Concatenate 2 string using operator+= in class C++ - c++

Question

I've seen some similar questions before asking, but I'm still stuck at the part of concatenating two strings using operator+=.
Currently, I can get separate strings correctly by constructor method. But when I compile code, the line str[length+i] = s[i]; in the method String& String::operator+= (const String& s) shows an error:
no match for ‘operator[]’ (operand types are ‘const String’ and ‘unsigned int’)
So I need your help to fix this bug.
Here's my code:
#include <iostream>
#include <cstring>
class String {
// Initialise char array
char* data;
unsigned length;
public:
// Constructor without arguments
String();
// Constructor with 1 arguments
String(char* s);
// Copy Constructor
String(const String& source);
// Move Constructor
String(String&& source);
// Destructor
~String() { delete[] data; }
/*!
* #brief String length.
* #return Value in String #c length.
*/
unsigned len ( ) const;
/*!
* #brief Append to String.
* #param[in] s A String object.
* #return A String reference to *this.
* #post String will equal the concatenation of itself with #a s.
*/
String& operator+= (const String& s);
};
// Constructor with no arguments
String::String()
: data{ nullptr }
{
data = new char[1];
data[0] = '\0';
}
// Constructor with one arguments
String::String(char* s)
{
if (s == nullptr) {
data = new char[1];
data[0] = '\0';
}
else {
data = new char[strlen(s) + 1];
// Copy character of s[]
// using strcpy
strcpy(data, s);
data[strlen(s)] = '\0';
std::cout << data << "\n";
}
}
// Copy Constructor
String::String(const String& source)
{
data = new char[strlen(source.data) + 1];
strcpy(data, source.data);
data[strlen(source.data)] = '\0';
}
// Move Constructor
String::String(String&& source)
{
data = source.data;
source.data = nullptr;
}
unsigned String::len ( ) const
{
return length;
}
String& String::operator+= (const String& s)
{
unsigned len = length + s.len();
char* str = new char[len];
for (unsigned j=0; j < length; j++)
str[j] = data[j];
for (unsigned i=0; i < s.len(); i++)
str[length+i] = s[i];
delete data;
length = len;
data = str;
return *this;
}
int main()
{
// Constructor with no arguments
String a;
// Convert string literal to
// char array
char temp[] = "Hello world.";
// Constructor with one argument
std::cout << "s1: ";
String s1{ temp };
// Copy constructor
String s11{ a };
char temp1[] = "Goodbye!";
std::cout << "s2: ";
String s2{ temp1 };
String s3 = String s1 + String s2;
return 0;
}
Another way of writing main function:
int main()
{
String s1("Hello World.");
String s2("Goodbye!");
std::cout << "s1: " << s1 << std::endl;
std::cout << "s2: " << s2 << std::endl;
String s3 = s1 + s2;
std::cout << "s3: " << s3 << std::endl;
std::cout << "The last char of s3: " << s3[s3.size()-1] << std::endl;
return 0;
}
Expected result:
s1: Hello World.
s2: Goodbye!
s3: Hello World.Goodbye!
The last char of s3: !
How can I modify my code to get s3 and last char of s3 correctly?

Answer 1

In many of your constructors, you do not set length which leaves it with an indeterminate value - and reading such values makes the program have undefined behavior. So, first fix that:
#include <algorithm> // std::copy_n
// Constructor with no arguments
String::String() : data{new char[1]{'\0'}}, length{0} {}
// Constructor with one argument
String::String(const char* s) { // note: const char*
if (s == nullptr) {
data = new char[1]{'\0'};
length = 0;
} else {
length = std::strlen(s);
data = new char[length + 1];
std::copy_n(s, length + 1, data);
}
}
// Copy Constructor
String::String(const String& source) : data{new char[source.length + 1]},
length{source.length}
{
std::copy_n(source.data, length + 1, data);
}
// Move Constructor
String::String(String&& source) : String() {
std::swap(data, source.data);
std::swap(length, source.length);
}
In operator+= you are trying to use the subscript operator, String::operator[], but you haven't added such an operator so instead of s[i], use s.data[i]:
String& String::operator+=(const String& s) {
unsigned len = length + s.length;
char* str = new char[len + 1];
for (unsigned j = 0; j < length; j++) str[j] = data[j];
for (unsigned i = 0; i < s.length; i++) str[length + i] = s.data[i];
str[len] = '\0';
delete[] data; // note: delete[] - not delete
length = len;
data = str;
return *this;
}
If you want to be able to use the subscript operator on String objects, you would need to add a pair of member functions:
class String {
public:
char& operator[](size_t idx);
char operator[](size_t idx) const;
};
char& String::operator[](size_t idx) { return data[idx]; }
char String::operator[](size_t idx) const { return data[idx]; }
And for String s3 = s1 + s2; to work, you need a free operator+ overload:
String operator+(const String& lhs, const String& rhs) {
String rv(lhs);
rv += rhs;
return rv;
}
Also, to support printing a String like you try in your alternative main function, you need an operator<< overload. Example:
class String {
friend std::ostream& operator<<(std::ostream& os, const String& s) {
os.write(s.data, s.length);
return os;
}
};
Full demo

Answer 2

For starters neither constructor sets the data member length.
So the operator
String& String::operator+= (const String& s)
{
unsigned len = length + s.len();
char* str = new char[len];
//...
has undefined behavior.
Also provided that the data member length was initialized you need to write
char* str = new char[len + 1];
instead of
char* str = new char[len];
to reserve memory for the terminating zero character '\0' because you are using the standard C string function strcpy in the copy constructor
strcpy(data, source.data);
And the class does not have the subscript operator used in this for loo[
for (unsigned i=0; i < s.len(); i++)
str[length+i] = s[i];
And you forgot to append the terminating zero character '\0'.
Pay attention to that there is no member function size in the class used in this expression
s3[s3.size()-1]
And this construction
String s3 = String s1 + String s2;
is invalid. At least you should write
String s3 = s1 + s2;
and correspondingly define the operator +.