ostream: class that outputs either on cout or on a file

Question

I need to write a program that output either to the std::cout or to some file. I was reading this post to see how to do. However I would like to separate the manage

Answer 1

Since they both inherit from std::ostream, you can just assign it to a std::ostream&.

In your case, you can simply do something like this:

#include 
#include 

void do_stuff(const char* filename = nullptr) {
    std::ofstream _f;
    std::ostream& os = filename ? (_f.open(filename), _f) : std::cout;

    os << "Output normally";

    // If you want to check if it is a file somewhere else
    if (std::ofstream* fp = dynamic_cast(&os)) {
        std::ofstream& f = *fp;

        // But here you can probably check the condition used to make the file
        // (e.g. here `filename != nullptr`)
    }

    // After returning, `os` is invalid because `_f` dies, so you can't return it.
}

A simpler approach would be to not worry about this at all. Just put all of your code that outputs stuff inside one function that takes a std::ostream& parameter, and call it with a std::ofstream or another std::ostream:

void do_stuff(std::ostream& os) {
    os << "Write string\n";
}

int main() {
    if (using_file) {
        std::ofstream f("filename");
        do_stuff(f);
    } else {
        do_stuff(std::cout);
    }
}

If you want to be able to return the object without the file closing and becoming a dangling reference, you need to store it somewhere. This example stores it in a struct:

#include 
#include 
#include 
#include 
#include 

struct sw_ostream {
private:
    // std::optional f;
    // Use raw storage and placement new pre-C++17 instead of std::optional
    alignas(std::fstream) unsigned char f[sizeof(std::fstream)];
    std::ostream* os;

    bool did_construct_fstream() const noexcept {
        // If `os` is a pointer to `f`, we placement new`d, so we need to destruct it
        return reinterpret_cast(os) == f;
    }
    // Destroys currently held std::fstream
    // (Must have been constructed first and have `os` point to it)
    void destruct() noexcept {
        static_cast(*os).~basic_fstream();
    }
public:
    sw_ostream() = default;
    sw_ostream(std::ostream& os_) : os(&os_) {}
    template
    explicit sw_ostream(Args&&... args) {
        os = new (f) std::fstream(std::forward(args)...);
    }
    sw_ostream(std::fstream&& f) : os(nullptr) {
        *this = std::move(f);
    }
    sw_ostream(sw_ostream&& other) noexcept {
        *this = std::move(other);
    }

    sw_ostream& operator=(sw_ostream&& other) {
        if (did_construct_fstream()) {
            if (other.did_construct_fstream()) {
                static_cast(*os) = std::move(static_cast(*(other.os)));
            } else {
                destruct();
                os = other.os;
            }
        } else {
            if (other.did_construct_fstream()) {
                os = new (f) std::fstream(std::move(static_cast(*other.os)));
            } else {
                os = other.os;
            }
        }
        return *this;
    }
    sw_ostream& operator=(std::ostream& other) {
        if (did_construct_fstream()) {
            destruct();
        }
        os = &other;
        return *this;
    }
    sw_ostream& operator=(std::fstream&& other) {
        if (did_construct_fstream()) {
            static_cast(*os) = std::move(other);
        } else {
            os = new (f) std::fstream(std::move(other));
        }
        return *this;
    }

    std::ostream& operator*() const noexcept {
        return *os;
    }
    std::ostream* operator->() const noexcept {
        return os;
    }
    operator std::ostream&() const noexcept {
        return *os;
    }
    std::fstream* get_fstream() const noexcept {
        if (did_construct_fstream()) return &static_cast(*os);
        return dynamic_cast(os);
    }

    // `s << (...)` is a shorthand for `*s << (...)` (Where `s` is a `sw_ostream`)
    template
    const sw_ostream& operator<<(T&& o) const {
        *os << std::forward(o);
        return *this;
    }
    template
    sw_ostream& operator<<(T&& o) {
        *os << std::forward(o);
        return *this;
    }

    ~sw_ostream() {
        if (did_construct_fstream()) {
            destruct();
        }
    }
};

int main() {
    sw_ostream s;
    if (opening_file) {
        s = std::fstream("filename");
    } else {
        s = std::cout;
    }

    if (std::fstream* fp = s.get_fstream()) {
        assert(fp->is_open());
    }

    s << "Hello, world!\n";
    s->flush();
}

I also came up with another solution that uses std::unique_ptr so that you can use any derived class of std::ostream, but that unnecessarily uses dynamic memory if you only want an existing std::ostream (Like std::cout) or a std::fstream. See here.