From 67b36b32e19e17db9b1e5c72deb8db7202a0f41b Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Thu, 9 Aug 2018 09:36:23 +0200
Subject: Add support for returning optional<T> from (buildfile) function
 implementations

---
 build2/function.hxx |  6 +++++-
 build2/variable.hxx |  7 +++++++
 build2/variable.ixx | 20 ++++++++++++++++++++
 3 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/build2/function.hxx b/build2/function.hxx
index 139f1fa..5b84a69 100644
--- a/build2/function.hxx
+++ b/build2/function.hxx
@@ -46,12 +46,16 @@ namespace build2
   // are conceptually "moved" and can be reused by the implementation.
   //
   // A function can also optionally receive the current scope by having the
-  // first argument of the const scope* type. It may be NULL is the function
+  // first argument of the const scope* type. It may be NULL if the function
   // is called out of any scope (e.g., command line).
   //
   // Note also that we don't pass the location to the function instead
   // printing the info message pointing to the call site.
   //
+  // A function can return value or anything that can be converted to value.
+  // In particular, if a function returns optional<T>, then the result will be
+  // either NULL or value of type T.
+  //
   // Normally functions come in families that share a common qualification
   // (e.g., string. or path.). The function_family class is a "registrar"
   // that simplifies handling of function families. For example:
diff --git a/build2/variable.hxx b/build2/variable.hxx
index 401cc77..7a0e531 100644
--- a/build2/variable.hxx
+++ b/build2/variable.hxx
@@ -250,10 +250,17 @@ namespace build2
     explicit
     value (names); // Create untyped value.
 
+    explicit
+    value (optional<names>);
+
     template <typename T>
     explicit
     value (T); // Create value of value_traits<T>::value_type type.
 
+    template <typename T>
+    explicit
+    value (optional<T>);
+
     // Note: preserves type.
     //
     value&
diff --git a/build2/variable.ixx b/build2/variable.ixx
index 8c16559..bf5fb92 100644
--- a/build2/variable.ixx
+++ b/build2/variable.ixx
@@ -24,6 +24,14 @@ namespace build2
     new (&data_) names (move (ns));
   }
 
+  inline value::
+  value (optional<names> ns)
+      : type (nullptr), null (!ns), extra (0)
+  {
+    if (!null)
+      new (&data_) names (move (*ns));
+  }
+
   template <typename T>
   inline value::
   value (T v)
@@ -33,6 +41,18 @@ namespace build2
     null = false;
   }
 
+  template <typename T>
+  inline value::
+  value (optional<T> v)
+      : type (&value_traits<T>::value_type), null (true), extra (0)
+  {
+    if (v)
+    {
+      value_traits<T>::assign (*this, move (*v));
+      null = false;
+    }
+  }
+
   inline value& value::
   operator= (reference_wrapper<value> v)
   {
-- 
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