detecting typedef at compile time (template metaprogramming)

Question

I am currently doing some template metaprogramming. In my case I can handle any \"iteratable\" type, i.e. any type for which a typedef foo const_iterator exists in

Answer 1

There is a couple of ways to do this. In C++03, you could use boost and enable_if to define the trait (docs, source):

BOOST_MPL_HAS_XXX_TRAIT_DEF(const_iterator);

template 
struct Foo;

template 
struct Foo< T, typename boost::enable_if >::type>{ 
   void do_stuff(){ ... }
};

template
struct Foo >::type> {
    void do_stuff(){ ... }
};

In C++11, you could use Tick like this:

TICK_TRAIT(has_const_iterator)
{
    template
    auto require(const T&) -> valid<
        has_type
    >;
};

template 
struct Foo;

template 
struct Foo< T, TICK_CLASS_REQUIRES(std::is_fundamental::value)>{ 
   void do_stuff(){ ... }
};

template
struct Foo())> {
    void do_stuff(){ ... }
};

Also with Tick you can further enhance the trait to actually detect that the const_iterator is actually an iterator, as well. So say we define a simple is_iterator trait like this:

TICK_TRAIT(is_iterator,
    std::is_copy_constructible<_>)
{
    template
    auto require(I&& i) -> valid<
        decltype(*i),
        decltype(++i)
    >;
};

We can then define has_const_iterator trait to check that the const_iterator type matches the is_iterator trait like this:

TICK_TRAIT(has_const_iterator)
{
    template
    auto require(const T&) -> valid<
        has_type>
    >;
};