nullptr cleanup for -Wzero-as-null-pointer-constant

Fixes #106.

* m4/gccwarn.m4: Use -Wzero-as-null-pointer-constant if supported.
* src/bin/autfilt.cc, src/bin/common_aoutput.cc,
src/bin/common_finput.cc, src/bin/common_finput.hh,
src/bin/common_output.cc, src/bin/common_output.hh,
src/bin/common_post.cc, src/bin/common_r.hh, src/bin/common_range.hh,
src/bin/common_setup.cc, src/bin/common_trans.cc, src/bin/dstar2tgba.cc,
src/bin/genltl.cc, src/bin/ltl2tgba.cc, src/bin/ltl2tgta.cc,
src/bin/ltlcross.cc, src/bin/ltldo.cc, src/bin/ltlfilt.cc,
src/bin/ltlgrind.cc, src/bin/randaut.cc, src/bin/randltl.cc,
src/bin/spot-x.cc, src/kripke/kripkeexplicit.cc, src/ltlast/formula.cc,
src/ltlvisit/randomltl.cc, src/ltlvisit/randomltl.hh,
src/ltlvisit/relabel.cc, src/ltlvisit/relabel.hh,
src/ltlvisit/simplify.cc, src/ltlvisit/snf.hh, src/misc/fixpool.hh,
src/misc/mspool.hh, src/misc/hash.hh, src/misc/optionmap.cc,
src/misc/satsolver.cc, src/misc/tmpfile.cc, src/misc/tmpfile.hh,
src/priv/bddalloc.cc, src/ta/ta.hh, src/ta/taexplicit.cc,
src/ta/taexplicit.hh, src/ta/taproduct.cc, src/ta/tgtaexplicit.hh,
src/ta/tgtaproduct.cc, src/taalgos/minimize.cc,
src/taalgos/reachiter.cc, src/taalgos/tgba2ta.cc,
src/tests/complementation.cc, src/tests/ikwiad.cc,
src/tests/randtgba.cc, src/tests/reduc.cc, src/twa/bdddict.cc,
src/twa/bddprint.cc, src/twa/taatgba.cc, src/twa/twa.cc,
src/twa/twagraph.hh, src/twa/twaproduct.cc,
src/twa/twasafracomplement.cc, src/twaalgos/bfssteps.cc,
src/twaalgos/compsusp.cc, src/twaalgos/dtgbasat.cc,
src/twaalgos/emptiness.cc, src/twaalgos/gtec/ce.cc,
src/twaalgos/gv04.cc, src/twaalgos/ltl2taa.cc,
src/twaalgos/ltl2tgba_fm.cc, src/twaalgos/ltl2tgba_fm.hh,
src/twaalgos/magic.cc, src/twaalgos/minimize.cc,
src/twaalgos/ndfs_result.hxx, src/twaalgos/postproc.cc,
src/twaalgos/postproc.hh, src/twaalgos/reachiter.cc,
src/twaalgos/reducerun.cc, src/twaalgos/safety.hh,
src/twaalgos/sccfilter.hh, src/twaalgos/se05.cc,
src/twaalgos/simulation.cc, src/twaalgos/tau03.cc,
src/twaalgos/tau03opt.cc, src/twaalgos/translate.cc,
src/twaalgos/translate.hh, utf8/utf8/core.h,
bench/stutter/stutter_invariance_formulas.cc, iface/ltsmin/ltsmin.cc,
iface/ltsmin/modelcheck.cc: Use nullptr to fix the warning.

iface/ltsmin/ltsmin.cc

@@ -376,7 +376,7 @@ namespace spot
 
 	  char* name = (char*) malloc(str.size() + 1);
 	  char* name_p = name;
-	  char* lastdot = 0;
+	  char* lastdot = nullptr;
 	  while (*s && (*s != '=') && *s != '<' && *s != '!'  && *s != '>')
 	    {
 
@@ -608,17 +608,18 @@ namespace spot
 	  d_(d),
 	  state_size_(d_->get_state_size()),
 	  dict_(dict), ps_(ps),
-	  compress_(compress == 0 ? 0
+	  compress_(compress == 0 ? nullptr
 		    : compress == 1 ? int_array_array_compress
 		    : int_array_array_compress2),
-	  decompress_(compress == 0 ? 0
+	  decompress_(compress == 0 ? nullptr
 		      : compress == 1 ? int_array_array_decompress
 		      : int_array_array_decompress2),
-	  uncompressed_(compress ? new int[state_size_ + 30] : 0),
-	  compressed_(compress ? new int[state_size_ * 2] : 0),
+	  uncompressed_(compress ? new int[state_size_ + 30] : nullptr),
+	  compressed_(compress ? new int[state_size_ * 2] : nullptr),
 	  statepool_(compress ? sizeof(spins_compressed_state) :
 		     (sizeof(spins_state) + state_size_ * sizeof(int))),
-	  state_condition_last_state_(0), state_condition_last_cc_(0)
+	  state_condition_last_state_(nullptr),
+	  state_condition_last_cc_(nullptr)
       {
 	vname_ = new const char*[state_size_];
 	format_filter_ = new bool[state_size_];
@@ -770,7 +771,7 @@ namespace spot
 			    : static_cast<const void*>(&statepool_));
 	cc->compress = compress_;
 	cc->compressed = compressed_;
-	t = d_->get_successors(0, const_cast<int*>(vars),
+	t = d_->get_successors(nullptr, const_cast<int*>(vars),
 			       compress_
 			       ? transition_callback_compress
 			       : transition_callback,
@@ -790,7 +791,7 @@ namespace spot
 	  {
 	    state_condition_last_state_->destroy();
 	    delete state_condition_last_cc_; // Might be 0 already.
-	    state_condition_last_cc_ = 0;
+	    state_condition_last_cc_ = nullptr;
 	  }
 
 	const int* vars = get_vars(st);
@@ -853,7 +854,7 @@ namespace spot
 	if (state_condition_last_cc_)
 	  {
 	    cc = state_condition_last_cc_;
-	    state_condition_last_cc_ = 0; // Now owned by the iterator.
+	    state_condition_last_cc_ = nullptr; // Now owned by the iterator.
 	  }
 	else
 	  {
@@ -1032,7 +1033,7 @@ namespace spot
             if (verbose)
               std::cerr << "Failed to compile `" << file_arg
               << "'." << std::endl;
-            return 0;
+            return nullptr;
           }
       }
 
@@ -1040,7 +1041,7 @@ namespace spot
       {
 	if (verbose)
 	  std::cerr << "Failed to initialize libltdl." << std::endl;
-	return 0;
+	return nullptr;
       }
 
     lt_dlhandle h = lt_dlopen(file.c_str());
@@ -1049,7 +1050,7 @@ namespace spot
 	if (verbose)
 	  std::cerr << "Failed to load `" << file << "'." << std::endl;
 	lt_dlexit();
-	return 0;
+	return nullptr;
       }
 
     spins_interface* d = new spins_interface;
@@ -1117,7 +1118,7 @@ namespace spot
 		    << file << "'\n";
 	delete d;
 	lt_dlexit();
-	return 0;
+	return nullptr;
       }
 
     if (d->have_property && d->have_property())
@@ -1127,7 +1128,7 @@ namespace spot
         	    << std::endl;
         delete d;
         lt_dlexit();
-        return 0;
+        return nullptr;
       }
 
     spot::prop_set* ps = new spot::prop_set;
@@ -1138,7 +1139,7 @@ namespace spot
 	dict->unregister_all_my_variables(d);
 	delete d;
 	lt_dlexit();
-	return 0;
+	return nullptr;
       }
 
     return std::make_shared<spins_kripke>(d, dict, ps, dead, compress);

iface/ltsmin/modelcheck.cc

@@ -72,7 +72,7 @@ checked_main(int argc, char **argv)
   bool accepting_run = false;
   bool expect_counter_example = false;
   bool deterministic = false;
-  char *dead = 0;
+  char *dead = nullptr;
   int compress_states = 0;
 
   const char* echeck_algo = "Cou99";