Generic iterators to specific types

Rennie deGraaf <>
Tue, 08 May 2007 23:11:37 -0600
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I would like to write a function that reads a sequence of unsigned
shorts from /any/ container, converts them to pairs of unsigned chars,
and writes them to /any/ container. In other words, something like this:=

#include <iterator>
#include <vector>
#include <iostream>
#include <cassert>
template <typename InIt, typename Out>
getBytes(const InIt& begin, const InIt& end, const typename std::back_i=

nsert_iterator<Out>& o)

    typename std::back_insert_iterator<Out> out = o;
        unsigned short u16;
        unsigned char u8[2];
    } elmt;
    for (InIt i=begin; i!=end; ++i)
        elmt.u16 = *i; // disregard endianness for now
        out = std::copy(elmt.u8, elmt.u8+2, out);
int main()
    std::vector<unsigned short> vec;
    std::vector<unsigned char> bytes;
    getBytes(vec.begin(), vec.end(), std::back_inserter(bytes));
    return 0;

That code works with the test driver provided. However, it /also/
compiles and executes if the input container holds something other than
unsigned short or if the output container takes something other than
unsigned char for which implicit conversions are defined. For instance,
getBytes() will compile with inputs from an std::set<double>, despite
the conversion only working properly for unsigned short.

The work-around that I'm currently using is to use BOOST_STATIC_ASSERT
to verify that the types are of the correct sizes in the beginning of

BOOST_STATIC_ASSERT(sizeof(typename Out::value_type) == sizeof(unsi=

gned char));

BOOST_STATIC_ASSERT(sizeof(typename InIt::value_type) == sizeof(uns=

igned short));
However, this is non-standard, makes assumptions which may not be safe,
and only works with container classes for which value_type is defined.

What I would like would be a way to define getBytes() to take iterators
to unsigned short and unsigned char but allow the container to vary.
I'd like it to support both STL containers and arrays. Does anyone have
any ideas on how to accomplish this?

Rennie deGraaf

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