error: passing const std::string as this argument

The Following is my Code

FileNotReadableException Signature

Code:

class FileNotReadableException : public CGIException{
	protected:
		const string& fileName;
	public:
		FileNotReadableException();
		FileNotReadableException(const string& file);
		~FileNotReadableException();
		void setFileName(const string& file);
		const string& getFileName() const;
};

FileNotReadableException Implementation

Code:

FileNotReadableException::FileNotReadableException():fileName(""), CGIException(){

}

FileNotReadableException::FileNotReadableException(const string& file):fileName(file), CGIException("CGI::System::FileUnReadable", ("File: "+fileName+" Couldn't be opened for reading")){

}

void FileNotReadableException::setFileName(const string& file){
	fileName = file;//Line 43 Here is teh Error
}

const string& FileNotReadableException::getFileName() const{
	return fileName;
}

FileNotReadableException::~FileNotReadableException(){

}

It yields the following Compiler Error.

Code:

Line 43: error: passing ‘const std::string’ as ‘this’ argument of ‘std::basic_string<_CharT, _Traits, _Alloc>& std::basic_string<_CharT, _Traits, _Alloc>::operator=(const std::bas.........

solved by using string instead of const string&
I can understand why the const was making the problem (const-correctness)
But cant understand why the reference was making a Problem cause I am initializing all of them on teh Constructor.

Your member variable shouldn't be const if you are modifying it. It can be a reference, that wasn't causing that problem. But it shouldn't be because you want the string to be owned by the object, not refer to some other string from outside. Making filename a string is the correct choice.