scripts/wrap/ docs/: added operator bool() to non-pod wrapper classes.

Trace diagnostics for operator bool() are generated in debug builds if
MUPDF_trace=1.

docs/src/language-bindings.rst

@@ -121,6 +121,22 @@ instead of MuPDF C structs:
 
 * Class-aware functions are declared in `mupdf/platform/c++/include/mupdf/classes2.h`.
 
+*
+  Wrapper classes for reference-counted MuPDF structs:
+
+  *
+    The C++ wrapper classes will have a public `m_internal` member that is a
+    pointer to the MuPDF struct.
+
+  *
+    If a MuPDF functions returns a null pointer to a MuPDF struct, the
+    class-aware C++ wrapper will return an instance of the wrapper class with a
+    null `m_internal` member.
+
+  *
+    The C++ wrapper class wil have an `operator bool()` that returns true if
+    the `m_internal` member is non-null.
+
 Usually it is more convenient to use the class-aware C++ API rather than the
 low-level C++ API.
 

scripts/wrap/cpp.py

@@ -3306,9 +3306,13 @@ def class_custom_method(
         return_space = ''
         comment = 'Custom destructor.'
         is_destructor = True
+    elif extramethod.name_args.startswith('operator '):
+        name_args = extramethod.name_args
+        comment = 'Custom operator.'
+        return_space = ''
     else:
         # Constructor.
-        assert extramethod.name_args.startswith( '('), f'bad constructor/destructor in classname={classname}'
+        assert extramethod.name_args.startswith( '('), f'bad constructor/destructor in {classname=}: {extramethod.name_args=}'
         name_args = f'{classname}{extramethod.name_args}'
         return_space = ''
         comment = 'Custom constructor.'
@@ -4509,7 +4513,7 @@ class has custom begin() and end() methods; <has_to_string> is true if we
                 )
 
     # If class has '{structname}* m_internal;', provide access to m_iternal as
-    # an integer, for use by python etc.
+    # an integer, for use by python etc, and provide `operator bool()`.
     if not extras.pod:
         class_custom_method(
                 tu,
@@ -4530,6 +4534,35 @@ class has custom begin() and end() methods; <has_to_string> is true if we
                 out_cpp,
                 refcheck_if,
                 )
+        class_custom_method(
+                tu,
+                register_fn_use,
+                struct_cursor,
+                classname,
+                classes.ExtraMethod(
+                    '',
+                    'operator bool()',
+                    f'''
+                    {{
+                        {refcheck_if}
+                        if (s_trace)
+                        {{
+                            std::cerr << __FILE__ << ":" << __LINE__ << ":"
+                                    << " {classname}::operator bool() called,"
+                                    << " m_internal=" << m_internal << "."
+                                    << "\\n";
+                        }}
+                        #endif
+                        return m_internal ? true : false;
+                    }}
+                    ''',
+                    '/** Return true iff `m_internal` is not null. */',
+                    ),
+                out_h,
+                out_cpp,
+                refcheck_if,
+                )
+
     # Class members.
     #
     out_h.write( '\n')

scripts/wrap/swig.py

@@ -1579,7 +1579,7 @@ def FzDocumentWriter__init__1(self, *args):
                         if isinstance( arg, FzOutput2):
                             assert not out, "More than one FzOutput2 passed to FzDocumentWriter.__init__()"
                             out = arg
-                    if out:
+                    if out is not None:
                         self._out = out
                     return FzDocumentWriter__init__0(self, *args)
                 FzDocumentWriter.__init__ = FzDocumentWriter__init__1