Re: Upgrading C++ library features.

On 9/24/26 09:46, Zaphod Beeblebrox wrote:
> I'm currently running FreeBSD 15.1.  I found out that in c++20 you can
> now say:
> 
> for(auto [i, j] : v | std::views:enumerate)
> 
> ... which I would very much like to.  FreeBSD 15.1 has clang 19 as
> standard, which supposedly support c++20 and more, but some searching
> leads me to conclude that enumerate is a late addition to clang.
> 
> So I installed clang23 to see.  This still kicks out the error --- but
> I'm expecting this is about include files.  Does clang23 come with new
> include files --- or can I access them.  I suppose there is a similar
> question about libc++ ... even though this specific problem doesn't
> involve the library proper, AFAIK.
> 
> Help?

For the most part, libc++ is include files. Something like
std::views:enumerate would be from the libc++ headers.

The FreeBSD OS supports only its own libc++ header files, no matter
which LLVM or clang++ compiler version is in use. That includes if one
uses a devel/llvm* port (or package).

The LLVM development is not linear in C++ standard versions. There are
still things missing that are defined in c++17 (so somewhat under a
decade ago). There are other things from much more recent standard
versions that are implemented. One has to track status item by item,
rather than a C++ standard version as a a unit. (Some things have
"nothing to do" because they were implemented before the standard for
whatever the issue was.)

You do not give the actual error message(s). Some of the notation in:

for(auto [i, j] : v | std::views:enumerate)

is not from libc++ and other parts of it are. There is no clue given
about which parts ended up being referenced as wrong/unsupported by the
error message(s).

For:

#include <iostream>
#include <vector>
#include <ranges>
#include <tuple>

int main() {
    std::vector<std::tuple<int,uint>> t = {};
    std::vector<int>             v = {};

    for (auto [i, j] : t) {
        std::cout << "Index: " << i << ", Value: " << j << "\n";
    }

    for (auto [i, j] : std::views::enumerate(v)) {
        std::cout << "Index: " << i << ", Value: " << j << "\n";
    }

    for (auto [i, j] : v | std::views::enumerate) {
        std::cout << "Index: " << i << ", Value: " << j << "\n";
    }

    return 0;
}


On a main (a.k.a. CURRENT) FreeBSD OS I get:

# c++ -std=c++23 c23_structured_binding_example.cc
c23_structured_binding_example.cc:14:36: error: no member named
'enumerate' in namespace 'std::ranges::views'
   14 |     for (auto [i, j] : std::views::enumerate(v)) {
      |                                    ^~~~~~~~~
c23_structured_binding_example.cc:18:40: error: no member named
'enumerate' in namespace 'std::ranges::views'
   18 |     for (auto [i, j] : v | std::views::enumerate) {
      |                                        ^~~~~~~~~
2 errors generated.

# c++ -std=c++23 -fexperimental-library \
c23_structured_binding_example.cc
c23_structured_binding_example.cc:14:36: error: no member named
'enumerate' in namespace 'std::ranges::views'
   14 |     for (auto [i, j] : std::views::enumerate(v)) {
      |                                    ^~~~~~~~~
c23_structured_binding_example.cc:18:40: error: no member named
'enumerate' in namespace 'std::ranges::views'
   18 |     for (auto [i, j] : v | std::views::enumerate) {
      |                                        ^~~~~~~~~
2 errors generated.


Those are complaints about something missing in the libc++ headers. Note
that the first "for(. . .)" got no complaints at all. Nor were there any
complaints about std::views of itself.


For Reference:

# c++ -v
FreeBSD clang version 21.1.8 (https://github.com/llvm/llvm-project.git
llvmorg-21.1.8-0-g2078da43e25a)
Target: aarch64-unknown-freebsd16.0
Thread model: posix
InstalledDir: /usr/bin

https://libcxx.llvm.org/Status/Cxx23.html#cxx23-status reports:

P2164R9 views::enumerate 2023-02 (Issaquah) Complete 23 #105251

That 23 is for LLVM23 and its libc++23 being the "First released
version". Some things are incompletely implemented well before they are
fully implemented/released. -fexperimental-library sometimes allows use
of some incompletely implemented status items.

Until the FreeBSD OS version of interest is based o LLVM23 and its
libc++23 (or later versions of such), std::views::enumerate will not be
operational as far as I can tell.


-- 
===
Mark Millard
marklmi at yahoo.com