What's wrong with my String concatenation in C++? - c++

Question

I am learning C++ by creating a String class for an embedded project and I have a problem with my String class' concatenation.
Here is my main method and the output
#include <iostream>
#include "string.hpp"
using namespace std;
int main() {
String s1("hello "), s2("world");
String s3 = s1 + s2;
cout << "s1=" << s1 << endl;
cout << "s2=" << s2 << endl;
cout << "s3=" << s3 << endl;
return 0;
}
s1=hello
s2=world
s3=hello
Rather than print out "hello world", it prints just "hello "
Here is my string.hpp class:
#pragma once
#include <cstring>
#include <iostream>
class String {
public:
String() : c_str(NULL), len(0)
{
}
String(const String& str) : c_str(new char[str.len]), len(str.len)
{
strncpy(c_str, str.c_str, len);
c_str[len] = '\0';
}
String(const char* str) : String(str, strlen(str))
{
strncpy(c_str, str, len);
c_str[len] = '\0';
}
String(const char* str, const int n) : len(n), c_str(new char[n+1])
{
strncpy(c_str, str, len);
}
~String()
{
delete[] c_str;
}
const char* get_c_str()
{
return c_str;
}
bool contains(const String &cmd, const size_t pos)
{
return strncmp(c_str+pos, cmd.c_str, cmd.len) == 0;
}
size_t length()
{
return len;
}
friend std::ostream& operator<<(std::ostream& os, const String obj)
{
os << obj.c_str;
return os;
}
friend void swap(String& s1, String& s2)
{
using std::swap;
swap(s1.c_str, s2.c_str);
swap(s1.len, s2.len);
}
bool operator==(const String& str)
{
return strncmp(c_str, str.c_str, len) == 0;
}
char operator[](const size_t i)
{
return c_str[i];
}
String& operator=(const String& src)
{
String tmp(src);
swap(*this, tmp);
return *this;
}
String operator+(const String& rhs)
{
const size_t new_len = len + rhs.len;
char* new_c_arr = new char[new_len+1];
strcpy(new_c_arr, c_str);
strcat(new_c_arr, rhs.c_str);
printf("new_c_arr=%s\n", new_c_arr);
return String(new_c_arr, len);
}
String operator+(const char* rhs)
{
const size_t new_len = len + strlen(rhs) + 1;
char* new_c_arr = new char[new_len];
strcpy(new_c_arr, c_str);
strcat(new_c_arr, rhs);
return String(new_c_arr, new_len);
}
private:
char* c_str;
int len;
};
I read about "the big 3" on SO while looking for a similar question and not sure if it because of this.

Answer 1

This code is broken in numerous ways.
#pragma once
Actual include guards are more portable than #pragma once.
String() : c_str(NULL), len(0)
{
}
Your default constructor makes c_str null; your other functions never check for this case. Remember that even an empty C string has one character.
String(const String& str) : c_str(new char[str.len]), len(str.len)
{
strncpy(c_str, str.c_str, len);
c_str[len] = '\0';
}
You only allocated str.len characters for c_str, yet you are accessing c_str[len].
String(const char* str) : String(str, strlen(str))
{
strncpy(c_str, str, len);
c_str[len] = '\0';
}
The constructor to which you delegated already performed a copy. Why are you calling strncpy here again?
String(const char* str, const int n) : len(n), c_str(new char[n+1])
{
strncpy(c_str, str, len);
}
and here you didn't make sure that your string is null-terminated.
const char* get_c_str()
{
return c_str;
}
Should be marked const.
bool contains(const String &cmd, const size_t pos)
{
return strncmp(c_str+pos, cmd.c_str, cmd.len) == 0;
}
Ditto. And you didn't check that pos is in range.
size_t length()
{
return len;
}
const.
friend std::ostream& operator<<(std::ostream& os, const String obj)
{
os << obj.c_str;
return os;
}
obj should be passed by const reference, not by value.
bool operator==(const String& str)
{
return strncmp(c_str, str.c_str, len) == 0;
}
const again, and the logic isn't even correct. By this logic, "something" == "something else" since you are only comparing the first len characters.
char operator[](const size_t i)
{
return c_str[i];
}
If you are returning a copy, then it should be const. If you want to allow users to modify the characters stored in the string, then it should return a char &. (Even better, have two separate overloads, one const and one non-const.)
String operator+(const String& rhs)
{
const size_t new_len = len + rhs.len;
char* new_c_arr = new char[new_len+1];
strcpy(new_c_arr, c_str);
strcat(new_c_arr, rhs.c_str);
printf("new_c_arr=%s\n", new_c_arr);
return String(new_c_arr, len);
}
You are constructing the new string with the wrong length, and also leaking new_c_arr. And this function should be const (and really should be implemented in terms of operator+=, which you don't have).
String operator+(const char* rhs)
{
const size_t new_len = len + strlen(rhs) + 1;
char* new_c_arr = new char[new_len];
strcpy(new_c_arr, c_str);
strcat(new_c_arr, rhs);
return String(new_c_arr, new_len);
}
Leaking new_c_arr again; also, new_len here includes the null terminator, while the version in the other operator+ doesn't. Your constructor taking a length does not appear to include the null terminator as part of the length.

Answer 2

This is the line of problem in the method String operator+(const String& rhs):
return String(new_c_arr, len);
You need to changed to this:
return String(new_c_arr, new_len+1);
You are initializing the returning String with a length equal to the first part only. Not the whole concatenated string.
See that the other operator is OK.
By the way you're creating a lot of new char[] in the operator+ and the constructor.
In the case of the operator+ you are creating a new char[] in every call and passing to the constructor of string (not deleting after), and in the constructor are creating a new char[] to storage the string that is deleted in the destructor, the new char[] created in the operators are leaked.