[cogman10]

> as long as the C++ sources of GCC are compatible with older and newer versions of the C++ standard.

I've worked on a number of pretty large projects. If the target for the source code changes it can be really hard to keep C++20 features from creeping in. It means that you either need to explicitly build targeting 11, or whoever does code reviews needs to have encyclopedic knowledge of whether or not a change leaked in a future feature.

It is "doable" but why would you do it when you can simply keep the compiler targeting 11 and let it do the code review for you.

[bluGill]

Compilers often allow things in 11 that technically are not there until some later standard. Or sometimes things they have always allowed finally got standardized in a later version. Setting your standard to 11 if that is what you want to target it a good first step but don't depend on it - the real tests is all the compilers you care to support compile your code.

Even if you only target 11, there may be advantages to setting a newer version anyway. Sometimes the standard finally allows some optimization that would work, or disallows something that was always error prone anyway. I would recommend you set your standard to the latest the compiler supports and fix any bugs. Solve your we have to support older standards problem by having your CI system build with an older compiler (and also the newest one). C++ is very good at compatibility so this will rarely be a problem.

[quietbritishjim]

> ... why would you do it when you can simply keep the compiler targeting 11 ...

It doesn't appear to me that the parent comment was implying otherwise.

The default is changing for any compilation that doesn't explicitly specify a standard version. I would have thought that the build process for a compiler is likely careful enough that it does explicitly specify a version.

[cogman10]

> It's the type of dog fooding they should be doing! It's one reason why people care so much about self-hosted compilers, it's a demonstration of maturity of the language/compiler.

I could be misreading this, but unless they have a different understanding of what it means to dog fooding than I do then it seems like the proposal is to use C++20 features in the compiler bootstraping.

[ziotom78]

I believe they are really referring to the default mode used by GCC when no standard is explicitly stated.

The email mentions that the last time they changed it was 5 years ago in GCC 11, and the link <https://gcc.gnu.org/projects/cxx-status.html#cxx17> indeed says

> C++17 mode is the default since GCC 11; it can be explicitly selected with the -std=c++17 command-line flag, or -std=gnu++17 to enable GNU extensions as well.

which does not imply a change in an obscure feature (bootstrapping) that would only affect a few users.

[eddd-ddde]

The answer is obvious, YES, specify your language version. Every single compiler invocation for production (for example ci builds) should explicitly select a version. Otherwise you are asking for trouble.

[stabbles]

Yeah, developers should specify what language and dialect a project is written in. In practice though, support for that in build systems is cumbersome.

For example in CMake the natural variable is CMAKE_CXX_STANDARD, but it's implemented backwards: if you set it to 14 but your compiler supports only C++11, they'll add -std=gnu++11. You have to also set CMAKE_CXX_STANDARD_REQUIRED to ON, which not man projects do. I don't think there's an easy way to say "this project requires C++14 or higher".

[menaerus]

There is - you simply build your code with -std=c++XY, and if your toolchain doesn't support it (which one btw doesn't support at least c++17?), it will simply error out, no? That should be a pretty strong signal that your code requires XY standard without having to go into the CMake territory. Even if you want to, I see nothing wrong with the way they are implemented. Two simple variables doing two simple things.