Improved handling of NULL strings

 * Safely assert upon passing NULL string without length
   (requires usage of RAPIDJSON_ASSERT within an expression)
 * Allow using a NULL string together with an explicit length 0
   (GenericStringRef, GenericValue::SetString, ...), see #817
 * Add GenericValue::SetString(StringRefType, Allocator&) overload
 * Add tests for the various cases

include/rapidjson/document.h

@@ -308,7 +308,7 @@ struct GenericStringRef {
      */
 #endif
     explicit GenericStringRef(const CharType* str)
-        : s(str), length(internal::StrLen(str)){ RAPIDJSON_ASSERT(s != 0); }
+        : s(str), length(((RAPIDJSON_ASSERT(str != 0)), internal::StrLen(str))) {}
 
     //! Create constant string reference from pointer and length
 #ifndef __clang__ // -Wdocumentation
@@ -320,7 +320,7 @@ struct GenericStringRef {
      */
 #endif
     GenericStringRef(const CharType* str, SizeType len)
-        : s(str), length(len) { RAPIDJSON_ASSERT(s != 0); }
+        : s(RAPIDJSON_LIKELY(str) ? str : emptyString), length(len) { RAPIDJSON_ASSERT(str != 0 || len == 0u); }
 
     GenericStringRef(const GenericStringRef& rhs) : s(rhs.s), length(rhs.length) {}
 
@@ -331,6 +331,9 @@ struct GenericStringRef {
     const SizeType length; //!< length of the string (excluding the trailing NULL terminator)
 
 private:
+    /// Empty string - used when passing in a NULL pointer
+    static const Ch emptyString[];
+
     //! Disallow construction from non-const array
     template<SizeType N>
     GenericStringRef(CharType (&str)[N]) /* = delete */;
@@ -338,6 +341,9 @@ private:
     GenericStringRef& operator=(const GenericStringRef& rhs) /* = delete */;
 };
 
+template<typename CharType>
+const CharType GenericStringRef<CharType>::emptyString[] = { CharType() };
+
 //! Mark a character pointer as constant string
 /*! Mark a plain character pointer as a "string literal".  This function
     can be used to avoid copying a character string to be referenced as a
@@ -352,7 +358,7 @@ private:
 */
 template<typename CharType>
 inline GenericStringRef<CharType> StringRef(const CharType* str) {
-    return GenericStringRef<CharType>(str, internal::StrLen(str));
+    return GenericStringRef<CharType>(str);
 }
 
 //! Mark a character pointer as constant string
@@ -1762,7 +1768,7 @@ public:
         \return The value itself for fluent API.
         \post IsString() == true && GetString() != s && strcmp(GetString(),s) == 0 && GetStringLength() == length
     */
-    GenericValue& SetString(const Ch* s, SizeType length, Allocator& allocator) { this->~GenericValue(); SetStringRaw(StringRef(s, length), allocator); return *this; }
+    GenericValue& SetString(const Ch* s, SizeType length, Allocator& allocator) { return SetString(StringRef(s, length), allocator); }
 
     //! Set this value as a string by copying from source string.
     /*! \param s source string. 
@@ -1770,7 +1776,15 @@ public:
         \return The value itself for fluent API.
         \post IsString() == true && GetString() != s && strcmp(GetString(),s) == 0 && GetStringLength() == length
     */
-    GenericValue& SetString(const Ch* s, Allocator& allocator) { return SetString(s, internal::StrLen(s), allocator); }
+    GenericValue& SetString(const Ch* s, Allocator& allocator) { return SetString(StringRef(s), allocator); }
+
+    //! Set this value as a string by copying from source string.
+    /*! \param s source string reference
+        \param allocator Allocator for allocating copied buffer. Commonly use GenericDocument::GetAllocator().
+        \return The value itself for fluent API.
+        \post IsString() == true && GetString() != s.s && strcmp(GetString(),s) == 0 && GetStringLength() == length
+    */
+    GenericValue& SetString(StringRefType s, Allocator& allocator) { this->~GenericValue(); SetStringRaw(s, allocator); return *this; }
 
 #if RAPIDJSON_HAS_STDSTRING
     //! Set this value as a string by copying from source string.
@@ -1780,7 +1794,7 @@ public:
         \post IsString() == true && GetString() != s.data() && strcmp(GetString(),s.data() == 0 && GetStringLength() == s.size()
         \note Requires the definition of the preprocessor symbol \ref RAPIDJSON_HAS_STDSTRING.
     */
-    GenericValue& SetString(const std::basic_string<Ch>& s, Allocator& allocator) { return SetString(s.data(), SizeType(s.size()), allocator); }
+    GenericValue& SetString(const std::basic_string<Ch>& s, Allocator& allocator) { return SetString(StringRef(s), allocator); }
 #endif
 
     //@}

test/unittest/valuetest.cpp

@@ -857,9 +857,46 @@ TEST(Value, String) {
 }
 
 // Issue 226: Value of string type should not point to NULL
-TEST(Value, SetStringNullException) {
-    Value v;
-    EXPECT_THROW(v.SetString(0, 0), AssertException);
+TEST(Value, SetStringNull) {
+
+    MemoryPoolAllocator<> allocator;
+    const char* nullPtr = 0;
+    {
+        // Construction with string type creates empty string
+        Value v(kStringType);
+        EXPECT_NE(v.GetString(), nullPtr); // non-null string returned
+        EXPECT_EQ(v.GetStringLength(), 0u);
+
+        // Construction from/setting to null without length not allowed
+        EXPECT_THROW(Value(StringRef(nullPtr)), AssertException);
+        EXPECT_THROW(Value(StringRef(nullPtr), allocator), AssertException);
+        EXPECT_THROW(v.SetString(nullPtr, allocator), AssertException);
+
+        // Non-empty length with null string is not allowed
+        EXPECT_THROW(v.SetString(nullPtr, 17u), AssertException);
+        EXPECT_THROW(v.SetString(nullPtr, 42u, allocator), AssertException);
+
+        // Setting to null string with empty length is allowed
+        v.SetString(nullPtr, 0u);
+        EXPECT_NE(v.GetString(), nullPtr); // non-null string returned
+        EXPECT_EQ(v.GetStringLength(), 0u);
+
+        v.SetNull();
+        v.SetString(nullPtr, 0u, allocator);
+        EXPECT_NE(v.GetString(), nullPtr); // non-null string returned
+        EXPECT_EQ(v.GetStringLength(), 0u);
+    }
+    // Construction with null string and empty length is allowed
+    {
+        Value v(nullPtr,0u);
+        EXPECT_NE(v.GetString(), nullPtr); // non-null string returned
+        EXPECT_EQ(v.GetStringLength(), 0u);
+    }
+    {
+        Value v(nullPtr, 0u, allocator);
+        EXPECT_NE(v.GetString(), nullPtr); // non-null string returned
+        EXPECT_EQ(v.GetStringLength(), 0u);
+    }
 }
 
 template <typename T, typename Allocator>