Problem with template and lambda C++0x

Question

#include <iostream>
#include <algorithm>
#include <array>

using namespace std;

template<class T>
void func(T beg, T end)
{
    typedef decltype(*beg) type;
    std::for_each(beg, end, [](type t) { cout << t << endl; });
}

int main()
{
    std::array<int, 4> arr = { 1,2,3,4 };
    func(arr.begin(), arr.end());

    return 0;
}

Is decltype the way to go when telling the lambda expression what type is going to use?

what is the error you are getting. It is running fine here: ideone.com/zI3iL — Naveen
– Naveen, Commented Apr 13, 2011 at 9:29
its no error, Im just wondering when using a lambda with a template, is decltype the only way to tell the lambda the type of the template, is the the correct way to code? — hidayat
– hidayat, Commented Apr 13, 2011 at 9:31

ildjarn · Accepted Answer · 2011-04-13 09:32:58Z

2

That's probably acceptable, however as your code appears to expect iterators exclusively, I think that the following would be more appropriate:

typedef typename std::iterator_traits<T>::value_type type;

Or even better (given how you're using it):

typedef typename std::add_reference<
    typename std::add_const<
        typename std::iterator_traits<T>::value_type
    >::type
>::type type;

answered Apr 13, 2011 at 9:32

ildjarn

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2 Comments

Andres Jaan Tack Over a year ago

I much prefer the way the OP did it.

ildjarn Over a year ago

@AndresJaanTack : Fair statement – given the year and a half since I answered this originally, my level of comfort with C++11 idioms has evolved enough that at this point I would probably use the OP's approach as well. :-] That said, I do still have some lingering concerns with the OP's approach in regards to iterators that return proxy objects (e.g. std::vector<bool>::iterator), though I don't know if those concerns are truly well-founded.

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