boost::array of boost::reference_wrapper not compiling

Question

I have a class with a boost::array type that stores references to std::vector. I want to construct an array of references (I'm trying to DI, or so I think), but haven't succeeded so far.

class RefHolder
{

public:

    boost::array<boost::reference_wrapper<std::vector<int> >, 3> sumsArray;

};

class OriginalHolder
{

public:

    OriginalHolder(boost::array<boost::reference_wrapper<std::vector<int> >, 3> & pSumsArray)
    {
        sumsArray[0] = boost::ref(_sums);
    }

private:

    std::vector<int> _sums;

}

int main()
{
    RefHolder ref;

    OriginalHolder original(ref.sumsArray);

    return 0;
}

I don't really get what the compiler is telling me. Does array even work with reference_wrapper?

C:/ide-4.6-workspace/chunkybacon/boost/boost/array.hpp:60: error: no matching function for call to 'boost::reference_wrapper<std::vector<int, std::allocator<int> > >::reference_wrapper()'
C:/ide-4.6-workspace/chunkybacon/boost/boost/ref.hpp:43: note: candidates are: boost::reference_wrapper<T>::reference_wrapper(T&) [with T = std::vector<int, std::allocator<int> >]
C:/ide-4.6-workspace/chunkybacon/boost/boost/ref.hpp:33: note:                 boost::reference_wrapper<std::vector<int, std::allocator<int> > >::reference_wrapper(const boost::reference_wrapper<std::vector<int, std::allocator<int> > >&)
C:/ide-4.6-workspace/chunkybacon/TEST/TEST.cc: In constructor 'RefHolder::RefHolder()':
C:/ide-4.6-workspace/chunkybacon/TEST/TEST.cc:6: note: synthesized method 'boost::array<boost::reference_wrapper<std::vector<int, std::allocator<int> > >, 3u>::array()' first required here 
C:/ide-4.6-workspace/chunkybacon/TEST/TEST.cc: In function 'int main()':
C:/ide-4.6-workspace/chunkybacon/TEST/TEST.cc:32: note: synthesized method 'RefHolder::RefHolder()' first required here
cc: C:/QNX641/host/win32/x86/usr/lib/gcc/i386-pc-nto-qnx6.4.0/4.3.3/cc1plus caught signal 1

Of course I could just use plain pointers, and it works, but I still have the doubt and, if I can fix it, I'd rather use references.

I'm working with QNX 6.4.1 and GCC 4.3.3.

Thank you in advance for any help =)

reference_wrappers have to be initialized with what to refer to. — Qaz
– Qaz, Commented Apr 16, 2014 at 14:55

Dale Wilson · Accepted Answer · 2014-04-16 15:35:41Z

1

When you initially create the array you have "empty" entries. A reference cannot really be empty (OK, it could be, but it's a very bad idea), so references do not work as array entries.

Consider using boost::smart_ptr<vectorint> > instead. I realize that's not exactly what you want (pointer vs reference) but it works which is a big point in its favor.

------------Response to question in the comments---------

The difference is that a vector of references must be referencing objects that exist somewhere else -- somewhere that manages the lifetime of these objects and ensures that they live longer than the references to them do.

A vector of smart pointers must be pointing to heap-allocated objects, and manages the lifetime of those objects.

Both of these approaches have valid uses.

edited Apr 16, 2014 at 15:35

answered Apr 16, 2014 at 15:00

Dale Wilson

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4 Comments

ArthurChamz Over a year ago

I replaced boost::array with std::vector and it to works now. It now creates the references as it pushes them into the vector. Maybe this is another question, but I wanted to know your opinion. I'm not quite sure if this is better or worse, comparing it to your approach. What represents more overhead, the reserved vector or the smart_ptr?

Dale Wilson Over a year ago

I tried to answer as a comment, but it got too long. I'll edit my answer to add a response.

ArthurChamz Over a year ago

Thank you very much. For now, I'll go with what I think it's more readable. If performance suffers, I, uh... well, I'll think of something if and when that happens.

Dale Wilson Over a year ago

An excellent approach! Make it correct, make it readable, THEN if performance is an issue, profile it and hand-optimize it.

Salgar · Accepted Answer · 2014-04-16 15:00:32Z

1

boost::arrays constructor is trying to initialize all of its members with the default constructor of the defined type, which in this case is a boost::reference_wrapper, which has no default constructor, because references can not be undefined.

answered Apr 16, 2014 at 15:00

Salgar

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