Here's a link to relevant code:
#include <iostream>
#include <string>
#include <vector>
#include <type_traits>
int main()
{
std::vector<int> v{1, 2, 3, 4, 5};
auto iter = begin(std::move(v));
if(std::is_const<typename std::remove_reference<decltype(*iter)>::type>::value)
std::cout<<"is const\n";
return 0;
}
http://coliru.stacked-crooked.com/a/253c6373befe8e50
I ran into this behavior because of a declval<Container>()
in a decltype
expression with std::begin
. Both gcc and clang return iterators which yield const references when dereferenced. It probably makes sense since r-value references usually bind to expiring objects that you don't want to mutate. However, I could not find any documentation on this to determine whether it's mandated by the standard. I couldn't find any relevant overloads of begin()
or ref-qualified overloads of Container::begin()
.
Update: The answers clarified what's happening but the interactions can be subtle as demonstrated below:
#include <iostream>
#include <string>
#include <vector>
#include <type_traits>
int main()
{
if(std::is_const<typename std::remove_reference<decltype(*begin(std::declval<std::vector<std::string>>()))>::type>::value)
std::cout<<"(a) is const\n";
if(!std::is_const<typename std::remove_reference<decltype(*std::declval<std::vector<std::string>>().begin())>::type>::value)
std::cout<<"(b) is not const\n";
if(!std::is_const<typename std::remove_reference<decltype(*begin(std::declval<std::vector<std::string>&>()))>::type>::value)
std::cout<<"(c) is not const\n";
return 0;
}
http://coliru.stacked-crooked.com/a/15c17b288f8d69bd
Naively, you wouldn't expect different results for (a) and (b) when ::begin is just defined in terms of calling vector::begin. However the absence of std::begin overloads that take non-const r-value reference and return iterator (or ref-qualified vector::begin overloads which return const_iterator) cause exactly that to happen.
As you can see in http://en.cppreference.com/w/cpp/iterator/begin the interesting overloads are:
template<class C> auto begin(C& c) -> decltype(c.begin());
template<class C> auto begin(const C& c) -> decltype(c.begin());
and std::vector<int>&&
can only bind to the second overload (so returns const_iterator
).
Let's try to analyze what happens, step by step:
You're calling std::begin(std::vector<int>&&)
, but std::begin
has no overload that takes an rvalue :
template< class C > auto begin( C& c ) -> decltype(c.begin()); template< class C > auto begin( const C& c ) -> decltype(c.begin());
Due to reference collapsing , a temporary (xvalue) will only bind to a const
lvalue reference:
If you call Fwd with an xvalue, we again get Type&& as the type of v. This will not allow you to call a function that takes a non-const lvalue, as an xvalue cannot bind to a non-const lvalue reference. It can bind to a const lvalue reference, so if Call used a const&, we could call Fwd with an xvalue.
(From the linked answer) .
Therefore, the
template<class C> auto begin(const C& c) -> decltype(c.begin());
overload is being called, which returns a const
iterator.
Why?
Because std::begin(v)
calls v.begin()
, which returns a const_iterator
when called on const
instances of std::vector
.
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