in any case we should drop the bool forms to avoid such
subtle problems.


Roberto Bagnara <bagnara at cs.unipr.it> writes:
>
> I did it again and this time is even worse.  Without looking at the documentation
> I thought there should be an assignment operator for assigning an mpz_t to
> a class_mpz.  So I did it, the compiler did not complain even with
> -Wall -W and then strange behavior started.  The guilty is now the
> following line in <gmpxx.h>
>
>      __gmp_expr & operator=(bool b) { mpz_set_ui(mp, b); return *this; }
>
> Strictly speaking, I am not reporting anything new with respect to my previous
> message.  What makes things worse is that I now realize that it is much
> easier to be bitten by this problem, since, IMHO, the user is somewhat
> legitimate to assume that mpz_t objects can be used to initialize/assign to
> mpz_class objects.
>
> My revised proposal is that these two lines in <gmpxx.hh>
>
>      __gmp_expr(bool b) { mpz_init_set_ui(mp, b); }
>      __gmp_expr & operator=(bool b) { mpz_set_ui(mp, b); return *this; }
>
> are _replaced_ by their analagous taking an mpz_t argument.
>
> Here is a small test program that exemplifies the problem:
>
> ///////////////////////////////////////////////////////////////////////////
> #include <gmp.h>
> #include <gmpxx.h>
> #include <iostream>
>
> using namespace std;
>
> int main() {
>    mpz_class n;
>    mpz_t m;
>    mpz_init_set_si(m, 123);
>
>    // I thought I was assigning 123 to n...
>    n = m;
>    // ... but I was assigning 1!
>    cout << n << endl;
> }
> ///////////////////////////////////////////////////////////////////////////
>
> All the best
>
>        Roberto
>
> -- 
> Prof. Roberto Bagnara
> Computer Science Group
> Department of Mathematics, University of Parma, Italy
> http://www.cs.unipr.it/~bagnara/
> mailto:bagnara at cs.unipr.it