error: passing const std::string as this argument

The Following is my Code

FileNotReadableException Signature

Code:

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

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FileNotReadableException Implementation

Code:

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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(){

}

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It yields the following Compiler Error.

Code:

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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.........

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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.