Reputation: 1033
I have a function where I need an iterator's underlying data type as return type, like this:
#include <iostream>
#include <vector>
#include <iterator>
template<class T, class ForwardIterator>
std::vector<T> get_odd(ForwardIterator data_begin, ForwardIterator data_end)
{
std::vector<T> ret;
std::copy_if(data_begin, data_end, std::back_inserter(ret), [](int x) { return x % 2; });
return ret;
}
int main()
{
std::vector<int> vi { 1, 2, 3, 4, 5 };
for (auto i : get_odd<int>(vi.begin(), vi.end()))
std::cout << i << ", ";
std::cout << std::endl;
std::vector<unsigned int> vui{ 9UL, 8UL, 7UL, 6UL, 5UL };
// Note the 'char' I provided to the function, this will print weird chars
for (auto i : get_odd<char>(vui.begin(), vui.end()))
std::cout << i << ", ";
std::cout << std::endl;
return 0;
}
In the main(), I have to explicitly provide data type 'int' or 'char' to the get_odd function, is there a way to find out iterator's underlying data type, and let template automatically deduct the correct data type?
Upvotes: 5
Views: 305
Reputation: 42594
This comes up often enough that I use a convenient template alias to wrap iterator_traits
:
template <typename I>
using ValueType = typename std::iterator_traits<I>::value_type;
template<class ForwardIterator>
std::vector<ValueType<ForwardIterator>>
get_odd(ForwardIterator data_begin, ForwardIterator data_end)
{
std::vector<ValueType<ForwardIterator>> ret;
std::copy_if(data_begin, data_end, std::back_inserter(ret), [](int x) {
return x % 2;
});
return ret;
}
Upvotes: 2
Reputation: 9403
If your compiler supports C++11, you can use:
#include <type_traits>
// ...
typename std::remove_reference<decltype(*data_begin)>::type
And probably typedef
for convenience:
typedef typename std::remove_reference<decltype(*data_begin)>::type ContainedType;
std::vector<ContainedType> ret;
Upvotes: 2
Reputation: 48527
Use std::iterator_traits:
template<class ForwardIterator>
std::vector<typename std::iterator_traits<ForwardIterator>::value_type> get_odd(ForwardIterator data_begin, ForwardIterator data_end)
{
....
}
Upvotes: 0
Reputation: 171197
Iterator properties can be queried using std::iterator_traits
:
template<class ForwardIterator>
std::vector<typename std::iterator_traits<ForwardIterator>::value_type>
get_odd(ForwardIterator data_begin, ForwardIterator data_end)
{
std::vector<typename std::iterator_traits<ForwardIterator>::value_type> ret;
std::copy_if(data_begin, data_end, std::back_inserter(ret), [](int x) { return x % 2; });
return ret;
}
Upvotes: 6