Hi everyone, According to standard

8.6.4 The range-based for statement

The range-based for statement

for ( init-statementopt for-range-declaration : for-range-initializer ) statement

is equivalent to

{
init-statementopt auto &&range = for-range-initializer ;
auto begin = begin-expr ;

auto end = end-expr ;

for ( ; begin != end ; ++begin ) {

for-range-declaration = * begin ;

statement

}

}

begin-expr and end-expr are determined as follows:

if the for-range-initializer is an expression of class type C, the unqualified-ids begin and end are looked up in the scope of C as if by class member access lookup (6.5.5), and if both find at least one declaration, begin-expr and end-expr are range.begin() and range.end(), respectively;

This means that if I have code like this, it will not compile, regardless of the fact that I provided begin() and end() in my class.

#include <iostream>
#include <memory>
#include <vector>

template<typename Iter>
struct range {

range(Iter b, Iter e)
: _begin(b)
, _end(e)
{}

Iter begin() && {
return _begin;
}

Iter end() && {
return _end;
}

private:
Iter _begin;
Iter _end;
};

template<std::size_t N, typename Container>
auto head(const Container& cnt) {
auto b = cnt.begin();
auto e = b;
std::advance(e, N);
return range(b, e);
}

int main() {

std::vector<int> v = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

for(auto v : head<7>(v)) {
std::cout << v << std::endl;
}

return 0;
}

The code introduces a template class range designed to represent a range of elements using iterators. Notably, it employs rvalue references (`&&`) for the begin() and end() member functions. Additionally, the code includes a template function head that extracts the first N elements from a container.

Proposal

If I rewrite the range-based for like this

int main() {

std::vector<int> v = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

// for(auto v : head<7>(v)) {
// std::cout << v << std::endl;
// }

{
auto&& range = head<7>(v);
auto begin = std::forward<decltype(range)>(range).begin();
auto end = std::forward<decltype(range)>(range).end();
for ( ; begin != end ; ++begin ) {
auto v = * begin ;
std::cout << v << std::endl;
}
}

return 0;
}

it will work properly, and the `begin()` and `end()` functions that are provided will be called.

I think this might be beneficial in cases where we want to avoid iterator invalidation, and we can encourage our class users to use it as an rvalue in such a context.

Why is the range variable not forwarded? Is there a specific reason for this?