Get BOUT++ ready for C++20 #3264

dschwoerer · 2026-02-03T12:04:58Z

No description provided.

Otherwise fmt tries to do compile-time checks, but fails as it cannot be checked at compile time. fmtlib/fmt#4179

With C++ std::accumulate uses std::move, which fails with the reference.

isatty is provided by both cpptrace and unistd.h, so we should only include one.

ZedThree · 2026-02-03T12:14:41Z

Awesome, thanks @dschwoerer! Disappointing that we have to use fmt::runtime in many places. It might be the case that we can make at least some of them constexpr instead?

github-actions

clang-tidy made some suggestions

github-actions · 2026-02-03T12:14:44Z

include/bout/boutexception.hxx

  template <class S, class... Args>
  BoutException(S&& format, Args&&... args)
-      : BoutException(fmt::format(std::forward<S>(format),
+      : BoutException(fmt::format(fmt::runtime(std::forward<S>(format)),


warning: no header providing "fmt::runtime" is directly included [misc-include-cleaner]

include/bout/boutexception.hxx:7:

- #include "fmt/core.h" + #include "fmt/base.h" + #include "fmt/core.h"

github-actions · 2026-02-03T12:14:45Z

include/bout/msg_stack.hxx

  template <class S, class... Args>
  int push(const S& format, const Args&... args) {
-    return push(fmt::format(format, args...));
+    return push(fmt::format(fmt::runtime(format), args...));


warning: do not implicitly decay an array into a pointer; consider using gsl::array_view or an explicit cast instead [cppcoreguidelines-pro-bounds-array-to-pointer-decay]

return push(fmt::format(fmt::runtime(format), args...)); ^

github-actions · 2026-02-03T12:14:45Z

include/bout/msg_stack.hxx

  template <class S, class... Args>
  int push(const S& format, const Args&... args) {
-    return push(fmt::format(format, args...));
+    return push(fmt::format(fmt::runtime(format), args...));


warning: no header providing "fmt::runtime" is directly included [misc-include-cleaner]

include/bout/msg_stack.hxx:26:

- class MsgStack; + #include "fmt/base.h" + class MsgStack;

github-actions · 2026-02-03T12:14:45Z

include/bout/optionsreader.hxx

  template <class S, class... Args>
  void read(Options* options, const S& format, const Args&... args) {
-    return read(options, fmt::format(format, args...));
+    return read(options, fmt::format(fmt::runtime(format), args...));


warning: no header providing "fmt::runtime" is directly included [misc-include-cleaner]

include/bout/optionsreader.hxx:31:

- class OptionsReader; + #include "fmt/base.h" + class OptionsReader;

github-actions · 2026-02-03T12:14:45Z

include/bout/output.hxx

  template <class S, class... Args>
  Output(const S& format, Args&&... args)
-      : Output(fmt::format(format, std::forward<decltype(args)>(args)...)) {}
+      : Output(fmt::format(fmt::runtime(format), std::forward<decltype(args)>(args)...)) {


warning: no header providing "fmt::runtime" is directly included [misc-include-cleaner]

include/bout/output.hxx:25:

- class Output; + #include "fmt/base.h" + class Output;

github-actions · 2026-02-03T12:14:45Z

src/sys/options.cxx

  for (const auto& child : children) {
    if (child.second.isValue()) {
-      fmt::format_to(ctx.out(), format_string, child.second);
+      fmt::format_to(ctx.out(), fmt::runtime(format_string), child.second);


warning: no header providing "fmt::format_to" is directly included [misc-include-cleaner]

fmt::format_to(ctx.out(), fmt::runtime(format_string), child.second); ^

github-actions · 2026-02-03T12:14:45Z

src/sys/options/options_ini.cxx


  // Call recursive function to write to file
-  fout << fmt::format("{:uds}", *options);
+  fout << fmt::format(fmt::runtime("{:uds}"), *options);


warning: no header providing "fmt::format" is directly included [misc-include-cleaner]

src/sys/options/options_ini.cxx:52:

+ #include "fmt/format.h"

github-actions · 2026-02-03T12:14:45Z

src/sys/options/options_ini.cxx


  // Call recursive function to write to file
-  fout << fmt::format("{:uds}", *options);
+  fout << fmt::format(fmt::runtime("{:uds}"), *options);


warning: no header providing "fmt::runtime" is directly included [misc-include-cleaner]

src/sys/options/options_ini.cxx:52:

+ #include "fmt/base.h"

github-actions · 2026-02-03T12:14:45Z

tests/unit/fake_mesh.hxx

-                    .unique());
+    addRegion2D("RGN_BNDRY", std::accumulate(begin(boundary_names), end(boundary_names),
+                                             Region<Ind2D>{},
+                                             [&](Region<Ind2D> a, const std::string& b) {


warning: the parameter 'a' is copied for each invocation but only used as a const reference; consider making it a const reference [performance-unnecessary-value-param]

Suggested change

[&](Region<Ind2D> a, const std::string& b) {

[&](const Region<Ind2D>& a, const std::string& b) {

Can we avoid this wrong comment? With C++20 it is moved.

It might be that we need to compile with C++20 to avoid this

Once this is merged, fedora-ci will switch to gcc16, which will compile as C++20, so will notice at least if we break it.

tests/unit/fake_mesh.hxx

dschwoerer · 2026-02-03T12:19:44Z

I have not tested whether constexpr is possible. I just changed everything to fmt::runtime, where the compiler complained.

I need this to get BOUT++ building for fedora again, see e.g.:
https://github.com/dschwoerer/bout-container-base/actions/runs/21592270382/job/62328541345
https://copr.fedorainfracloud.org/coprs/davidsch/testing/build/10086815/

ZedThree · 2026-02-04T18:10:40Z

tests/unit/fake_mesh.hxx

+    addRegion2D("RGN_BNDRY", std::accumulate(begin(boundary_names), end(boundary_names),
+                                             Region<Ind2D>{},
+                                             [&](Region<Ind2D> a, const std::string& b) {
+                                               return a + getRegion2D(b);


I think we probably also need

Suggested change

return a + getRegion2D(b);

return std::move(a) + getRegion2D(b);

(and on L245 below) to take advantage of C++20's std::move in accumulate. Although it gets moved in, our use here is still an l-value.

std::move is only a cast, so this should always be correct and also keep clang-tidy happy

dschwoerer added 4 commits February 3, 2026 13:00

Use fmt::runtime

97516d4

Otherwise fmt tries to do compile-time checks, but fails as it cannot be checked at compile time. fmtlib/fmt#4179

Expose erase even with C++20

2435b9f

Do not pass by reference

cccb29d

With C++ std::accumulate uses std::move, which fails with the reference.

Do not include unneeded header

53c6d3e

isatty is provided by both cpptrace and unistd.h, so we should only include one.

github-actions bot reviewed Feb 3, 2026

View reviewed changes

dschwoerer mentioned this pull request Feb 3, 2026

Switch to C++20 #3265

Open

ZedThree reviewed Feb 4, 2026

View reviewed changes

	[&](Region<Ind2D> a, const std::string& b) {
	[&](const Region<Ind2D>& a, const std::string& b) {

	return a + getRegion2D(b);
	return std::move(a) + getRegion2D(b);

Get BOUT++ ready for C++20 #3264

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dschwoerer commented Feb 3, 2026

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dschwoerer commented Feb 3, 2026

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ZedThree Feb 4, 2026 •

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