implemented remaining methods and added checks to neccessary methods

(some methods, like size() could segfault on empty Strings)

String.cpp

@@ -33,7 +33,8 @@ String& String::operator=(String other)     // take parameter by value: can bind
 
 String::~String()
 {
-    _str->operator--(); // the ref-counting is done by StringValue, no need to do that here
+    if (_str)
+        _str->operator--(); // the ref-counting is done by StringValue, no need to do that here
 }
 
 void swap(String& s1, String& s2) noexcept
@@ -70,23 +71,69 @@ String& String::operator+=(const String& other)
     return *this;
 }
 
-// TODO: continue implementing missing stuff from the middle
+String String::operator+(char c) const
+{
+    auto data = new char[this->size() + 2];          // new char + null terminator
+    std::strcpy(data, this->c_str());
+    data[this->size()] = c;
+    data[this->size()+1] = '\0';
+
+    String ret;
+    ret._str = new StringValue(data);
+
+    return ret;
+}
+
+String& String::operator+=(char c)
+{
+    auto data = new char[this->size() + 2];          // new char + null terminator
+    std::strcpy(data, this->c_str());
+    data[this->size()] = c;
+    data[this->size()+1] = '\0';
+
+    _str->operator--();
+    _str = new StringValue(data);
+
+    return *this;
+}
 
 bool String::operator==(const String& other) const
-{ return static_cast<bool>(std::strcmp(this->c_str(), other.c_str())); }    // explicit cast to silence warning
+{
+    if (_str)
+        return !static_cast<bool>(std::strcmp(this->c_str(), other.c_str()));     // explicit cast to silence warning
+    else
+        throw std::runtime_error("You cannot compare uninitialized Strings!");
+}
 
 const char& String::operator[](size_t index) const
-{ return (const_cast<const StringValue*>(_str))->operator[](index); }       // cast to use const operator[]
+{
+    if (_str)
+        return (const_cast<const StringValue*>(_str))->operator[](index);   // cast to use const operator[]
+    else
+        throw std::runtime_error("You cannot index an uninitialized String!");
+}
 
 // reuse const operator[]
 char& String::operator[](size_t index)
-{ return const_cast<char&>(const_cast<const StringValue*>(_str)->operator[](index)); }
+{
+    return const_cast<char&>(const_cast<const String*>(this)->operator[](index));
+}
 
 size_t String::size() const
-{ return _str->size(); }
+{
+    if (_str)
+        return _str->size();
+    else
+        return 0;
+}
 
 const char* String::c_str() const
-{ return *_str; }   // invokes StringValue::operator const char*()
+{
+    if (_str)
+        return *_str;   // invokes StringValue::operator const char*()
+    else
+        throw std::runtime_error("You cannot get the content of an uninitialized string!");
+}
 
 std::istream& operator>>(std::istream& is, String& str)
 {