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Why does std::declval<int&>() returns lvalue reference?

RicoJ (24)
Below the return value is “is lvalue”. However, in the documentation, (https://www.cplusplus.com/reference/utility/declval/), it seems that we always get rvalue reference from std::declval. why? 

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if (std::is_lvalue_reference<decltype(std::declval<int>())>::value) cout<<"is lvalue ref"<<endl;
            else cout<<"not lvalue ref"<<endl;



Last edited on
deleted account xyzzy (5768)
The URL parser here fooked up your link. Put a space before at the end of a link with trailing characters so when someone clicks the link it isn't a 404.

You can also use BB tags instead of the space(s) (https://www.cplusplus.com/reference/utility/declval/). I use empty opening and closing teletype tags after the link and before the closing ).
deleted account xyzzy (5768)
mbozzi (3932)
Below the return value is “is lvalue”

No, the type decltype(std::declval<int>()) is exactly int&& as evidenced by this program:
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#include <type_traits>
#include <utility>
static_assert(std::is_same_v<decltype(std::declval<int>()), int&&>);

https://coliru.stacked-crooked.com/a/f33f7ea5775c936c

It seems that we always get rvalue reference from std::declval. why?

Not true, if you specify an lvalue reference type as a template argument.

To give a concrete example, here is the signature for std::declval:
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template <class T>
  typename add_rvalue_reference<T>::type declval() noexcept;

As the template shows, the return type of std::declval<int&> is typename add_rvalue_reference<int&>::type, which is an lvalue reference type as shown below:

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#include <type_traits>
#include <utility>

static_assert(std::is_same_v<std::add_rvalue_reference_t<int>, int&&>);
static_assert(std::is_same_v<std::add_rvalue_reference_t<int&>, int&>); // <--
static_assert(std::is_same_v<std::add_rvalue_reference_t<int&&>, int&&>);
using lvalue_ref = int&;
using rvalue_ref = int&&;
static_assert(std::is_same_v<lvalue_ref&, int&>);
static_assert(std::is_same_v<lvalue_ref&&, int&>);
static_assert(std::is_same_v<rvalue_ref&, int&>);
static_assert(std::is_same_v<rvalue_ref&&, int&&>);

These are examples of reference collapsing. Reference collapsing was introduced in C++11 to support perfect forwarding.

One of these days I'm going to write a tutorial for this. We get a slow but steady stream of questions about this topic, and they're not easy.
Last edited on
seeplus (6591)
Scott Myers has a whole section on reference collapsing in his book "Effective Modern C++" - Item 28. The while of part 5 deals with move/forward/universal references etc.

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