Add config.{c,cxx}.std

These values override {c,cxx}.std specified at the project level. In
particular, this allows us to force a specific standard for all the
projects in a build configuration, for example:

b create: conf/,cc config.cxx=g++ config.cxx.std=experimental

5 files changed, 26 insertions, 8 deletions

diff --git a/build2/c/init.cxx b/build2/c/init.cxx
index dbaff78..438cd3b 100644
--- a/build2/c/init.cxx
+++ b/build2/c/init.cxx
@@ -164,6 +164,7 @@ namespace build2
         v.insert<string>  ("config.c.id",       true),
         v.insert<string>  ("config.c.version",  true),
         v.insert<string>  ("config.c.target",   true),
+        v.insert<string>  ("config.c.std",      true),
         v.insert<strings> ("config.c.poptions", true),
         v.insert<strings> ("config.c.coptions", true),
         v.insert<strings> ("config.c.loptions", true),
@@ -174,6 +175,7 @@ namespace build2
         v.insert<dir_paths>    ("c.sys_lib_dirs"),
         v.insert<dir_paths>    ("c.sys_inc_dirs"),
 
+        v.insert<string>       ("c.std", variable_visibility::project),
         v.insert<strings>      ("c.poptions"),
         v.insert<strings>      ("c.coptions"),
         v.insert<strings>      ("c.loptions"),
@@ -208,8 +210,6 @@ namespace build2
         v.insert<string>   ("c.preprocessed"), // See cxx.preprocessed.
         nullptr,                               // No __symexport (no modules).
 
-        v.insert<string>   ("c.std", variable_visibility::project),
-
         v.insert<string>   ("c.id"),
         v.insert<string>   ("c.id.type"),
         v.insert<string>   ("c.id.variant"),
diff --git a/build2/cc/common.hxx b/build2/cc/common.hxx
index dec71ed..19733cf 100644
--- a/build2/cc/common.hxx
+++ b/build2/cc/common.hxx
@@ -43,6 +43,7 @@ namespace build2
       const variable& config_x_id;      // <type>[-<variant>]
       const variable& config_x_version;
       const variable& config_x_target;
+      const variable& config_x_std;
       const variable& config_x_poptions;
       const variable& config_x_coptions;
       const variable& config_x_loptions;
@@ -53,6 +54,7 @@ namespace build2
       const variable& x_sys_lib_dirs; // System library search directories.
       const variable& x_sys_inc_dirs; // System header search directories.
 
+      const variable& x_std;
       const variable& x_poptions;
       const variable& x_coptions;
       const variable& x_loptions;
@@ -87,8 +89,6 @@ namespace build2
       const variable& x_preprocessed; // x.preprocessed
       const variable* x_symexport;    // x.features.symexport
 
-      const variable& x_std;
-
       const variable& x_id;
       const variable& x_id_type;
       const variable& x_id_variant;
diff --git a/build2/cc/module.cxx b/build2/cc/module.cxx
index 1e3806a..7527da2 100644
--- a/build2/cc/module.cxx
+++ b/build2/cc/module.cxx
@@ -286,9 +286,24 @@ namespace build2
       const compiler_info& ci (*ci_);
       const target_triplet& tt (cast<target_triplet> (rs[x_target]));
 
-      // Translate x_std value (if any) to the compiler option(s) (if any).
+      // config.x.std overrides x.std
       //
-      tstd = translate_std (ci, rs, cast_null<string> (rs[x_std]));
+      {
+        lookup l (config::omitted (rs, config_x_std).first);
+
+        const string* v;
+        if (l.defined ())
+        {
+          v = cast_null<string> (l);
+          rs.assign (x_std) = v;
+        }
+        else
+          v = cast_null<string> (rs[x_std]);
+
+        // Translate x_std value (if any) to the compiler option(s) (if any).
+        //
+        tstd = translate_std (ci, rs, v);
+      }
 
       // Extract system header/library search paths from the compiler and
       // determine if we need any additional search paths.
diff --git a/build2/cxx/init.cxx b/build2/cxx/init.cxx
index 2491d90..ff104d3 100644
--- a/build2/cxx/init.cxx
+++ b/build2/cxx/init.cxx
@@ -389,6 +389,7 @@ namespace build2
         v.insert<string>  ("config.cxx.id",       true),
         v.insert<string>  ("config.cxx.version",  true),
         v.insert<string>  ("config.cxx.target",   true),
+        v.insert<string>  ("config.cxx.std",      true),
         v.insert<strings> ("config.cxx.poptions", true),
         v.insert<strings> ("config.cxx.coptions", true),
         v.insert<strings> ("config.cxx.loptions", true),
@@ -409,6 +410,7 @@ namespace build2
         v.insert<dir_paths>    ("cxx.sys_lib_dirs"),
         v.insert<dir_paths>    ("cxx.sys_inc_dirs"),
 
+        v.insert<string>   ("cxx.std", variable_visibility::project),
         v.insert<strings>  ("cxx.poptions"),
         v.insert<strings>  ("cxx.coptions"),
         v.insert<strings>  ("cxx.loptions"),
@@ -456,8 +458,6 @@ namespace build2
 
         nullptr, // cxx.features.symexport (set in init() below).
 
-        v.insert<string>   ("cxx.std", variable_visibility::project),
-
         v.insert<string>   ("cxx.id"),
         v.insert<string>   ("cxx.id.type"),
         v.insert<string>   ("cxx.id.variant"),
diff --git a/build2/variable.hxx b/build2/variable.hxx
index 5e6ad2f..01e7038 100644
--- a/build2/variable.hxx
+++ b/build2/variable.hxx
@@ -339,6 +339,9 @@ namespace build2
     template <typename T> value& operator= (T);
     template <typename T> value& operator+= (T);
 
+    template <typename T> value& operator= (T* v) {
+      return v != nullptr ? *this = *v : *this = nullptr;}
+
     template <typename T> value& operator+= (T* v) {
       return v != nullptr ? *this += *v : *this;}