Building failure: ISO C++17 does not allow 'register' storage class specifier [-Wregister]

[wilstoff]

Describe the bug
When running cmake/make with

cmake -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/usr -Wno-dev

it seems to generate a bunch of code with the register keyword that seems to be deprecated.
Which errors out when building:

/build/bstoffel/rpmbuild/BUILD/doxygen-1.10.0/generated_src/mscgen_lexer.cpp:899:2: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
        register yy_state_type yy_current_state;
        ^~~~~~~~~
/build/bstoffel/rpmbuild/BUILD/doxygen-1.10.0/generated_src/mscgen_lexer.cpp:900:2: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
        register char *yy_cp, *yy_bp;
...

Expected behavior
I expect the code to compile appropriately

Version
Building version 1.10.0

Additional context
Running on centos 7 machine with clang 10.0.0 installed.
I have tried adjusting the C++ version to 14/11 and still has the same errors.
I'm sure there is an option I'm missing or can alter some environmental variable in order for it to generate valid C++17 code.

[albert-github]

When I build doxygen I see in the resulting mscgen_lexer.cpp file:

/** The main scanner function which does all the work.
 */
YY_DECL
{
        yy_state_type yy_current_state;
        char *yy_cp, *yy_bp;
        int yy_act;

I'm using flex version 2.6.4 locally.
On GitHub actions we also use the CLang compiler, version 10.0.0 with flex 2.6.4 (see e.g. https://github.com/doxygen/doxygen/actions/runs/7877930596/job/21495098205) and here I don't see any problems there.

• which version of flex are you using? or are you using lex?
• can you post the resulting mscgen_lexer.cpp file in a, compressed, tar or zip file attached to the issue?

Looking in the file CMakeLists.txt we see for the MacOs Darwin version:

    set(CMAKE_CXX_FLAGS "-Wno-deprecated-register -mmacosx-version-min=${MACOS_VERSION_MIN} ${CMAKE_CXX_FLAGS}")
    set(CMAKE_C_FLAGS "-Wno-deprecated-register -mmacosx-version-min=${MACOS_VERSION_MIN} ${CMAKE_C_FLAGS}")

I see that CLang has also an option (https://releases.llvm.org/10.0.0/tools/clang/docs/ClangCommandLineReference.html):

-Wdeprecated, -Wno-deprecated¶

Enable warnings for deprecated constructs and define __DEPRECATED

so maybe something like

    set(CMAKE_CXX_FLAGS "-Wno-deprecated ${CMAKE_CXX_FLAGS}")
    set(CMAKE_C_FLAGS "-Wno-deprecated ${CMAKE_C_FLAGS}")

should be added, conditionally, to the CMakeLists.txt file, but first we have to see what the version of flex etc. is that is used.

[matthewjmiller1]

I am seeing this too in a legacy build environment. It looks like its flex is 2.5.39. Using -Wno-deprecated didn't seem to help.

[matthewjmiller1]

Here is the resulting mscgen_lexer.cpp file (I replaced the real workspace path in some of the #line lines with /workspace):
mscgen_lexer.cpp.tar.gz

[albert-github]

@matthewjmiller1
Looks like to be a problem with the a bit older flex versions.

• Do you see this problem only with mscgen_lexer.cpp / mscgen_lexer.l or also with other .l files and or .cpp / .c files?
• You wrote about -Wno-deprecated but in my comment (Building failure: ISO C++17 does not allow 'register' storage class specifier [-Wregister] #10649 (comment)) I wrote about -Wno-deprecated-register (though I don't see the -Wno-deprecated-registerin the gcc manual page I have) what did you use?
• What happens when you place #define register as line 38 in the file mscgen_lexer.l?

[matthewjmiller1]

@albert-github
It seems to be other files too, but not sure how many b/c the build bails after several errors. xml.cpp failed after mscgen_lexer.cpp. I'd guess there may be others after that.

I tried -Wno-deprecated-register and that didn't help.

I tried adding #define register to line 38 of mscgen_lexer.l and that did seem to get past the mscgen_lexer.cpp issue, but then I hit the xml.cpp issue.

I wrote an Ubuntu 24.04 Dockerfile that you can use to repro this:

doxygen_cmake.tar.gz

There is a README.md in there that tells you the commands to use. It creates two clones of doxygen: one to test -Wno-deprecated-register and the other to test the modified mscgen_lexer.l file.

[albert-github]

It is like I expected

• with #define resister the problem disappears for that file
• the problem is present in other wfiles as well

For the solution it would be good to now whether the problem only occurs in files generated from .l files or also on other places. I see the word register also in deps/libmscgen/gd.c.
You cans see this by means of make -i which will compile all files and issue the warnings but won't stop compiling. In this way you can see all the files that will give problems.

My expectations are:

• deps/libmscgen/gd.c won't give problems
• all .l file will give a compilation warning
• no further files will give a problem.

So the completer list probably would be:

• config_doxyw.cpp
• mscgen_lexer.cpp
• xml.cpp
• code.cpp
• commentcnv.cpp
• commentscan.cpp
• configimpl.cpp
• constexp.cpp
• declinfo.cpp
• defargs.cpp
• doctokenizer.cpp
• fortrancode.cpp
• fortranscanner.cpp
• lexcode.cpp
• lexscanner.cpp
• pre.cpp
• pycode.cpp
• pyscanner.cpp
• scanner.cpp
• sqlcode.cpp
• vhdlcode.cpp
• xmlcode.cpp

@matthewjmiller1 can you confirm this?

As additional information he c++ 2023 standard stes:

The register keyword is unused but is reserved for future use

For the first 3 files I think the way to go would be to add the line #define register in the file
Fort the other files (all on the src directory I think the way to go is to add the line #define register to the file util.h or doxygen.h

Another way would be to add the -Dregister to the CMAKE_CXX_FLAGS in case of a flex versions < 2.6.0
(In the NEWS file of the https://github.com/westes/flex repository I see

* version 2.6.0 released 2015-11-17
...
*** Removed deprecated 'register' storage class specifier

@doxygen what do you think, which way to choose?

[matthewjmiller1]

@albert-github
This is the list of files I saw in my build with make -i:

xmlcode_intermediate.cpp
vhdlcode_intermediate.cpp
sqlcode_intermediate.cpp
scanner_intermediate.cpp
pyscanner_intermediate.cpp
pycode_intermediate.cpp
pre_intermediate.cpp
lexscanner_intermediate.cpp
lexcode_intermediate.cpp
fortranscanner_intermediate.cpp
fortrancode_intermediate.cpp
doctokenizer_intermediate.cpp
defargs_intermediate.cpp
declinfo_intermediate.cpp
constexp_intermediate.cpp
commentscan_intermediate.cpp
commentcnv_intermediate.cpp
code_intermediate.cpp
configimpl_intermediate.cpp
xml.cpp
mscgen_lexer.cpp

Adding -DCMAKE_CXX_FLAGS="-Dregister=''" to the cmake CLI seemed to work for those errors (I saw linking errors later, not sure if they were related).

Adding -DCMAKE_CXX_FLAGS="-Dregister" didn't, giving errors like:

/git_repos/doxygen_no_deprecated/cmake_build/generated_src/mscgen_lexer.cpp:1509:10: error: expected ';' after expression
 1509 |         register int number_to_move, i;
      |                 ^
      |                 ;

The Dockerfile tarball from #10649 (comment) can be used to test all these permutations.

[albert-github]

Perfect, exactly the list I expected (actually I expected one more file: config_doxyw_intermediate.cpp, but you didn't get this one as you didn't enable the doxywizard).

-DCMAKE_CXX_FLAGS="-Dregister=''" won't work as the line:

register int number_to_move, i;

would be translated into:

'' int number_to_move, i;

and this is not valid C / C++

I thought it would be easy to fix but MSVC gives some problems:

D:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.16.27023\include\xkeycheck.h(221): warning C4005: 'register': macro redefinition
...\doxygen\deps\spdlog\src\spdlog.cpp: note: see previous definition of 'register'
D:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Tools\MSVC\14.16.27023\include\xkeycheck.h(260): fatal error C1189: #error:  The C++ Standard Library forbids macroizing keywords. Enable warning C4005 to find the forbidden macro.

and in https://learn.microsoft.com/en-us/cpp/porting/visual-cpp-change-history-2003-2015?view=msvc-170I found:

C++11 17.6.4.3.1 [macro.names]/2 forbids macro replacement of keywords when C++ Standard Library headers are included. The headers now emit compiler errors if they detect macro-replaced keywords. (Defining _ALLOW_KEYWORD_MACROS allows such code to compile, but we strongly discourage that usage.)

I've just pushed a proposed patch, pull request #11113, though I would advise also to update the flex version to 2.6.4 (which anyway is also already from 2017, but is the newest release available)