ParseResult: improve bool conversion and add operator!=

 * Use safe-bool idiom for boolean conversion to avoid accidental
   misuse of ParseResult values (closes #989)
 * Add operator!= to support more comparison expressions
   (previously silently/erroneously used operator bool)

include/rapidjson/error/error.h

@@ -104,6 +104,8 @@ enum ParseErrorCode {
     \see GenericReader::Parse, GenericDocument::Parse
 */
 struct ParseResult {
+    //!! Unspecified boolean type
+    typedef bool (ParseResult::*BooleanType)() const;
 public:
     //! Default constructor, no error.
     ParseResult() : code_(kParseErrorNone), offset_(0) {}
@@ -115,8 +117,8 @@ public:
     //! Get the error offset, if \ref IsError(), 0 otherwise.
     size_t Offset() const { return offset_; }
 
-    //! Conversion to \c bool, returns \c true, iff !\ref IsError().
-    operator bool() const { return !IsError(); }
+    //! Explicit conversion to \c bool, returns \c true, iff !\ref IsError().
+    operator BooleanType() const { return !IsError() ? &ParseResult::IsError : NULL; }
     //! Whether the result is an error.
     bool IsError() const { return code_ != kParseErrorNone; }
 
@@ -124,6 +126,10 @@ public:
     bool operator==(ParseErrorCode code) const { return code_ == code; }
     friend bool operator==(ParseErrorCode code, const ParseResult & err) { return code == err.code_; }
 
+    bool operator!=(const ParseResult& that) const { return !(*this == that); }
+    bool operator!=(ParseErrorCode code) const { return !(*this == code); }
+    friend bool operator!=(ParseErrorCode code, const ParseResult & err) { return err != code; }
+
     //! Reset error code.
     void Clear() { Set(kParseErrorNone); }
     //! Update error code and offset.

test/unittest/documenttest.cpp

@@ -128,8 +128,14 @@ TEST(Document, UnchangedOnParseError) {
     Document doc;
     doc.SetArray().PushBack(0, doc.GetAllocator());
 
+    ParseResult noError;
+    EXPECT_TRUE(noError);
+
     ParseResult err = doc.Parse("{]");
     EXPECT_TRUE(doc.HasParseError());
+    EXPECT_NE(err, noError);
+    EXPECT_NE(err.Code(), noError);
+    EXPECT_NE(noError, doc.GetParseError());
     EXPECT_EQ(err.Code(), doc.GetParseError());
     EXPECT_EQ(err.Offset(), doc.GetErrorOffset());
     EXPECT_TRUE(doc.IsArray());
@@ -138,6 +144,9 @@ TEST(Document, UnchangedOnParseError) {
     err = doc.Parse("{}");
     EXPECT_FALSE(doc.HasParseError());
     EXPECT_FALSE(err.IsError());
+    EXPECT_TRUE(err);
+    EXPECT_EQ(err, noError);
+    EXPECT_EQ(err.Code(), noError);
     EXPECT_EQ(err.Code(), doc.GetParseError());
     EXPECT_EQ(err.Offset(), doc.GetErrorOffset());
     EXPECT_TRUE(doc.IsObject());
@@ -488,15 +497,19 @@ TYPED_TEST(DocumentMove, MoveConstructorParseError) {
     a.Parse("{ 4 = 4]");
     ParseResult error(a.GetParseError(), a.GetErrorOffset());
     EXPECT_TRUE(a.HasParseError());
+    EXPECT_NE(error, noError);
+    EXPECT_NE(error.Code(), noError);
     EXPECT_NE(error.Code(), noError.Code());
     EXPECT_NE(error.Offset(), noError.Offset());
 
     D b(std::move(a));
     EXPECT_FALSE(a.HasParseError());
     EXPECT_TRUE(b.HasParseError());
+    EXPECT_EQ(a.GetParseError(), noError);
     EXPECT_EQ(a.GetParseError(), noError.Code());
-    EXPECT_EQ(b.GetParseError(), error.Code());
     EXPECT_EQ(a.GetErrorOffset(), noError.Offset());
+    EXPECT_EQ(b.GetParseError(), error);
+    EXPECT_EQ(b.GetParseError(), error.Code());
     EXPECT_EQ(b.GetErrorOffset(), error.Offset());
 
     D c(std::move(b));