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Reference Wrappers Explained
Wednesday, 14 July 2010
The
upcoming C++0x
standard includes reference wrappers in the form of
the std::reference_wrapper<T> class template, and
the helper function templates std::ref()
and std::cref(). As I mentioned in my blog post
on Starting
Threads with Member Functions and Reference Arguments, these
wrappers can be used to pass references to objects across interfaces
that normally require copyable (or at least movable) objects
— in that blog post, std::ref was used for passing
references to objects over to the new thread, rather than copying the
objects. I was recently asked what the difference was
between std::ref and std::cref, and how they
worked, so I thought I'd elaborate.
Deducing the Referenced Type
std::ref is a function template, so automatically
deduces the type of the wrapped reference from the type of the
supplied argument. This type deduction includes
the const-ness of the supplied object:
int x=3; const int y=4; std::reference_wrapper<int> rx=std::ref(x); // std::reference_wrapper<int> ry=std::ref(y); // error std::reference_wrapper<const int> rcy=std::ref(y);
On the other hand, though std::cref also deduces the
type of the wrapped reference from the supplied argument,
it always wraps a const reference:
int x=3; const int y=4; // std::reference_wrapper<int> rx=std::cref(x); // error std::reference_wrapper<const int> rcx=std::cref(x); // std::reference_wrapper<int> ry=std::cref(y); // error std::reference_wrapper<const int> rcy=std::cref(y);
Since a no-const-reference can always be bound to
a const reference, you can thus
use std::ref in pretty much every case where you would
use std::cref, and your code would work the same. Which
begs the question: why would you ever choose to
use std::cref?
Using std::cref to prevent modification
The primary reason for choosing std::cref is because
you want to guarantee that the source object is not modified through
that reference. This can be important when writing multithreaded
code — if a thread should not be modifying some data then it
can be worth enforcing this by passing a const
reference rather than a mutable reference.
void foo(int&); // mutable reference int x=42; // Should not be modified by thread std::thread t(foo,std::cref(x)); // will fail to compile
This can be important where there are overloads of a function such
that one takes a const reference, and the other a
non-const reference: if we don't want the object
modified then it is important that the overload taking
a const reference is chosen.
struct Foo
{
void operator()(int&) const;
void operator()(int const&) const;
};
int x=42;
std::thread(Foo(),std::cref(x)); // force const int& overload
References to temporaries
std::cref has another property missing
from std::ref — it can bind to temporaries, since
temporary objects can bind to const references. I'm
not sure this is a good thing, especially when dealing with multiple
threads, as the referenced temporary is likely to have been
destroyed before the thread has even started. This is therefore
something to watch out for:
void bar(int const&); std::thread t(bar,std::cref(42)); // oops, ref to temporary
Documentation
Finally, std::cref serves a documentation purpose,
even where std::ref would suffice — it declares
in big bold letters that this reference cannot be used to modify the
referenced object, which thus makes it easier to reason about the
code.
Recommendation
I would recommend that you use std::cref in preference
to std::ref whenever you can — the benefits as
documentation of intent, and avoiding accidental modification
through the reference make it a clear winner in my opinion. Of
course, if you do want to modify the referenced
object, then you need to use std::ref, but such usage
now stands out, and makes it clear that this is the intent.
You do still need to be careful to ensure that you don't try and
wrap references to temporaries, particularly when
applying std::cref to the result of a function call,
but such uses should stand out — I expect most uses to be
wrapping a reference to a named variable rather than wrapping a
function result.
Posted by Anthony Williams
[/ cplusplus /] permanent link
Tags: reference wrappers, ref, cref, cplusplus
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20 Comments
Very informative blog. I like your blog because of unique content. thanks for sharing such a useful information.
You might be interested to learn that the latest C++0x draft contains deleted versions of both ref and cref:
template <class T> void ref(const T&& t) = delete; template <class T> void cref(const T&& t) = delete;
This prevents ref and cref from binding to temporaries.This post includes all information that I have always required. Although on internet we find a lot of information but the practical information is limited which you have originated in this post.
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