Add Value::consume() to create an Array-typed Value without copying

Currently few places construct arrays, but more are coming soon. Note
that this can't be used for things like $push's getValue() since that
can be called multiple times so the array needs to be preserved.

5 files changed, 17 insertions, 5 deletions

diff --git a/src/mongo/db/pipeline/document.h b/src/mongo/db/pipeline/document.h
index 375cd48faa3..77c04bafb3f 100644
--- a/src/mongo/db/pipeline/document.h
+++ b/src/mongo/db/pipeline/document.h
@@ -467,7 +467,7 @@ namespace mongo {
             return *this << Value(val);
         }
 
-        Value done() { return Value(_array); }
+        Value done() { return Value::consume(_array); }
 
     private:
         vector<Value> _array;
diff --git a/src/mongo/db/pipeline/document_source.cpp b/src/mongo/db/pipeline/document_source.cpp
index 6c46604e1ba..32a4ced05b9 100644
--- a/src/mongo/db/pipeline/document_source.cpp
+++ b/src/mongo/db/pipeline/document_source.cpp
@@ -167,7 +167,7 @@ namespace mongo {
             }
         }
 
-        return Value(values);
+        return Value::consume(values);
     }
 
     Document DocumentSource::documentFromBsonWithDeps(const BSONObj& bson,
diff --git a/src/mongo/db/pipeline/document_source_sort.cpp b/src/mongo/db/pipeline/document_source_sort.cpp
index 20e7948af4c..08ea0e85dd6 100644
--- a/src/mongo/db/pipeline/document_source_sort.cpp
+++ b/src/mongo/db/pipeline/document_source_sort.cpp
@@ -286,7 +286,7 @@ namespace mongo {
         for (size_t i=0; i < sp.size(); i++) {
             keys.push_back(sp[i]->evaluate(d));
         }
-        key = Value(keys);
+        key = Value::consume(keys);
     }
 
     int DocumentSourceSort::compare(const KeyAndDoc & lhs, const KeyAndDoc & rhs) const {
diff --git a/src/mongo/db/pipeline/expression.cpp b/src/mongo/db/pipeline/expression.cpp
index c5b45981f72..3c5a52ddea7 100644
--- a/src/mongo/db/pipeline/expression.cpp
+++ b/src/mongo/db/pipeline/expression.cpp
@@ -1118,7 +1118,7 @@ namespace mongo {
                     result.push_back(doc.freezeToValue());
                 }
 
-                out.addField(field.first, Value(result));
+                out.addField(field.first, Value::consume(result));
             }
             else {
                 verify( false );
@@ -1300,7 +1300,7 @@ namespace mongo {
                 result.push_back(nested);
         }
 
-        return Value(result);
+        return Value::consume(result);
     }
     Value ExpressionFieldPath::evaluatePath(size_t index, const Document& input) const {
         // Note this function is very hot so it is important that is is well optimized.
diff --git a/src/mongo/db/pipeline/value.h b/src/mongo/db/pipeline/value.h
index 26113876265..86cb7fa570c 100644
--- a/src/mongo/db/pipeline/value.h
+++ b/src/mongo/db/pipeline/value.h
@@ -97,6 +97,16 @@ namespace mongo {
         */
         static Value createIntOrLong(long long value);
 
+        /** Construct an Array-typed Value from consumed without copying the vector.
+         *  consumed is replaced with an empty vector.
+         *  In C++11 this would be spelled Value(std::move(consumed)).
+         */
+        static Value consume(vector<Value>& consumed) {
+            RCVector* vec = new RCVector();
+            std::swap(vec->vec, consumed);
+            return Value(ValueStorage(Array, vec));
+        }
+
         /** A "missing" value indicates the lack of a Value.
          *  This is similar to undefined/null but should not appear in output to BSON.
          *  Missing Values are returned by Document when accessing non-existent fields.
@@ -233,6 +243,8 @@ namespace mongo {
         template <typename InvalidArgumentType>
         explicit Value(const InvalidArgumentType& invalidArgument);
 
+        explicit Value(const ValueStorage& storage) :_storage(storage) {}
+
         // does no type checking
         StringData getStringData() const; // May contain embedded NUL bytes