SERVER-12132 Stop using stringstream in Value::coerceToString()

This speeds up the function by two changes:
1) Using StringBuilder (through str::stream) rather than stringstream.
2) Not constructing a StringBuilder or stringstream when not needed.

At least with libstdc++, constructing and destructing a stringstream is
extremely expensive.

1 files changed, 4 insertions, 9 deletions

diff --git a/src/mongo/db/pipeline/value.cpp b/src/mongo/db/pipeline/value.cpp
index 9139f0cdbed..4b91603ce18 100644
--- a/src/mongo/db/pipeline/value.cpp
+++ b/src/mongo/db/pipeline/value.cpp
@@ -477,19 +477,15 @@ namespace mongo {
     }
 
     string Value::coerceToString() const {
-        stringstream ss;
         switch(getType()) {
         case NumberDouble:
-            ss << _storage.doubleValue;
-            return ss.str();
+            return str::stream() << _storage.doubleValue;
 
         case NumberInt:
-            ss << _storage.intValue;
-            return ss.str();
+            return str::stream() << _storage.intValue;
 
         case NumberLong:
-            ss << _storage.longValue;
-            return ss.str();
+            return str::stream() << _storage.longValue;
 
         case Code:
         case Symbol:
@@ -497,8 +493,7 @@ namespace mongo {
             return getStringData().toString();
 
         case Timestamp:
-            ss << getTimestamp().toStringPretty();
-            return ss.str();
+            return getTimestamp().toStringPretty();
 
         case Date:
             return tmToISODateString(coerceToTm());