| Issue |
127307
|
| Summary |
libc++: `std::ranges::to` does not match standards.
|
| Labels |
libc++
|
| Assignees |
|
| Reporter |
AnonymousPC
|
This code below can work on gcc, but compile failed on clang++ (version=19.1.7).
```cpp
#include <ranges>
struct my_vector
{
void emplace_back(auto&&...) { /*assume we do something here*/; }
};
int main ( )
{
auto my_vec = std::vector<int>() | std::ranges::to<my_vector>();
}
```
- According to `https://en.cppreference.com/w/cpp/ranges/to`,
- A container is `ranges::toable` (non-recursive) iff container implements
- constructor `container(range&&)`, or
- constructor `container(std::from_range_t, range&&)`, or
- constructor `container(iterator, sentinel)`, or
- satisfies concept `container-appendable`.
- We now focus on the 4th case.
- In standard, A container is `container-appendable` iff container has member function `emplace_back()` or `push_back()` or `emplace()` or `insert()`.
- Then, we should use a for-loop to put the items into this container one-by-one.
- What libc++ did:
- in file `.../c++/v1/__ranges/to.h`
- define `__container_insertable` concept, which **only accepts a container who implements either `push_back()` or `insert()`**. libc++ did not care `emplace_back()` and `emplace()`.
- put items into container through `ranges::copy(__range, ranges::__container_inserter<range_reference_t<_Range>>(__result));`, which then calls `back_insert_iterator`(if push_back) or `insert_iterator`(if insert).
- back_insert_iterator has `operator=(container::value_type&&)` to *do* this push_back(), but **what if the container(which might be trivial) who does not typedef value_type?** same do insert_iterator.
- What is expected:
- Accepts all 4 kinds of "container-appendable" container.
- get rid of the fancy insert_iterator, an use a **trivial for-loop** to append elements one by one. (it only takes <= 5 lines)
# Thank you
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