Questions tagged [reference-wrapper]
107 questions
145
votes
4 answers
What is the difference between std::reference_wrapper and a simple pointer?
Why is there a need to have std::reference_wrapper? Where should it be used? How is it different from a simple pointer? How its performance compares to a simple pointer?
Laurynas Lazauskas
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18
votes
2 answers
Why can template instances not be deduced in `std::reference_wrapper`s?
Suppose I have some object of type T, and I want to put it into a reference wrapper:
int a = 5, b = 7;
std::reference_wrapper p(a), q(b); // or "auto p = std::ref(a)"
Now I can readily say if (p < q), because the reference wrapper has a…
Kerrek SB
- 464,522
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18
votes
2 answers
Issue with std::reference_wrapper
The issue is clear with the following code:
#include
#include
#include
int main() {
//std::vector a, b;
int a = 0, b = 0;
auto refa = std::ref(a);
auto refb = std::ref(b);
std::cout << (refa < refb)…
Lingxi
- 14,579
- 2
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- 93
16
votes
4 answers
Why does std::reference_wrapper not accept a temporary?
Normally, rvalues can bind to const references (const SomeType&). It's built into the language. However, std::reference_wrapper does not accept an rvalue as its constructor argument since the corresponding overload is deliberately deleted.…
isarandi
- 3,120
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14
votes
1 answer
Why standard containers use function templates instead of non-template Koenig operators
This question is inspired by Issue with std::reference_wrapper. Let' say, for example, operator< for std::vector. It's defined as a function template as
template< class T, class Alloc >
bool operator<( const vector& lhs,
…
Lingxi
- 14,579
- 2
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- 93
11
votes
1 answer
Constant reference_wrapper
In the following code, the aim is to have a reference_wrapper b such that when a changes, b also changes however, the opposite should not be allowed that is, a should not change when b changes. I tried two ways: Lines 7 and 8. Line 7 caused…
Shibli
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11
votes
3 answers
Why does std::make_tuple turn std::reference_wrapper arguments into X&?
In the C++11 standard it states that (see cppreference.com, see also section 20.4.2.4 of the standard) it states that
template< class... Types >
tuple make_tuple( Types&&... args );
Creates a tuple object, deducing the target type from…
Ralph Tandetzky
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- 120
10
votes
1 answer
Use std::reference_wrapper in std::map
I thought maps and reference_wrappers would go easy but I tripped over something weird:
#include
WorldSEnder
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9
votes
2 answers
Is there any guarantee on the size of an std::reference_wrapper?
Consider the case in which I've enough storage to host a void * and therefore to construct in-place a pointer.
Is there any guarantee that the same storage is big enough to always store also an std::reference_wrapper?
Kind of (written out of my…
skypjack
- 49,335
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9
votes
1 answer
Should std::reference_wrapper contain the default comparator "<" operator?
The STL uses the "less than" as the default comparator. An STL comparator call on an object wrapped with the reference_wrapper<> does not compile, even if the underlying class has the "<" operator defined.
It seems, this is because there is no…
VSOverFlow
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8
votes
2 answers
Ambiguous constructor taking std::reference_wrapper when compiling with -pedantic
I have a class with both a copy constructor and a constructor taking a std::reference_wrapper:
#include
#include
class Class {
public:
Class() {
std::cout << "Class()" << std::endl;
}
Class(Class const &)…
zennehoy
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7
votes
1 answer
std::reference_wrapper, constructor implementation explaination
I have been trying to understand the implementation of std::reference_wrapper, from here, which is as follows:
namespace detail {
template constexpr T& FUN(T& t) noexcept { return t; }
template void FUN(T&&) = delete;
}
…
warrior_monk
- 383
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- 14
7
votes
2 answers
Why can't I sort a vector of reference_wrapper?
I want a sorted view of a std::vector but I don't want to modify the original container. std::reference_wrapper seems perfect for this and it works just fine for a vector of integers.
I've created this small…
Jesper Juhl
- 30,449
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7
votes
2 answers
How to convert std::vector > to std::vector
I have a local std::vector > and now I want to return a real copy of its elements (i.e std::vector). Is there a better way than a loop?
Example:
std::vector foobar() {
std::vector >…
Dmytrii S.
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6
votes
1 answer
std::optional> - is it OK?
Is
std::optional>
is conforming to the Standard (or draft) of C++17?
The Standard explicitly says, that std::optional for a reference type is ill-formed. But does it include reference_wrapper?
vladon
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