[c++] C++: constructor initializer for arrays

You can use C++0x auto keyword together with template specialization on for example a function named boost::make_array() (similar to make_pair()). For the case of where N is either 1 or 2 arguments we can then write variant A as

namespace boost
{
/*! Construct Array from @p a. */
template <typename T>
boost::array<T,1> make_array(const T & a)
{
    return boost::array<T,2> ({{ a }});
}
/*! Construct Array from @p a, @p b. */
template <typename T>
boost::array<T,2> make_array(const T & a, const T & b)
{
    return boost::array<T,2> ({{ a, b }});
}
}

and variant B as

namespace boost {
/*! Construct Array from @p a. */
template <typename T>
boost::array<T,1> make_array(const T & a)
{
    boost::array<T,1> x;
    x[0] = a;
    return x;
}
/*! Construct Array from @p a, @p b. */
template <typename T>
boost::array<T,2> make_array(const T & a, const T & b)
{
    boost::array<T,2> x;
    x[0] = a;
    x[1] = b;
    return x;
}
}

GCC-4.6 with -std=gnu++0x and -O3 generates the exact same binary code for

auto x = boost::make_array(1,2);

using both A and B as it does for

boost::array<int, 2> x = {{1,2}};

For user defined types (UDT), though, variant B results in an extra copy constructor, which usually slow things down, and should therefore be avoided.

Note that boost::make_array errors when calling it with explicit char array literals as in the following case

auto x = boost::make_array("a","b");

I believe this is a good thing as const char* literals can be deceptive in their use.

Variadic templates, available in GCC since 4.5, can further be used reduce all template specialization boiler-plate code for each N into a single template definition of boost::make_array() defined as

/*! Construct Array from @p a, @p b. */
template <typename T, typename ... R>
boost::array<T,1+sizeof...(R)> make_array(T a, const R & ... b)
{
    return boost::array<T,1+sizeof...(R)>({{ a, b... }});
}

This works pretty much as we expect. The first argument determines boost::array template argument T and all other arguments gets converted into T. For some cases this may undesirable, but I'm not sure how if this is possible to specify using variadic templates.

Perhaps boost::make_array() should go into the Boost Libraries?