How to overload operator in base-derived classes?

Question

As shown in https://godbolt.org/z/WhEqjaehc

Code is like

#include <filesystem>
#include <fstream>
#include <iostream>

class BaseFile {
protected:
    std::ofstream file;

public:
    template<typename T>
    BaseFile &operator<<(const T &v) {
        file << v;
        return *this;
    }
    BaseFile &operator<<(std::ostream &(*pf)(std::ostream &) ) {
        file << pf;
        return *this;
    }
};

class TmpFile : public BaseFile {
public:
    template<typename T>
    TmpFile &operator<<(const T &v) {
        //some additional operation into file
        file << v << "some suffix maybe";
        return *this;
    }
    TmpFile &operator<<(std::ostream &(*pf)(std::ostream &) ) {
        file << pf;
        return *this;
    }
};

int main(){
    TmpFile f("path");
    f << "123" << std::endl;
}

BaseFile is a base class and TmpFile is derived from it. If I want to change std::ofstream file in BaseFile into private, then how to implement operator<< in TmpFile without directly using file?

Answer 1

You can just call base class method explicitly:

    template<typename T>
    TmpFile &operator<<(const T &v) {
        //some additional operation into file
        // file << v << "some suffix maybe";
        BaseFile::operator<<(v);
        BaseFile::operator<<("some suffix maybe");
        return *this;
    }
    TmpFile &operator<<(std::ostream &(*pf)(std::ostream &) ) {
        BaseFile::operator<<(pf);
        return *this;
    }

Godbolt demo.

It is also possible to chain the calls:

BaseFile::operator<<(v).BaseFile::operator<<("some suffix maybe");
BaseFile::operator<<(v) << "some suffix maybe";

but for me it's less readable.