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

[Roberto Bagnara]

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

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