Why it's not possible to assign std::string to a subtring? - c++

Question

This is the code for my project which I want to compile. The error is stated at the end of the question.
#include <iostream>
#include <string>
#include <stdio.h>
class substring {
friend std::ostream& operator<<(std::ostream& output, substring const& sub);
public:
char *str;
int length;
substring();
~substring();
substring(std::string);
substring(const substring &);
substring& operator=(substring const& other);
substring& operator=(std::string const& strz);
substring& operator+=(substring const& other);
bool operator>(substring const& other) const;
bool operator<(substring const& other) const;
char& operator[](size_t idx);
char operator[](size_t idx) const;
};
std::ostream& operator<<(std::ostream& output, substring const& sub);
std::istream& operator >> (std::istream& input, substring const& sub);
bool operator==(substring const& one, substring const& another);
bool operator!=(substring const& one, substring const& another);
bool operator==(std::string const& str, substring const& sub);
bool operator==(substring const& sub, std::string const& str);
bool operator!=(std::string const& str, substring const& sub);
bool operator!=(substring const& sub, std::string const& str);
substring::substring()
{
length = 0;
}
substring::~substring()
{
delete str;
}
substring::substring(std::string)
{
}
substring::substring(const substring & sub)
{
str = new char[length];
}
std::ostream & operator<<(std::ostream & output, substring const & sub)
{
output << sub;
return output;
}
std::istream & operator >> (std::istream & input, substring const & sub)
{
std::cout << "Enter sub:";
input >> sub;
return input;
}
bool operator==(substring const & one, substring const & another)
{
if (one.length != another.length)
return false;
else
{
for (int i = 0; i < another.length; i++)
if (one[i] != another[i])
{
return false;
break;
}
}
return true;
}
bool operator!=(substring const & one, substring const & another)
{
if (one.length != another.length)
return true;
else
{
for (int i = 0; i < another.length; i++)
if (one[i] != another[i])
{
return true;
break;
}
}
return false;
}
bool operator==(std::string const & str, substring const & sub)
{
if (sub.length != str.length())
return false;
else
{
for (int i = 0; i < sub.length; i++)
if (str[i] != sub[i])
{
return false;
break;
}
}
return true;
}
bool operator==(substring const & sub, std::string const & str)
{
if (str.length() != sub.length)
return false;
else
{
for (unsigned int i = 0; i < str.length(); i++)
if (sub[i] != str[i])
{
return false;
break;
}
}
return true;
}
bool operator!=(std::string const & str, substring const & sub)
{
if (sub.length != str.length())
return true;
else
{
for (int i = 0; i < sub.length; i++)
if (str[i] != sub[i])
{
return true;
break;
}
}
return false;
}
bool operator!=(substring const & sub, std::string const & str)
{
if (sub.length != str.length())
return true;
else
{
for (int i = 0; i < sub.length; i++)
if (str[i] != sub[i])
{
return true;
break;
}
}
return false;
}
substring & substring::operator=(substring const & other)
{
delete str;
length = other.length;
str = new char[length];
for (int i = 0; i<length; i++)
{
str[i] = other.str[i];
}
return *this;
}
substring & substring::operator=(std::string const & strz)
{
length = strz.length();
str = new char[length];
for (int i = 0; i<length; i++)
{
str[i] = strz[i];
}
return *this;
}
substring & substring::operator+=(substring const & other)
{
char* new_str = new char[length + other.length];
for (int i = 0; i<length; i++)
{
new_str[i] = str[i];
}
for (int i = length; i<other.length; i++)
{
new_str[i] = other.str[i];
}
delete str;
str = new_str;
return *this;
}
bool substring::operator>(substring const & other) const
{
return true;
}
bool substring::operator<(substring const & other) const
{
return true;
}
char & substring::operator[](size_t idx)
{
return str[idx];
}
char substring::operator[](size_t idx) const
{
return str[idx];
}
int main()
{
std::string str = "abc";
substring sub = str;
std::cout << sub;
return 0;
}
The problem is that when I run this code, it seems that the compiler just skips this: substring sub = str;.
I cannot even change this line to substring sub = "aaa"; because it shows an error which says I cannot convert subtring to std::string (although there is an operation overloading for this in the code).

Answer 1

The problem is that when I run this code, it seems that the compiler just skips this: substring sub = str;
The compiler did not "skip it". It compiled succesfully, and the new object was created succesfully. However, the converting constructor that you defined leaves the objects members default initialized:
substring::substring(std::string)
{
}
I cannot even change this line to substring sub = "aaa"; because it shows an error which says I cannot convert subtring to std::string
I highly doubt that. I suspect that you misread.
My compiler says that const char [4] cannot be converted substring
(although there is an operation overloading for this in the code).
There certainly isn't a substring::substring(const char(&)[4]) (nor substring::substring(const char*)).