[ign to gz] C++ namespaces

[chapulina]

🎉 New feature

• Part of Rename Ignition to Gazebo gazebo-tooling/release-tools#698
• Subtask: C++ namespaces, directories and header guards from ignition to gz gazebo-tooling/release-tools#711

Summary

Changes:

• Change all namespaces from ignition to gz
• Add alias so that ignition still works for users (tick-tock's tick)
• Added a test to make sure ignition still works
• While at it, I added missing inline version namespaces in a couple of places

Ideally we'd issue deprecation warnings when using ignition. Unfortunately, C++ doesn't allow this:

namespace [[deprecated]] ignition = gz;

Clang's warning is very clear:

.../include/ignition/utils/config.hh:24:11: error: attributes cannot be specified on namespace alias
namespace [[deprecated]] ignition = gz;
          ^

So we'll need to either find a workaround, or don't issue deprecation warnings for namespaces.

In any case, we should wait until most of the stack is already using gz before we turn on these warnings, otherwise they would hide other warnings.

Test it

See the added test. I also expect downstream libraries not to need any modifications because of this PR.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial - We'll add complete migration tutorials at the end
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[scpeters]

Ideally we'd issue deprecation warnings when using ignition. Unfortunately, C++ doesn't allow this:

I assume we will rename the headers and provide ignition/utils/* headers with #pragma deprecation warnings inside. I would suggest placing the namespace ignition = gz; statement inside one of these headers

[azeey]

Can we use this trick?

namespace [[deprecated]] ignition {
  using namespace gz;
}

https://godbolt.org/z/4eYEco8qx

[chapulina]

Can we use this trick?

Brilliant! ✨ Works like a charm. I added it commented out in 5fee873 so we don't enable warnings just yet. It didn't work with IGN_DEPRECATED, so I used [[deprecated]].

I assume we will rename the headers and provide ignition/utils/* headers with #pragma deprecation warnings inside. I would suggest placing the namespace ignition = gz; statement inside one of these headers

I was thinking of adding symlinks so we don't duplicate the headers. But I'm open to suggestions about a different place to put the alias. I went for config.hh because that's common across all libraries and including it adds little overhead

[scpeters]

I assume we will rename the headers and provide ignition/utils/* headers with #pragma deprecation warnings inside. I would suggest placing the namespace ignition = gz; statement inside one of these headers

I was thinking of adding symlinks so we don't duplicate the headers. But I'm open to suggestions about a different place to put the alias. I went for config.hh because that's common across all libraries and including it adds little overhead

I wouldn't suggest using identical headers in ignition/utils/*. I would have some deprecation warnings and then a #include <gz/utils/___.hh> statement, similar to the following:

• https://github.com/ignitionrobotics/sdformat/blob/sdf12/include/sdf/sdf_config.h

[chapulina]

I wouldn't suggest using identical headers in ignition/utils/*. I would have some deprecation warnings and then a #include <gz/utils/___.hh> statement, similar to the following:

Ah ok. I'm not sure I fully understand though, do you think we should keep each one of the existing headers, and inside each of them, include the gz equivalent? Or would it be just one master ignition header that includes all the gz headers?

That may work better than symlinks because it's better for cross-platform 🤔

[methylDragon]

This was superceded by #61 because I had to do the namespace migration following the header migration.

[chapulina]

Closing in favor of #61