C++ range-for and boost::irange - c++11

Question

I'm using boost::irange and created a helper function to simplify the code by removing the need for explicit template parameters. I don't understand why it doesn't work. Here's the code:
#include <iostream>
#include <boost/range/irange.hpp>
template<typename T>
boost::irange<T> range_from_zero(T limit)
{
return boost::irange<T>(T(), limit);
}
int main() {
size_t end = 100;
for (auto i : range_from_zero(0,end))
std::cout << i << ' ';
return 0;
}
There's a live version here https://ideone.com/VVvW6e, which produces compilation errors
prog.cpp:5:8: error: 'irange<T>' in namespace 'boost' does not name a type
boost::irange<T> range_from_zero(T limit)
^
prog.cpp: In function 'int main()':
prog.cpp:12:41: error: 'range_from_zero' was not declared in this scope
for (auto i : range_from_zero(0,end))
If I use boost::irange directly in the range-for, then it works:
#include <iostream>
#include <boost/range/irange.hpp>
int main() {
size_t end = 100;
for (auto i : boost::irange<size_t>(0,end))
std::cout << i << ' ';
return 0;
}
this works fine: https://ideone.com/TOWY6H
I thought maybe is was a problem using range-for on the return of a function, but it isn't; this works using a std::vector:
#include <iostream>
#include <boost/range/irange.hpp>
template<typename T>
std::vector<T> range_from_zero(T limit)
{
auto range = boost::irange<T>(T(), limit);
return { std::begin(range), std::end(range) };
}
int main() {
size_t end = 100;
for (auto i : range_from_zero(end))
std::cout << i << ' ';
return 0;
}
See https://ideone.com/TYRXnC
Any ideas, please?

Answer 1

But, first off, what's wrong with Live On Coliru
for (size_t i : irange(0, 100))
or even Live On Coliru
size_t end = 100;
for (auto i : irange(0ul, end))
irange is a function template, and it cannot be used as a return type.
The return type is integer_range or strided_integer_range. As such, irange is already the function you were looking for.
Only, you didn't pass arguments that could be unambiguously deduced. If you can to allow this, "copy" irange() implementation using separate template argument types for the boundary values and use e.g. std::common_type<T1,T2>::type as the range element.
Here's my stab at writing range_from_zero without naming implementation details in the interface:
Live On Coliru
#include <iostream>
#include <boost/range/irange.hpp>
template <typename T>
auto izrange(T upper) -> decltype(boost::irange(static_cast<T>(0), upper)) {
return boost::irange(static_cast<T>(0), upper);
}
int main() {
size_t end = 100;
for (size_t i : izrange(end))
std::cout << i << ' ';
}