Compile-time iterator type

⁸/Jan 2008 By admin

Crazy snippets

Something we’ve been wondering at work one day (just for kicks, but being resident template freak I couldnt resist to give it a try). Challenge: given STL collection, find a way to determine ‘compatible’ iterator type for it. Example of intended usage:

std::vector v;
for (ITER_TYPE(int, v)::iterator it = v.begin(); it != v.end(); ++it)
    ; // do something here.

Do not read further if you want to try it by yourself, solution follows.Main problem is that everything has to be done at compile time. Solution? Template specialization to the rescue, rough code:

struct vector_type { char dummy[1]; };
struct list_type { char dummy[2]; };
template vector_type container_type(const std::vector&);
template list_type container_type(const std::list&);

template struct iter_adapter {};
template struct iter_adapter<T, sizeof(vector_type)>
{
    typedef typename std::vector::iterator		iterator;
    typedef typename std::vector::const_iterator	const_iterator;
};
template struct iter_adapter<T, sizeof(list_type)>
{
    typedef typename std::list::iterator			iterator;
    typedef typename std::list::const_iterator	const_iterator;
};

#define ITER_TYPE(t, c)	iter_adapter<c, sizeof(container_type(c))>

Note that container_type function is never executed, so body doesnt have to be defined, compiler needs it only to determine size of returned variable.

.m.m.s.

Maciej Sinilo

Compile-time iterator type

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