i cant understand this compile time error

[ozair]

hello there,
I am defining a class in a program similiar to 'string' class in standard c++ library <string>

here is the code..

#include <iostream>
using namespace std;

class mystring
{
private:
char* string;

public:
// constructor functions
mystring();
mystring(const char []);
mystring(const mystring &);

char* get() { return string; } // getter function
};

mystring::mystring()
{
string=new char[256];
string="\0";
}

mystring::mystring(const char str[])
{
string=new char[256];
strcpy(string, str);
}

mystring::mystring(const mystring &str)
{
string=new char[256];
strcpy(string, str.get()); // there is error on this line
}

int main()
{
mystring s1="red", s2=s1;

return 0;
}

the error says that
'get' : cannot convert 'this' pointer from 'const class mystring' to 'class mystring &'

I cant understand what is going wrong..

regards
ozair

[Bazzy]

The prototype of get should be const char* get() const so you can call it from const objects ( as you are doing in the copy constructor.
The first const means that the C string returned by get can't be modified externally, the second means that the function itself doesn't modify the object

[ozair]

you said that we need these const's so that we can call this function from const object but if i remove const from the copy construction parameter then it should not give a error..!

[Bazzy]

You must pass by const references to a copy constructor

[ozair]

can you explain briefly...?

thanks

[Bazzy]

A copy constructor must copy the contents of an existing object ( without modifying it ) into the new object

The alternatives are:
- passing by value: It will cause an infinite copy loop since the argument to the constructor itself must be copied to get it's value
- passing by reference: It may modify the argument and it can't take rvalues or const objects
- passing by const reference: Will work