Added static assert of std::string::value_type size

* CodepointIterator only supports UTF-8 encoded single-byte input strings
** this should prevent CodepointIterator from compiling on systems with larger char sizes while providing a helpful error message
* improved const-correctness by marking currByte (iterator dereferencing cache) and helper method arguments as const

3 files changed, 16 insertions, 7 deletions

diff --git a/src/codepoint_iterator.cc b/src/codepoint_iterator.cc
index 59cb23b..b96423a 100644
--- a/src/codepoint_iterator.cc
+++ b/src/codepoint_iterator.cc
@@ -40,8 +40,10 @@ std::ptrdiff_t CodepointIterator::operator-(
 }
 
 char32_t CodepointIterator::operator*() {
-	std::uint8_t  currByte  = *(this->iterator_);
-	char32_t codePoint      = 0;
+	const std::uint8_t currByte(
+		static_cast<std::uint8_t>(*(this->iterator_))
+	);
+	char32_t codePoint{};
 
 	if ( match(currByte, dtl::CodeUnitType::CONTINUATION) ) {
 		if ( match(currByte, dtl::CodeUnitType::THREE) ) {
@@ -94,7 +96,9 @@ char32_t CodepointIterator::operator*() {
 }
 
 CodepointIterator& CodepointIterator::operator++() {
-	std::uint8_t currByte(*(this->iterator_));
+	const std::uint8_t currByte(
+		static_cast<std::uint8_t>(*(this->iterator_))
+	);
 	std::string::difference_type offset(1);
 
 	if ( match(currByte, dtl::CodeUnitType::CONTINUATION) ) {
diff --git a/src/codepoint_iterator.h b/src/codepoint_iterator.h
index d1806e5..77600b7 100644
--- a/src/codepoint_iterator.h
+++ b/src/codepoint_iterator.h
@@ -10,6 +10,11 @@ namespace UTF8 {
 class CodepointIterator : public std::iterator<std::bidirectional_iterator_tag,
                                                char32_t,
                                                std::string::difference_type> {
+	static_assert(
+		sizeof(std::string::value_type) == 1,
+		"CodepointIterator only supports single-byte UTF-8 encoded input"
+	);
+
 	public:
 		CodepointIterator(std::string::const_iterator);
 		CodepointIterator(const CodepointIterator&);
diff --git a/src/utility.h b/src/utility.h
index 8aa46a0..75811f5 100644
--- a/src/utility.h
+++ b/src/utility.h
@@ -20,14 +20,14 @@ enum class CodePoint : std::uint8_t {
 	FOUR         = (UINT8_MAX >> 5), // 00000111
 };
 
-inline bool match(std::uint8_t unit, CodeUnitType type) {
+inline bool match(const std::uint8_t unit, const CodeUnitType type) {
 	return unit & static_cast<std::uint8_t>(type);
 }
 
 inline void write(char32_t& point,
-                  std::uint8_t unit,
-                  CodePoint mask,
-                  std::uint8_t offset) {
+                  const std::uint8_t unit,
+                  const CodePoint mask,
+                  const std::uint8_t offset) {
 	point += (unit & static_cast<std::uint8_t>(mask)) << offset;
 }