Error : const copy constructor invoking non const methods

[dany2704]

I have modified the copy constructor as constant in order to fix the compilation error.Again the compiler throws an error,because calling a non-const method from a const method.



String.h

class String
{
private:
char *_text;
friend class String_Iterator;
public:
// ctors and dtor
explicit String ();
explicit String (char *);
explicit String (int );
String (String const & );
~String();

/////////////////
// conversions:
/////////////////
// to C-like type char *
operator char *() {return _text;}

/////////////////
// overloads:
/////////////////
// assignment String = String
String &operator=(String &);
}

String.c

String::String()
{
_text = new char[1024];
}
String::String(char *ch)
{
if (ch == NULL )
{
// if input char* is empty - allocate short buffer
// and set it to ""
_text = new char[2];
strcpy(_text, "");
}
else
{

_text = new char[strlen(ch) + 1];

if(_text)
strcpy(_text, ch);
else
{
_text = new char[2];
strcpy(_text, "");
}
}

}
String::String(int iLen)
{
_text = new char[iLen+1];
}
String::String(String const& string)//modified copy cons as const
{
_text = new char[string.length() + 1];

strcpy(_text,string);
}
String::~String()
{
delete[] _text;
}
String &String::operator=(String &ptrStr)
{

delete[] _text;
_text = new char[ptrStr.length() + 1];

strcpy(_text, ptrStr);
return *this;
}
String &String::operator=(char *ch)
{
delete[] _text;
_text = new char[strlen(ch) + 1];

strcpy(_text, ch);
return *this;
} Error:
String.c: In copy constructor 'String::String(const String&)':
String.c:49: error: passing 'const String' as 'this' argument of 'int String::length()' discards qualifiers
String.c:51: error: passing 'const String' as 'this' argument of 'String::operator char*()' discards qualifiers
String.c:51: error: invalid conversion from 'char*' to 'const char*'
String.c:51: error: initializing argument 2 of 'char* strcpy(char*, const char*)'
make: *** [all] Error 1
Please check and help me to resolve the issue ..
Thanks for looking into this

[spjackson]

String.c: In copy constructor 'String::String(const String&)':
String.c:49: error: passing 'const String' as 'this' argument of 'int String::length()' discards qualifiers
String.c:51: error: passing 'const String' as 'this' argument of 'String::operator char*()' discards qualifiers
String.c:51: error: invalid conversion from 'char*' to 'const char*'
String.c:51: error: initializing argument 2 of 'char* strcpy(char*, const char*)'
make: *** [all] Error 1

You don't show the declaration of length(). I'm guessing that it's something like:


class String {
public:
int length();
};

You want:


class String {
public:
int length() const;
};

This says that length() does not modify its object, so it can be called on const object.

The conversion operator to char * has probably two faults:


class String {
public:
operator char *() {return _text;}
};

First, again since the function is not declared const, you are saying that the function may change its object. Second, do you really want it to return a pointer to an address that can be written to? I would guess not. So you probably want:


class String {
public:
operator const char *() const {return _text;}
};

Your assignment operator should also have a const parameter.

Finally, what's wrong with std::string?

[dany2704]

Thank you!!! your solution fix the problem