Why doesn't range-based for loop match rvalue-reference modifier of this?

Question

#include <vector>

class MyContainer {
public:
    std::vector<int> data;

    // begin() only defined for rvalues
    auto begin() && { return data.begin(); }
    auto end() && { return data.end(); }
};

MyContainer container;
container.data = {1, 2, 3};

// why not working ? compile error;
for (int x : MyContainer(container)) {
      std::cout << x << " ";
}

clang c++ 20 complain:

'this' argument to member function 'begin' is an lvalue, but function has rvalue ref-qualifier;

but why ? MyContainer(container) is rvalue, it SHOULD match begin() &&, right ?
I know using MyContainer(container).begin() is ok, but how to use it in for range loop directly?

update: why i want those to work ?

for iter = xxx.begin(); if xxx is rvalue, *iter return rvalue too; so, for

for (auto &&e : Container()) {
    x = e;         // call move directly;
}

for (auto &&e : container) {
    x = e;         // call copy directly;
}

it's very convenient for implements many move aware of algorithm; but seems standard not aware of it for now;
hops it update in future!

solution for current c++ standard (20)

for (auto itb = move(container).begin(), ite = move(container).end(); itb != ite; ++ itb) {
    x = *itb;            // ok, call move;
}

for (auto &&e : container) {
    x = e;              // ok, call copy as it is;
}

wohlstad · Accepted Answer · 2025-06-01 05:01:51Z

2

As you can see in cppreference.com, a ranged-based for loop is equivalent to a regular for loop, scoped with some variables defined, and with specific init-statement, condition etc.

There are some changes depending on the C++ version, but for C++20 that you use it is:

{
    init-statement
    auto&& /* range */ = range-initializer ;
    auto /* begin */ = /* begin-expr */;
    auto /* end */ = /* end-expr */;
    for ( ; /* begin */ != /* end */; ++/* begin */)
    {
        item-declaration = */* begin */;
        statement
    }
}

It is a bit different in earlier C++ versions, but anyway since C++11 there is always an equivalent for a variable:

auto&& /* range */ = range-initializer ;

This variable is the one used for calling begin() and end().
But it has a name and is therefore an lvalue (even if you supply an rvalue as your original range-like object).

This is the reason the compiler rejects calling the rvalue ref (&&) qualified begin() and end() with it.

In order to use a ranged-based for loop you should remove the && qualification from them.

edited Jun 1 at 5:01

answered May 31 at 14:46

wohlstad

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9 Comments

Ashcoll Ash May 31 at 14:57

i don't think c++ specification says it always a lvalue; even a variable is lvalue, but it can forward to rvalue;

wohlstad May 31 at 15:13

Not sure I follow. It is specified that the ranged-based loop is equivalent to a normal for loop with some variables. The variables are lvalues, so you cannot call your begin(),end() with it.

Jarod42 May 31 at 15:35

Spec might have added some std::forward to make OP's code works...

Jarod42 May 31 at 15:59

Just speculation, but they already missed some temporary lifetime extension in C++11, move was young at that stage.

Eljay May 31 at 17:53

I would not expect to be able to call end() on a temporary that had already been consumed by a begin() and left in a valid-but-unspecified state.

|

Joel · Accepted Answer · 2025-05-31 15:08:47Z

8.6.4.1 [stmt.ranged]

The range-based for statement

for ( init-statement for-range-declaration : for-range-initializer ) statement

is equivalent to

{
  init-statement

  auto &&range = for-range-initializer ;
  auto begin = begin-expr ;
  auto end = end-expr ;
  for ( ; begin != end; ++begin ) {
      for-range-declaration = * begin ;
      statement
  }
}

The above quote from the C++ standard suggests that your loop will look like the following snippet:

{
    auto&& range = MyContainer(container);
    auto begin = range.begin();
    auto end = range.end();
    for(; begin != end; ++begin) {
        int x = *begin;
        std::cout << x << " ";
    }
}

As you can see, begin() and end() are both called on range, which is a named l-value at this point.

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Why doesn't range-based for loop match rvalue-reference modifier of this?

update: why i want those to work ?

solution for current c++ standard (20)

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update: why i want those to work ?

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