How to compile Objective C++ in GCC?

Question

I installed GNUStep and gobjc in Ubuntu 16.04. I can compile Objective-C code like this:

gcc codefile.m `gnustep-config --objc-flags` -lobjc -lgnustep-base -o codefile

But I want to compile Objective-C++ code in GCC. How can I do it?

Community · Accepted Answer · 2017-05-23 11:46:10Z

6

Documentation of objc/objc++ dialects in GCC of your version of gcc / gobjc don't list specific option to select the dialect variant.

Just use standard file extension ".mm" or ".M" (additionally you can replace gcc with g++ to automatically add C++ libraries into linking stage; both variants untested):

gcc codefile.mm `gnustep-config --objc-flags` -lobjc -lgnustep-base -o codefile
g++ codefile.mm `gnustep-config --objc-flags` -lobjc -lgnustep-base -o codefile

And ObjC++ is "simply source code that mixes Objective-C classes and C++ classes"

Gcc will select mode based on file extension, there is huge table of file extensions and corresponding language modes with Objective-C for .m and Objective-C++ for .mm/.M: https://github.com/gcc-mirror/gcc/blob/gcc-4_7_4-release/gcc/gcc.c#L913

static const struct compiler default_compilers[] =
{
  /* Add lists of suffixes of known languages here.  If those languages
     were not present when we built the driver, we will hit these copies
     and be given a more meaningful error than "file not used since
     linking is not done".  */
  {".m",  "#Objective-C", 0, 0, 0}, {".mi",  "#Objective-C", 0, 0, 0},
  {".mm", "#Objective-C++", 0, 0, 0}, {".M", "#Objective-C++", 0, 0, 0},
  {".mii", "#Objective-C++", 0, 0, 0},
 . . .
  /* Next come the entries for C.  */
  {".c", "@c", 0, 0, 1},

There is also -x option of g++ with allowed argument of "objective-c++" to manually select language dialect (gcc -x objective-c++ ... or g++ -x objective-c++): https://github.com/gcc-mirror/gcc/blob/1cb6c2eb3b8361d850be8e8270c597270a1a7967/gcc/cp/g%2B%2Bspec.c#L167

case OPT_x:
 . . . 
      && (strcmp (arg, "c++") == 0
      || strcmp (arg, "c++-cpp-output") == 0
      || strcmp (arg, "objective-c++") == 0

It is documented at https://gcc.gnu.org/onlinedocs/gcc/Overall-Options.html

 -x language
Specify explicitly the language for the following input files (rather than letting the compiler choose a default based on the file name suffix). This option applies to all following input files until the next -x option. Possible values for language are:
      c  . . .  c++ . . .
      objective-c  objective-c-header  objective-c-cpp-output
      objective-c++ objective-c++-header objective-c++-cpp-output

edited May 23, 2017 at 11:46

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answered Feb 17, 2017 at 23:01

osgx

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3 Comments

H. Al-Amri Over a year ago

My experience with Objective-C++ is limited to macOS, but the file needs to be named with extension .mm, otherwise it won't work. I think this is the correct answer.

İbrahim Over a year ago

@osgx Thanks for your detail information. I could compile Objective-C++ with g++ like this: g++ codefile.mm `gnustep-config --objc-flags` -lobjc -lgnustep-base -o codefile.

Clearer Over a year ago

@H.Al-Amri With clang on macOS you can use the -ObjC++ flag when compiling. This will override the filename based mode detection.

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