Support -std= command line option

SWIG now supports command line options -std=cXX and -std=c++XX to
specify the C/C++ standards version.  The only effect of these options
is to set appropriate values for __STDC_VERSION__ and __cplusplus
respectively, which is useful if you're wrapping headers which have
preprocessor checks based on their values.

Closes #2591

CHANGES.current

@@ -7,6 +7,26 @@ the issue number to the end of the URL: https://github.com/swig/swig/issues/
 Version 4.2.0 (in progress)
 ===========================
 
+2023-11-09: olly
+	    #2591 SWIG now supports command line options -std=cXX and
+	    -std=c++XX to specify the C/C++ standards version.  The only effect
+	    of these options is to set appropriate values for __STDC_VERSION__
+	    and __cplusplus respectively, which is useful if you're wrapping
+	    headers which have preprocessor checks based on their values.
+
+2023-11-09: olly
+	    SWIG now defines __STDC__ to 1 to match the behaviour of ISO C/C++
+	    compilers - previously it had an empty value.
+
+	    *** POTENTIAL INCOMPATIBILITY ***
+
+2023-11-09: olly
+	    When -c++ is used, SWIG now defines __cplusplus to be 199711L (the
+	    value for C++98) by default - previously its value was set to
+	    __cplusplus.
+
+	    *** POTENTIAL INCOMPATIBILITY ***
+
 2023-11-08: emmenlau
             #2480 [C#] Add std_unordered_map.i for wrapping std::std_unordered_map, implementing
             C# System.Collections.Generic.IDictionary<>.
@@ -555,22 +575,6 @@ Version 4.2.0 (in progress)
 	    purpose in over 20 years, and has never been documented outside of
 	    CHANGES.
 
-2023-05-18: olly
-	    SWIG now defines __STDC__ to 1 to match the behaviour of ISO C/C++
-	    compilers, rather than to have an empty value.
-
-	    *** POTENTIAL INCOMPATIBILITY ***
-
-2023-05-18: olly
-	    #2591 The value of __cplusplus SWIG defines can now be overridden
-	    on the command line, e.g. using -D__cplusplus=201103L which is
-	    useful if you're wrapping headers which have preprocessor checks
-	    based on its value.  By default SWIG now defines __cplusplus to
-	    199711L (the value for C++98) rather than setting its value to
-	    be __cplusplus.
-
-	    *** POTENTIAL INCOMPATIBILITY ***
-
 2023-05-18: olly
 	    #2591 Add new -U command line option to undefine a preprocessor
 	    symbol.

Doc/Manual/Preprocessor.html

@@ -164,8 +164,9 @@ In addition, SWIG defines the following set of standard C/C++ macros:
 <pre>
 __LINE__                        Current line number
 __FILE__                        Current file name
-__STDC__                        Defined to indicate ISO C
+__STDC__                        Defined to indicate ISO C/C++
-__cplusplus                     Defined when -c++ option used
+__cplusplus                     Defined when -c++ option used, value controlled by <tt>-std=c++NN</tt>
+__STDC_VERSION__                May be defined when -c++ option is not used, value controlled by <tt>-std=cNN</tt>
 </pre>
 </div>
 
@@ -177,13 +178,16 @@ have an empty value.
 
 <p>
 Since SWIG 4.2.0, <tt>__cplusplus</tt> is defined to <tt>199711L</tt>
-(the value for C++98) but you can override this by defining it
+(the value for C++98) by default. Before this SWIG always defined it to have
-explicitly on the SWIG command line if you are wrapping headers
+the <b>value</b> <tt>__cplusplus</tt>.
-which have preprocessor checks based on its value (e.g. use
+</p>
-<tt>-D__cplusplus=201103L</tt> to set it to the value for C++11).
+
-Before this SWIG always defined it to have the <b>value</b>
+<p>
-<tt>__cplusplus</tt> and attempting to use <tt>-D__cplusplus</tt>
+Since SWIG 4.2.0, SWIG supports command line options <tt>-std=cXX</tt> and
-to change this gave an error.
+<tt>-std=c++XX</tt> to specify the C/C++ standards version.  The only effect of
+these options is to set appropriate values for <tt>__STDC_VERSION__</tt> and
+<tt>__cplusplus</tt> respectively, which is useful if you're wrapping
+headers which have preprocessor checks based on their values.
 </p>
 
 <p>

Examples/test-suite/common.mk

@@ -375,6 +375,7 @@ CPP_TEST_CASES += \
 	pointer_reference \
 	preproc_constants \
 	preproc_cpp \
+	preproc_predefined_stdcpp \
 	primitive_ref \
 	private_assign \
 	proxycode \
@@ -824,6 +825,7 @@ C_TEST_CASES += \
 	preproc_include \
 	preproc_line_file \
 	preproc_predefined \
+	preproc_predefined_stdc \
 	register_par \
 	ret_by_value \
 	simple_array \
@@ -853,6 +855,8 @@ MULTI_CPP_TEST_CASES += \
 wallkw.cpptest: SWIGOPT += -Wallkw
 preproc_include.ctest: SWIGOPT += -includeall
 command_line_define.ctest: SWIGOPT += -DFOO -DBAR=123 -DBAZ -UBAZ -UNOTSET
+preproc_predefined_stdc.ctest: SWIGOPT += -std=c23
+preproc_predefined_stdcpp.cpptest: SWIGOPT += -std=c++23
 
 # Allow modules to define temporarily failing tests.
 C_TEST_CASES := $(filter-out $(FAILING_C_TESTS),$(C_TEST_CASES))

Examples/test-suite/preproc_predefined_stdc.i

@@ -0,0 +1,24 @@
+%module preproc_predefined_stdc
+
+// Test handling of -std=c23
+
+// __STDC__ should still have value 1.
+#ifndef __STDC__
+# error __STDC__ not defined at SWIG-time
+#endif
+#if __STDC__-0 != 1
+# error __STDC__ value not 1 at SWIG-time
+#endif
+
+// __cplusplus should not be defined.
+#ifdef __cplusplus
+# error __cplusplus defined at SWIG-time but should not be
+#endif
+
+// __STDC_VERSION__ should be suitably defined.
+#ifndef __STDC_VERSION__
+# error __STDC_VERSION__ not defined at SWIG-time
+#endif
+#if __STDC_VERSION__ != 202311L
+# error __STDC_VERSION__ value not 202311L at SWIG-time
+#endif

Examples/test-suite/preproc_predefined_stdcpp.i

@@ -0,0 +1,24 @@
+%module preproc_predefined_stdcpp
+
+// Test handling of -std=c++23
+
+// __STDC__ should still have value 1.
+#ifndef __STDC__
+# error __STDC__ not defined at SWIG-time
+#endif
+#if __STDC__-0 != 1
+# error __STDC__ value not 1 at SWIG-time
+#endif
+
+// __STDC_VERSION__ should not be defined.
+#ifdef __STDC_VERSION__
+# error __STDC_VERSION__ defined at SWIG-time but should not be
+#endif
+
+// __cplusplus should be suitably defined.
+#ifndef __cplusplus
+# error __cplusplus not defined at SWIG-time
+#endif
+#if __cplusplus != 202302L
+# error __cplusplus value not 202302L at SWIG-time
+#endif

Source/Modules/main.cxx

@@ -210,6 +210,8 @@ static String *external_runtime_name = 0;
 enum { STAGE1=1, STAGE2=2, STAGE3=4, STAGE4=8, STAGEOVERFLOW=16 };
 static List *libfiles = 0;
 static List *all_output_files = 0;
+static const char *stdcpp_define = NULL;
+static const char *stdc_define = NULL;
 
 /* -----------------------------------------------------------------------------
  * check_extension()
@@ -463,6 +465,46 @@ static void getoptions(int argc, char *argv[]) {
 	// Undocumented
 	Swig_cparse_cplusplusout(1);
 	Swig_mark_arg(i);
+      } else if (strncmp(argv[i], "-std=c", 6) == 0) {
+	const char *std = argv[i] + 6;
+	if (strncmp(std, "++", 2) == 0) {
+	  std += 2;
+	  if (strcmp(std, "98") == 0 || strcmp(std, "03") == 0) {
+	    stdcpp_define = "__cplusplus 199711L";
+	  } else if (strcmp(std, "11") == 0) {
+	    stdcpp_define = "__cplusplus 201103L";
+	  } else if (strcmp(std, "14") == 0) {
+	    stdcpp_define = "__cplusplus 201402L";
+	  } else if (strcmp(std, "17") == 0) {
+	    stdcpp_define = "__cplusplus 201703L";
+	  } else if (strcmp(std, "20") == 0) {
+	    stdcpp_define = "__cplusplus 202002L";
+	  } else if (strcmp(std, "23") == 0) {
+	    stdcpp_define = "__cplusplus 202302L";
+	  } else {
+	    Printf(stderr, "Unrecognised C++ standard version in option '%s'\n", argv[i]);
+	    Exit(EXIT_FAILURE);
+	  }
+	} else {
+	  if (strcmp(std, "89") == 0 || strcmp(std, "90") == 0) {
+	    stdc_define = NULL;
+	  } else if (strcmp(std, "95") == 0) {
+	    stdc_define = "__STDC_VERSION__ 199409L";
+	  } else if (strcmp(std, "99") == 0) {
+	    stdc_define = "__STDC_VERSION__ 199901L";
+	  } else if (strcmp(std, "11") == 0) {
+	    stdc_define = "__STDC_VERSION__ 201112L";
+	  } else if (strcmp(std, "17") == 0 || strcmp(std, "18") == 0) {
+	    // Both GCC and clang accept -std=c18 as well as -std=c17.
+	    stdc_define = "__STDC_VERSION__ 201710L";
+	  } else if (strcmp(std, "23") == 0) {
+	    stdc_define = "__STDC_VERSION__ 202311L";
+	  } else {
+	    Printf(stderr, "Unrecognised C standard version in option '%s'\n", argv[i]);
+	    Exit(EXIT_FAILURE);
+	  }
+	}
+	Swig_mark_arg(i);
       } else if (strcmp(argv[i], "-fcompact") == 0) {
 	Wrapper_compact_print_mode_set(1);
 	Swig_mark_arg(i);
@@ -890,10 +932,28 @@ int SWIG_main(int argc, char *argv[], const TargetLanguageModule *tlm) {
   Preprocessor_define("SWIG 1", 0);
   Preprocessor_define("__STDC__ 1", 0);
 
-  // Define __cplusplus to the C++98 value, but only if it's not already
+  if (CPlusPlus) {
-  // defined so the user to override with e.g. -D__cplusplus=202002L
+    // Default to C++98.
-  if (CPlusPlus && !Preprocessor_defined("__cplusplus"))
+    if (!stdcpp_define) stdcpp_define = "__cplusplus 199711L";
-    Preprocessor_define("__cplusplus 199711L", 0);
+    Preprocessor_define(stdcpp_define, 0);
+  } else {
+    if (stdcpp_define) {
+      Printf(stderr, "Option -std=c++XX was used without -c++\n");
+      Exit(EXIT_FAILURE);
+    }
+  }
+
+  if (!CPlusPlus) {
+    // Default to C90 which didn't define __STDC_VERSION__.
+    if (stdc_define) {
+      Preprocessor_define(stdc_define, 0);
+    }
+  } else {
+    if (stdc_define) {
+      Printf(stderr, "Option -std=cXX was used with -c++\n");
+      Exit(EXIT_FAILURE);
+    }
+  }
 
   String *vers = Swig_package_version_hex();
   Preprocessor_define(vers, 0);