After reading this question Easy bitset I/O. I wanted to see if there was a simpler way of doing it.

This is how I would define a way of outputting std::bitset<N> to the stream and reading it back in. It works perfectly with current containers.


// Include guards deliberately not included for simplicity.

#include <bitset>
#include <iostream>

template<int S>
class BitIO
    std::bitset<S>&     data;
    BitIO(std::bitset<S>& data)
        : data(data)

    friend std::ostream& operator<<(std::ostream& stream, BitIO const& value)
        // Currently using just an integer.
        // But we could easily take this a step further and use hex.
        // and pad with 0 to get an exact width (that way we don't need a
        // trailing space).
        // Using a fixed width means we may potentially waste space but can
        // be made to waste the least amount of space possible quite easily.
        // Amount of wasted space would be (4 - (S % 4)) % 4

        return stream << value.data.to_ullong() << " ";
    friend std::istream& operator>>(std::istream& stream, BitIO& value)
        // As the current output uses a white space separated
        // unsigned long long it is easy to read it back in using the
        // normal `operator>>`
        // Would be just as simple to convert this function to use
        // a fixed width format to mirror the output. 
        unsigned long long tmp;
        if (stream >> tmp)
            value.data  = std::bitset<S>(tmp);
        return stream;


#include "BitIO.h"
#include <vector>

int main()
    std::vector<std::bitset<16> >    values { std::bitset<16>(1), std::bitset<16>(2), std::bitset<16>(255) };

    std::copy(std::begin(values), std::end(values), std::ostream_iterator<BitIO<16>>(std::cout));

    std::bitset<16>  data { 129 };
    std::cout << BitIO<16>(data);


The code is relatively short and well-written, so I didn't find many nits to pick. With that said, here's my review:

Include all needed files

Since std::ostream_iterator is declared in <iterator> that should be among the includes in main:

#include <iterator>

Use appropriate type for template parameter

The std::bitset template is defined as taking std::size_t rather than int, so it would make sense to define the BitIO template similarly:

template<std::size_t S>
class BitIO
    // ...

One can, of course, still abuse the interface by passing negative numbers but at least the user of the code is forewarned that they will be interpreted as std::size_t rather than as signed numbers.

Consider implementing a move constructor

At the moment, this potential usage cannot compile:

std::cout << BitIO<16>(std::bitset<16>{ 99 });

It's easily fixed by adding this constructor:

BitIO(std::bitset<S>&& data)
    : data(data)

Maintain parallelism with existing practice

I was surprised to see spaces separating the outputs when none had been specified in the output ostream_iterator. Although having the space is arguably useful, the choice of separator (if any) should be left up to the user of the template instead of being hardcoded in the operator << function. I'd omit the trailing space to allow usage like this:

std::copy(std::begin(values), std::end(values), 
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  • \$\begingroup\$ The constructor could take its value by const reference and still work (have not tried). But that would make the move constructor redundant. \$\endgroup\$ – Martin York Dec 19 '14 at 20:20

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