Can I use a member variable of type ofstream initialized in the class constructor?

Question

I'm having issues declaring a constructor of an inherited class.

class Report{
public:
    string fileName;
    std::ofstream outputFile;

    Report(string fileName, ofstream outputFile) {

        fileName = fileName;
        outputFile = outputFile; //<-- error here
    }

    void returnFile(string, ofstream);

    void Report::returnFile(string name, ofstream file){

         file.open(name);
    }
};

class financialReport: public Report{
public:
    void electorateHappenings();
    void electorialImpact();
    double finances();
    void writetoFile();

    financialReport(string fileName, ofstream outputFile)
    :Report(fileName, outputFile) { } //<-- error here
};

the error occurs on the 3rd last line :Report(fileName, outputFile).

This line produces the error:

function "std::basic_ofstream<_CharT, _Traits>::basic_ofstream(const
 std::basic_ofstream<_CharT, _Traits> &) [with _CharT=char, 
_Traits=std::char_traits<char>]" (declared at line 848 of 
"C:\MinGW\lib\gcc\mingw32\9.2.0\include\c++\fstream") cannot be referenced 
-- it is a deleted function

Is it not possible to create a constructor including ofstream?

The error also occurs on line 9 with outputFile = outputFile.

Thank you.

The parameter names can't be the same as the member variables. — gavinb, May 14 '20 at 12:23
This is classic example of [XY problem](http://xyproblem.info/). What are you trying to achieve? You are trying to do this in incorrect way and ask to fix invalid solution. Sadly someone already made your wrong approach to compile (and work). — Marek R, May 14 '20 at 12:47

anastaciu · Accepted Answer · 2020-05-14T13:43:35.497

You can't pass it by copy, you can't copy one, but you can pass it by reference and initialize it in the initializer list of the class:

Demo

class Report {
public:
    string fileName;
    std::ofstream &outputFile; //reference here

    // reference parameter, and initializer list
    Report(string fileName, ofstream &outputFile) : outputFile(outputFile) {
        fileName = fileName;
    }
    //...
};

Do the same in financialReport:

financialReport(string fileName, ofstream& outputFile) : Report(fileName, outputFile) {}
                                         ^

Note that this is a solution to the problem posed in the question, as normal, but in a more deep analysis, though you don't go in detail about what you want to achieve, I wouldn't go so far as to say it's a wrong approach, but odds are you can structure your program in a better way.

Yksisarvinen · Answer 2 · 2020-05-14T12:42:52.490

Yes, you can, but the error is telling you that you cannot copy an object of std::ofstream.

Depending on what you want to do, there are two ways to handle it.

Pass the ownership of std::ofstream to your newly created object:

Report(string fileName, ofstream outputFile) :
    fileName{std::move(outputFile)},
    outputFile{std::move(outputFile)}
{
}

//creation of object:
std::ofstream ofs {"filename.txt"};
Report report {"filename.txt", std::move(ofs)};
//ofs is empty here, it's whole content has been transferred to report object

Pass a reference to existing std::ofstream object:

class Report{
public:
  string fileName;
  std::ofstream& outputFile;

Report(string fileName, ofstream& outputFile) :
    fileName{std::move(outputFile)},
    outputFile{outputFile}
{
}

//creation of object:
std::ofstream ofs {"filename.txt}";
Report report {"filename.txt", ofs};
//you can use ofs from both here and from inside of report, but 
//you have to ensure that ofs lives as long as report will use it or else you will enter Undefined Behaviour land

Note: If you want to have the same names for class members and for constructor arguments, you need to use member initializer list, like I did. If you decide to use references, you are required to use it as well.

Can I use a member variable of type ofstream initialized in the class constructor?

2 Answers2