Add string_set buildfile value type

This exposes the std::set<std::string> type to buildfiles.

New functions:

  $size(<string-set>)

Subscript returns true if the value is present and false otherwise (so
it is mapped to std::set::contains()). For example:

set = [string_set] a b c

if ($set[b])
  ...

Note that append (+=) and prepend (=+) have the same semantics
(std::set::insert()). For example:

set = [string_set] a b
set += c b              # a b c
set =+ d b              # a b c d

Example of iteration:

set = [string_set] a b c
for k: $set
  ...

1 files changed, 22 insertions, 0 deletions

diff --git a/libbuild2/variable.hxx b/libbuild2/variable.hxx
index d67098e..b157806 100644
--- a/libbuild2/variable.hxx
+++ b/libbuild2/variable.hxx
@@ -1177,6 +1177,26 @@ namespace build2
     static const pair_vector_value_type<K, V> value_type;
   };
 
+  // set<T>
+  //
+  template <typename T>
+  struct set_value_type;
+
+  template <typename T>
+  struct value_traits<set<T>>
+  {
+    static_assert (sizeof (set<T>) <= value::size_, "insufficient space");
+
+    static set<T> convert (names&&);
+    static void assign (value&, set<T>&&);
+    static void append (value&, set<T>&&);
+    static void prepend (value&, set<T>&&);
+    static bool empty (const set<T>& x) {return x.empty ();}
+
+    static const set<T> empty_instance;
+    static const set_value_type<T> value_type;
+  };
+
   // map<K, V>
   //
   // Either K or V can be optional<T> making the key or value optional.
@@ -1321,6 +1341,8 @@ namespace build2
   extern template struct LIBBUILD2_DECEXPORT
   value_traits<vector<pair<string, optional<bool>>>>;
 
+  extern template struct LIBBUILD2_DECEXPORT value_traits<set<string>>;
+
   extern template struct LIBBUILD2_DECEXPORT
   value_traits<map<string, string>>;