Need to make context available to C++ ostream insertion operators

Question

For an API that I am working on, I want to allow the user to insert custom objects into an ostream, but these objects have no meaning on their own, and are too memory constrained to include an additional pointer or reference for context. (Think tens of millions of 16-/32-/48-bit objects in an embedded system with limited memory.)

Suppose the user initializes the underlying context, and looks up one of these objects:

DDB ddb("xc5vlx330t");
Tilewire tw = ddb.lookUpTilewire("DSP_X34Y0", "DSP_IMUX_B5_3");
...
std::cout << complexDataStructure;

In an entirely different scope, possibly nested far away from the user's explicit code, we may need to insert the object into an ostream, with ddb unavailable.

os << tw;

The actual value encapsulated by tw is 97,594,974, but the desired output is this:

DSP_IMUX_B5_3@[263,84] DSP "DSP_X34Y0" (1488@77406)

In order for this to work, the appropriate insertion operator would need access to ddb, but it cannot rely on static or global variables or functions (for multithreading reasons). What I'd like to do is allow the user to request and use a stream wrapper kind of like this:

ostream& wrappedCout = ddb.getWrappedOstream(std::cout);

The returned subclass of ostream would include a reference to ddb for use by special stream inserters that needed it, and a reference to the original stream—std::cout in this case—where it would forward all of its output.

Unfortunately, the inheritance or composition schemes that I have come up with are messy to code up (not an enormous concern), and possibly problematic for the user (a much larger concern). Any suggestions on how to elegantly make ddb available to insertion operators? I am marginally aware of boost.Iostreams, but not sure that it will help me out here.

Answer 1

Write a custom stream manipulator that stores a reference to ddb using the iword/pword mechanism. Here is an example, you'd need to add locking around the iwork_indexes map in a multithreaded program.

class dbb
{
public:
    explicit dbb(int value) : m_value(value) {}
    int value() const { return m_value; }
private:
    int m_value;
};

class dbb_reliant_type
{
public:
    dbb_reliant_type(const std::string& value) : m_value(value) {}
    const std::string& value() const { return m_value; }
private:
    std::string m_value;
};

typedef std::map<std::ostream*, int> iword_map;
iword_map iword_indexes;

inline int get_iword_index(std::ostream& os)
{
    iword_map::const_iterator index = iword_indexes.find(&os);

    if(index == iword_indexes.end())
    {
        std::pair<iword_map::iterator, bool> inserted = iword_indexes.insert(std::make_pair(&os, os.xalloc()));
        index = inserted.first;
    }

    return index->second;
}


inline std::ostream& operator<<(std::ostream& os, const dbb& value)
{
    const int index = get_iword_index(os);

    if(os.pword(index) == 0)
        os.pword(index) = &const_cast<dbb&>(value);

    return os;
}

std::ostream& operator<<(std::ostream& os, const dbb_reliant_type& value)
{
    const int index = get_iword_index(os);
    dbb* deebeebee = reinterpret_cast<dbb*>(os.pword(index));
    os << value.value() << "(" << deebeebee->value() << ")";
    return os;
}

int main(int, char**)
{
    dbb deebeebee(5);
    dbb_reliant_type variable("blah");
    std::cout << deebeebee << variable << std::endl;
    return 0;
}

Answer 2

I'm not entirely sure if I understand what can be accessed at what time and what can and can't change, but....can you do something like this

struct TilewireFormatter {
    DDB *ddb;
    TilewireFormatter(DDB* d) : ddb(d) {}

    print(std::ostream& out, const Tilewire& obj) {
        // some formatting dependent on ddb
        out << obj;
    }
};

and replace out << tw; with formatter.print(out, tw);

then not provide any sort of << operator overload for Tilewire and pass an instance of TilewireFormatter around that's used to format them based on what ddb is?

Answer 3

Rather than fudging around and trying to find a way to pass contextual information while using the insertion operator, I suggest you make something like a print method like choobablue suggests. It's a nice and simple solution and anything fancier is probably more trouble than it's worth.

I also find it odd that you choose iostreams for an embedded system. They're one of the most bloated parts of the C++ standard library (not just by implementation, but by design) and if you are working on an embedded system, you could just as well roll your own alternative of this (still based on the basic design of iostreams) and can probably do it just as quickly as trying to use iostream effectively and across multiple threads.

Answer 4

I'm new at this, so in case providing my own answer gets in the way of me sharing the credit with Gary, well, Gary pointed out what I had just stumbled upon moments before through the same reference: Stream Storage for Private Use: iword, pword, and xalloc

#include <iostream>

// statically request a storage spot that can be associated with any stream
const int iosDdbIndex = std::ios_base::xalloc();

class DDB {
public:
    // give the stream a pointer to ourselves
    void bless(std::ostream& os) { os.pword(iosDdbIndex) = this; }
    // provide a function that the insertion operator can access
    int getSomething(void) { return 50; }
};

class Tilewire {
    friend std::ostream& operator<< (std::ostream& os, Tilewire tilewire);
    // encapsulate a dummy value
    int m;
public:
    // construct the Tilewire
    Tilewire(int m) : m(m) {}
};

std::ostream& operator<< (std::ostream& os, Tilewire tilewire) {
    // look up the pointer to the DDB object
    DDB* ddbPtr = (DDB*) os.pword(iosDdbIndex);
    // insert normally, and prove that we can access the DDB object's methods
    return os << "Tilewire(" << tilewire.m << ") with DDB param " << ddbPtr->getSomething();
}

int main (int argc, char * const argv[]) {
    DDB ddb;
    ddb.bless(std::cout);
    std::cout << Tilewire(0) << std::endl;
    return 0;
}