Asked  6 Months ago    Answers:  5   Viewed   20 times

The following link provides the 4 forms of reference collapsing (if I'm correct that these are the only 4 forms): http://thbecker.net/articles/rvalue_references/section_08.html.

From the link:

  1. A& & becomes A&
  2. A& && becomes A&
  3. A&& & becomes A&
  4. A&& && becomes A&&

Although I can make an educated guess, I would like a concise explanation for the rationale behind each of these reference-collapsing rules.

A related question, if I might: Are these reference-collapsing rules utilized in C++11 internally by such STL utilities such as std::move(), std::forward(), and the like, in typical real-world use cases? (Note: I'm specifically asking whether the reference-collapsing rules are utilized in C++11, as opposed to C++03 or earlier.)

I ask this related question because I am aware of such C++11 utilities as std::remove_reference, but I do not know if the reference-related utilities such as std::remove_reference are routinely used in C++11 to avoid need for the reference-collapsing rules, or whether they are used in conjunction with the reference-collapsing rules.

 Answers

90

The reference collapsing rules (save for A& & -> A&, which is C++98/03) exist for one reason: to allow perfect forwarding to work.

"Perfect" forwarding means to effectively forward parameters as if the user had called the function directly (minus elision, which is broken by forwarding). There are three kinds of values the user could pass: lvalues, xvalues, and prvalues, and there are three ways that the receiving location can take a value: by value, by (possibly const) lvalue reference, and by (possibly const) rvalue reference.

Consider this function:

template<class T>
void Fwd(T &&v) { Call(std::forward<T>(v)); }

By value

If Call takes its parameter by value, then a copy/move must happen into that parameter. Which one depends on what the incoming value is. If the incoming value is an lvalue, then it must copy the lvalue. If the incoming value is an rvalue (which collectively are xvalues and prvalues), then it must move from it.

If you call Fwd with an lvalue, C++'s type-deduction rules mean that T will be deduced as Type&, where Type is the type of the lvalue. Obviously if the lvalue is const, it will be deduced as const Type&. The reference collapsing rules mean that Type & && becomes Type & for v, an lvalue reference. Which is exactly what we need to call Call. Calling it with an lvalue reference will force a copy, exactly as if we had called it directly.

If you call Fwd with an rvalue (ie: a Type temporary expression or certain Type&& expressions), then T will be deduced as Type. The reference collapsing rules give us Type &&, which provokes a move/copy, which is almost exactly as if we had called it directly (minus elision).

By lvalue reference

If Call takes its value by lvalue reference, then it should only be callable when the user uses lvalue parameters. If it's a const-lvalue reference, then it can be callable by anything (lvalue, xvalue, prvalue).

If you call Fwd with an lvalue, we again get Type& as the type of v. This will bind to a non-const lvalue reference. If we call it with a const lvalue, we get const Type&, which will only bind to a const lvalue reference argument in Call.

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.

If you call Fwd with a prvalue, we again get Type&&, so everything works as before. You cannot pass a temporary to a function that takes a non-const lvalue, so our forwarding function will likewise choke in the attempt to do so.

By rvalue reference

If Call takes its value by rvalue reference, then it should only be callable when the user uses xvalue or rvalue parameters.

If you call Fwd with an lvalue, we get Type&. This will not bind to an rvalue reference parameter, so a compile error results. A const Type& also won't bind to an rvalue reference parameter, so it still fails. And this is exactly what would happen if we called Call directly with an lvalue.

If you call Fwd with an xvalue, we get Type&&, which works (cv-qualification still matters of course).

The same goes for using a prvalue.

std::forward

std::forward itself uses reference collapsing rules in a similar way, so as to pass incoming rvalue references as xvalues (function return values that are Type&& are xvalues) and incoming lvalue references as lvalues (returning Type&).

Tuesday, June 1, 2021
 
Crashthatch
answered 6 Months ago
43

They do introduce ambiguity for many algorithms. A lot of <algorithm> looks like

template<class iterator>
void do_something(iterator, iterator);

template<class iterator, class funct>
void do_something(iterator, iterator, funct);

If you add additional overloads

template<class container, class funct>
void do_something(container, funct);

the compiler will have some trouble figuring out what do_something(x, y) means. If x and y are of the same type, it will match both iterator = type and container = type, funct = type.*)

C++11 tried to solve this with "concepts" that could recognize the difference between a container and an iterator. However, these "concepts" turned out to be too complicated to make it into the standard, so neither did these overloads.

*) compilers disagree here, the Comeau compiler claims that it is ambiguous, g++ 4.5 and MSVC 10 calls the first function.


After an extremely long discussion in the comments, here is one example where it doesn't work as expected - using a container adapter that can also double as a predicate.

#include <iostream>
#include <vector>

template<class iterator>
void test(iterator, iterator)
{
   std::cout << "test iteratorn";
}

template<class iterator, class predicate>
void test(iterator, iterator, predicate)
{
   std::cout << "test iterator, predicaten";
}

template<class container, class predicate>
void test(const container& cont, predicate compare)
{
   std::cout << "test container, predicaten";

   test(cont.begin(), cont.end(), compare);
}

template<class container>
class adapter
{
public:
   typedef typename container::iterator   iterator;

   adapter(container* cont) : cont(cont)
   { }

   iterator begin() const
   { return cont->begin(); }

   iterator end() const
   { return cont->end(); }

   bool operator()(const iterator& one, const iterator& two)
   { return *one < *two; }

private:
   container* cont;
};

int main()
{
   std::vector<int>   v;

   adapter<std::vector<int>>   a(&v);

   test(a, a);

}

Output:

test iterator

http://ideone.com/wps2tZ

Saturday, July 3, 2021
 
fhonics
answered 5 Months ago
93

As noted in other answers and comments, you should just use vector's built-in functionality for this. But:

When you reserve() elements, the vector will allocate enough space for (at least?) that many elements. The elements do not exist in the vector, but the memory is ready to be used. This will then possibly speed up push_back() because the memory is already allocated.

When you resize() the vector, it will allocate enough space for those elements, but also add them to the vector.

So if you resize a vector to 100, you can access elements 0 - 99, but if you reserve 100 elements, they are not inserted yet, just ready to be used.

What you want is something like this:

vec2.reserve( vec1.size() );
copy(vec1.begin(), vec1.end(), std::back_inserter(vec2));

std::back_inserter is defined in <iterator>

Sunday, August 1, 2021
 
Pegues
answered 4 Months ago
90

The other answers weren't actually solutions to my problem and I never figured out what was REALLY causing the issue. However, this is how I solved it: My workaround was to copy the .9.pngs and styling for the EditText widget from Ice Cream Sandwich and hardcoded into my app for Honeycomb and Ice Cream Sandwich.

EDIT:

I created a file called res/drawable-nodpi/edit_text_holo_light.xml with the following:

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">

    <item android:state_multiline="true" android:state_window_focused="false" android:state_enabled="true"  android:drawable="@drawable/textfield_multiline_default_holo_light" />
    <item android:state_multiline="true" android:state_window_focused="false" android:state_enabled="false" android:drawable="@drawable/textfield_multiline_disabled_holo_light" />
    <item android:state_multiline="true" android:state_enabled="true" android:state_focused="true" android:drawable="@drawable/textfield_multiline_activated_holo_light" />
    <item android:state_multiline="true" android:state_enabled="true" android:state_activated="true" android:drawable="@drawable/textfield_multiline_focused_holo_light" />
    <item android:state_multiline="true" android:state_enabled="true" android:drawable="@drawable/textfield_multiline_default_holo_light" />
    <item android:state_multiline="true" android:state_focused="true" android:drawable="@drawable/textfield_multiline_disabled_focused_holo_light" />
    <item android:state_multiline="true" android:drawable="@drawable/textfield_multiline_disabled_holo_light" />

    <item android:state_window_focused="false" android:state_enabled="true" android:drawable="@drawable/textfield_default_holo_light" />
    <item android:state_window_focused="false" android:state_enabled="false" android:drawable="@drawable/textfield_disabled_holo_light" />
    <item android:state_enabled="true" android:state_focused="true" android:drawable="@drawable/textfield_activated_holo_light" />
    <iten android:state_enabled="true" android:state_activated="true" android:drawable="@drawable/textfield_focused_holo_light" />
    <item android:state_enabled="true" android:drawable="@drawable/textfield_default_holo_light" />
    <item android:state_focused="true" android:drawable="@drawable/textfield_disabled_focused_holo_light" />
    <item android:drawable="@drawable/textfield_disabled_holo_light" />
</selector>

Then I created a style in my styles.xml to set:

<item name="android:background">@drawable/edit_text_holo_light</item>

Then I copied the .9.png files from the android sdk and put them in res/drawable-*. The filenames are listed in the above xml.

Tuesday, August 3, 2021
 
Jubair
answered 4 Months ago
21

Ok - this one was pretty dumb. I changed my container from a LinearLayout to a FrameLayout and voilà: it's working.

Wednesday, August 4, 2021
 
Lime
answered 4 Months ago
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