added noexcept qualifier to Char ctor, and made notes on exception

safety

Char.cpp

@@ -6,6 +6,7 @@ Char::operator char() const
 { return _string._str->operator[](_index); }
 
 // copy-on-write magic happens here
+// provides strong exception guarantee
 Char& Char::operator=(char other)
 {
     auto data = new char[_string.size() + 1];   // space for old string & null-terminator

Char.h

@@ -17,7 +17,7 @@ private:
 
 public:
 
-    explicit Char(size_t index, String& string) :_index(index), _string(string) {}
+    explicit Char(size_t index, String& string) noexcept :_index(index), _string(string) {}
     operator char() const;
     Char& operator=(char other);
 };

String.cpp

@@ -24,6 +24,7 @@ String::String(const String& other) noexcept
 }
 
 // copy&move-assignment operator with copy-and-swap
+// provides strong exception guarantee
 String& String::operator=(String other)     // take parameter by value: can bind to copy and move-assignment
 {                                           //                          & the compiler can optimize copying
     swap(*this, other);
@@ -57,6 +58,7 @@ String String::operator+(const String& rhs) const
     return ret;
 }
 
+// provides strong exception guarantee
 String& String::operator+=(const String& rhs)
 {
     auto data = this->copy_and_concat(rhs);
@@ -87,6 +89,7 @@ String String::operator+(char rhs) const
     return ret;
 }
 
+// provides strong exception guarantee
 String& String::operator+=(char rhs)
 {
     auto data = this->copy_and_concat(rhs);
@@ -143,6 +146,7 @@ const char* String::c_str() const
         throw std::runtime_error("You cannot get the content of uninitialized Strings!");
 }
 
+// provides strong exception guarantee
 std::istream& operator>>(std::istream& is, String& str)
 {
     constexpr size_t BUFSIZE = 50;                // set some reasonable buffer size