Flag constant-string-class in clangd

clangd developers,

currently I’m working on an own small footprint Objective-C Class library completely unrelated to Cocoa. I use clangd [version 8.0.0 (tags/RELEASE_800/final)] from homebrew as the language server in my IDE.

Unfortunately I can’t clangd persuade to accept my own as a constant string class with the flag "-fconstant-string-class=NGStringLiteral". Ignoring the flag it complains: “Incompatible pointer types initializing 'NGStringLiteral *' with an expression of type 'NSString *’”.

To quiet this I declared ’NSString’ as ’NGStringLiteral’ (“typedef NSString NGStringLiteral”). Now clangd warns: “Incompatible pointer types initializing 'NSString *' (aka 'NGStringLiteral *') with an expression of type 'NSString *’”!

That’s annoying - like all the hard-wired Cocoa aka Apple stuff in clang software. Objective-C is an independent general purpose programming language. Not a Cocoa development tool and not an Apple software product. This should be true for clang, too.

So, what can I do (or what can you do) to get rid of - at least - this?

Thanks in advance.

Best regards,

Andreas Ostermeyer

Hi Andreas,

Are you getting this warning when building your code? Which compiler are you building your code with?


Hi Andreas,

Can you provide a minimal source file that exhibits the problem, and explain how you’re specifying the -fconstant-string-class flag? (It should be in the compile_commands.json or compile_flags.txt file)

It sounds like what you’re describing should work, but I think we need more detail to reproduce the issue.

I cant really speak to the layering between objc the language and the Foundation etc libraries. I’m sympathetic to the frustration but realistically clangd won’t diverge from clang here, so this would be an issue for experts on cfe-dev.

Cheers, Sam

Hi Alex,

thanks for rapid responding.

No, compilers (both clang and gcc) work like expected. It’s only clangd that simply ignores the constant-string-class flag (neither in compile_commands.json nor in compile_flags.txt).



Hi Sam,

thanks for rapid responding. Yes, you’re right. I’ve forgotten to provide some descriptive code.

I’ve issued the constant-string-class flag in both files: compile_flags.txt at first and than in compile_commands.json to no avail.

NGStringLiteral is declared exactly like NXConstantString and demanded in the LLVM documentation, i.e. Class, char*, and unsigned int. Like so:

@interface NGStaticObject
Class isa; /* A pointer to the instance’s class structure. */


@interface NGStringLiteral : NGStaticObject
char *c_string;
unsigned int len;

The test code in question is:

NGStringLiteral *test0 = @"Hello world”; // —> “Incompatible pointer types initializing ‘NGStringLiteral *’ with an expression of type 'NSString *’”

A first attempt to mitigate this was:

typedef NGStringLiteral NSString;

NSString *test1 = @"That’s me;)”; // —> “Incompatible pointer types initializing ‘NSString *’ (aka ‘NGStringLiteral *’) with an expression of type 'NSString *’”

Another attempt is subclassing NSString from NGStringLiteral. This quiets clangd, but now the compiler (gcc) complains “warning: initialization from distinct Objective-C type”. But this is another story.

All the above code would be irrelevant if clangd would regard the constant-string-class flag.



FWIW, I get this error from clang too:

$ cat str.m

@interface NGStaticObject
{ Class isa; }
@interface NGStringLiteral : NGStaticObject
{ char *c_string; unsigned int len; }
NGStringLiteral *t = @“Hello world”;

$ clang-7 -fsyntax-only str.m -fconstant-string-class=NGStringLiteral

str.m:7:18: warning: incompatible pointer types initializing ‘NGStringLiteral *’ with an expression of type ‘NSString *’ [-Wincompatible-pointer-types]

$ clang-7 --version

clang version 7.0.1-6 (tags/RELEASE_701/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

Similar results from tip-of-tree.

How are you running clang, and what’s your clang --version?

This may well need a solution regardless, but it’d be good to know if it’s really clangd-specific (which would be surprising).

Digging into the implementation (Sema::BuildObjCLiteral), I see two things:

  • the class is found by name lookup, so introducing an alias should work
  • the behavior depends on another flag, -fno-constant-cfstrings
  • when this flag is passed, -fconstant-string-class is used, falling back to “NSConstantString”
  • when this flag is not passed, “NSString” is always used

So two possible workarounds:

  • add @comptability_alias NSString NGStringLiteral; to your header
  • pass -fno-constant-cfstrings -fconstant-string-class=NGStringLiteral

I don’t have any deep understanding here, so I don’t know why these interactions exist, or which option you should choose. I’d guess the latter, though.
And AFAIK this should always be the same between (equal versions of) clang and clangd, so let me know if you can verify differences.

I use “clangd version 8.0.0 (tags/RELEASE_800/final)” from homebrew and run it without command line options through my IDE.

“-fno-constant-cfstrings” will do the trick. I used it formerly with clang and forgot it with clangd since I’m currently using gcc with this project. Sorry for this - my fault!

The next ten days I will be on travel and cannot verify your solution, but I’m sure you’re right. For me this issue is solved. Thanks a lot and sorry for bothering you and clangd-dev with it!

Back at home I will verify it and leave a note.