布尔值向量的基于范围的 for 循环

Question

std::vector<bool> uses proxy iterators.

So the following code will not compile (code taken from the accepted answer in related question ):

vector<bool> v = {true, false, false, true};
for (auto& x : v)
    x = !x;

In the related question, the accepted answer states that to modify the components of the vector in place, we must use

for (auto&& x : v)
    x = !x;

But if I simply do:

for (auto x : v)
    x = !x;

This produces identical results. So is the && not needed?

Further why does the following 2 codes not modify the components?

for (bool &&x : v)
    x = !x;

and

for (bool x : v)
    x = !x;

Answer 1

TL;DR: The proxy object knows how to read and write the single bits, regardless of how you keep it. Converting the proxy object to bool loses that information.

---

for (auto&& x : v)
    x = !x;

and

for (auto x : v)
    x = !x;

have the same behavior because in each case the proxy object ( std::vector<bool>::reference ) obtained from dereferencing a std::vector<bool>::iterator is stored in x . Whether the proxy object is stored by value or reference doesn't matter - its behavior of modifying the proxied bit is the same.

In

for (bool &&x : v)
    x = !x;

and

for (bool x : v)
    x = !x;

the proxy object is implicitly converted to a bool . This necessarily loses the information needed (and thus the capability) to affect the compressed bit.

Note that these are all implementation-defined. Your implementation is allowed to forego the space optimization too, in which case the behavior you see could be different. Only auto&& works in every case.