How to write vector of ostreams in C++ which takes in all the different output streams like cout, ostringstream and ofstream

Question

I am trying to implement a logger which can be registered with multiple streams like ostringstream, ofstream etc. I tried to implement the function like this

void register_stream(std::ostream& a);

The vector is as follows

std::vector<std::ostream> streams;

The register stream and operator overloading is as follows

void logger::register_stream(std::ostream &a)`

{

    streams.push_back(a);

}

template <typename T>

void logger::operator<<(T const& value)

{

    for (auto stream : streams)

    {

        (stream) << value;

    }

}

I am trying to implement a logger to write to all registered streams on a single operator "<<" call.

The following is the invocation code:

std::ostringstream os;
    std::ofstream f;
    logger l;
    l.register_stream(f);
    l.register_stream(os);
    l << "log this";

I am getting an error:

C2280: std::basic_ostream<char,std::char_traits<char>>::basic_ostream(const std::basic_ostream<char,std::char_traits<char>> &): attempting to reference a deleted function

Any help would be much appreciated.

Answer 1

You can't store copies of your streams, they are not copiable. You have to store std::reference_wrappers or pointers instead.

class logger {
    std::vector<std::reference_wrapper<std::ostream>> streams;
public:
    void register_stream(std::ostream &stream) {
        streams.emplace_back(stream);
    }

    template <typename T>
    void operator<<(T const &value) {
        for (auto &stream : streams) { // note the '&'
            stream.get() << value;
        }
    }
};

or if you want to be able to chain your calls like l << "log" << " this";:

    template <typename T>
    logger & operator<<(T const &value) {
        for (auto &stream : streams) {
            stream.get() << value;
        }
        return *this;
    }