Question

I am trying to write to file and stdout at the same time within c++ by overloading ofstream

test.h

 #pragma once 

#include <iostream>

using  std::ofstream;

class OutputAndConsole:public ofstream
{
public:
    std::string fileName;        
    OutputAndConsole(const std::string& fileName):ofstream(fileName),fileName(fileName){
    };
    template <typename T>
    OutputAndConsole& operator<<(T var);
};


template <typename T>
OutputAndConsole& OutputAndConsole::operator<<(T var)
{
    std::cout << var;
    ofstream::operator << (var);
    return (*this);
};

test.cpp

  OutputAndConsole file("output.txt");
  file << "test" ;

The output in the file is

01400930

but in the console is

test

I debugged the code it looks like it is entering into

_Myt& __CLR_OR_THIS_CALL operator<<(const void *_Val)

What am I doing wrong?

Answer 1

The problem

ofstream::operator << (var);

It’s your use of ofstream::operator<< as a qualified function call. You’re mandating that the function lookup locate a member function of ofstream; the best match that’s a member is the one for void*, whereas the specialisation for char* that prints the actual string contents is a free function (i.e. not a member function).

You’ll find the same problem if you do this with cout, too:

std::cout.operator<<(var);

The solution

This might do it:

static_cast<ofstream&>(*this) << var;

because you’re still using normal operator syntax (with all the overload resolution that this entails), but doing so with an ofstream as the LHS operand.

I haven’t actually tested it, though.

Conclusion

As an aside, your operator<< ought to be a free function too, in order to fit in with this convention.

So:

struct OutputAndConsole : std::ofstream
{
    OutputAndConsole(const std::string& fileName)
       : std::ofstream(fileName)
       , fileName(fileName)
    {};

    const std::string fileName;
};

template <typename T>
OutputAndConsole& operator<<(OutputAndConsole& strm, const T& var)
{
    std::cout << var;
    static_cast<std::ofstream&>(strm) << var;
    return strm;
};

I also took the liberty of making some minor syntax adjustments.