CODEWITHSUNDEEP
c++c++11iteratortype-traitsdecltype
Glen Low
Glen Low

Reputation: 4497

How to declare the value of an iterator through decltype

In C++98, I typically use the following to declare a variable in an iterator's value type:

typename std::iterator_traits<Iterator>::value_type value;

In C++11 we have decltype and I had thought the easiest way to deduce the value type is:

decltype(*iterator) value;

Unfortunately for most iterators, the type of *iterator is value_type& and not value_type. Any ideas, without the type modification classes, how to massage the above into yielding the value_type (and not any reference)?

I don't think the question is unreasonable given that the following is fairly robust but ends up creating another variable.

auto x = *iterator;
decltype(x) value;

Also note that I really want the deduced type and not just an instance e.g. if I wanted to declare a std::vector of these values.

Upvotes: 8

Views: 4331

Answers (2)

Martin Wirth
Martin Wirth

Reputation: 163

For this use-case i like std::decay. Typicall i'd be using 

std::vector< int > vec;
using value_type = typename std::decay< decltype(*begin(vec)) >::type; 
static_assert(std::is_same< int, value_type >::value, "expected int");

Upvotes: 1

aschepler
aschepler

Reputation: 72336

Keep using iterator_traits. decltype(*iterator) could even be some sort of weird proxy class in order to do special things in the expression *iter = something.

Example:

#include <iostream>
#include <iterator>
#include <typeinfo>
#include <vector>

template <typename T>
void print_type()
{
    std::cout << typeid(T).name() << std::endl;
}

template <typename Iterator>
void test(Iterator iter)
{
    typedef typename
        std::iterator_traits<Iterator>::value_type iter_traits_value;

    auto x = *iter;
    typedef decltype(x) custom_value;

    print_type<iter_traits_value>();
    print_type<custom_value>();
}

int main()
{
    std::vector<int> a;
    std::vector<bool> b;

    test(a.begin());
    test(b.begin());
}

Output on MSVC 2012:

int
int
bool
class std::_Vb_reference<struct std::_Wrap_alloc<class std::allocator<unsigned int>>>

They aren't the same.

Upvotes: 16

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