SERVER-23100 Allow StringBuilder and str::stream to return StringData views

Avoids unnecessary copies when passing result to a function.

5 files changed, 20 insertions, 12 deletions

diff --git a/src/mongo/bson/bsonobjbuilder.h b/src/mongo/bson/bsonobjbuilder.h
index 224deb1ee6d..72fd2d901af 100644
--- a/src/mongo/bson/bsonobjbuilder.h
+++ b/src/mongo/bson/bsonobjbuilder.h
@@ -428,7 +428,7 @@ public:
         return appendCode(fieldName, code.code);
     }
 
-    /** Append a std::string element.
+    /** Append a string element.
         @param sz size includes terminating null character */
     BSONObjBuilder& append(StringData fieldName, const char* str, int sz) {
         _b.appendNum((char)String);
@@ -437,15 +437,11 @@ public:
         _b.appendBuf(str, sz);
         return *this;
     }
-    /** Append a std::string element */
+    /** Append a string element */
     BSONObjBuilder& append(StringData fieldName, const char* str) {
         return append(fieldName, str, (int)strlen(str) + 1);
     }
-    /** Append a std::string element */
-    BSONObjBuilder& append(StringData fieldName, const std::string& str) {
-        return append(fieldName, str.c_str(), (int)str.size() + 1);
-    }
-    /** Append a std::string element */
+    /** Append a string element */
     BSONObjBuilder& append(StringData fieldName, StringData str) {
         _b.appendNum((char)String);
         _b.appendStr(fieldName);
diff --git a/src/mongo/bson/util/builder.h b/src/mongo/bson/util/builder.h
index e6d29ceb175..258e63cf52d 100644
--- a/src/mongo/bson/util/builder.h
+++ b/src/mongo/bson/util/builder.h
@@ -446,6 +446,15 @@ public:
         return std::string(_buf.buf(), _buf.l);
     }
 
+    /**
+     * Returns a view of this string without copying.
+     *
+     * WARNING: the view expires when this StringBuilder is modified or destroyed.
+     */
+    StringData stringData() const {
+        return StringData(_buf.buf(), _buf.l);
+    }
+
     /** size of current std::string */
     int len() const {
         return _buf.l;
diff --git a/src/mongo/db/namespace_string.cpp b/src/mongo/db/namespace_string.cpp
index 1128552ca67..95d600fb27b 100644
--- a/src/mongo/db/namespace_string.cpp
+++ b/src/mongo/db/namespace_string.cpp
@@ -117,15 +117,14 @@ bool NamespaceString::isListIndexesCursorNS() const {
 }
 
 NamespaceString NamespaceString::makeListCollectionsNSS(StringData dbName) {
-    NamespaceString nss(dbName, StringData(str::stream() << listCollectionsCursorCol));
+    NamespaceString nss(dbName, listCollectionsCursorCol);
     dassert(nss.isValid());
     dassert(nss.isListCollectionsCursorNS());
     return nss;
 }
 
 NamespaceString NamespaceString::makeListIndexesNSS(StringData dbName, StringData collectionName) {
-    NamespaceString nss(dbName,
-                        StringData(str::stream() << listIndexesCursorNSPrefix << collectionName));
+    NamespaceString nss(dbName, str::stream() << listIndexesCursorNSPrefix << collectionName);
     dassert(nss.isValid());
     dassert(nss.isListIndexesCursorNS());
     return nss;
diff --git a/src/mongo/db/pipeline/document_source_out.cpp b/src/mongo/db/pipeline/document_source_out.cpp
index 4bfc5e14247..b4ff0aa19bc 100644
--- a/src/mongo/db/pipeline/document_source_out.cpp
+++ b/src/mongo/db/pipeline/document_source_out.cpp
@@ -131,8 +131,8 @@ boost::optional<Document> DocumentSourceOut::getNext() {
 
     // We will write all results into a temporary collection, then rename the temporary collection
     // to be the target collection once we are done.
-    _tempNs = NamespaceString(StringData(str::stream() << _outputNs.db() << ".tmp.agg_out."
-                                                       << aggOutCounter.addAndFetch(1)));
+    _tempNs = NamespaceString(str::stream() << _outputNs.db() << ".tmp.agg_out."
+                                            << aggOutCounter.addAndFetch(1));
     auto renameCommandObj =
         BSON("renameCollection" << _tempNs.ns() << "to" << _outputNs.ns() << "dropTarget" << true);
 
diff --git a/src/mongo/util/mongoutils/str.h b/src/mongo/util/mongoutils/str.h
index 2aefea6f261..c1967003506 100644
--- a/src/mongo/util/mongoutils/str.h
+++ b/src/mongo/util/mongoutils/str.h
@@ -39,6 +39,7 @@
 #include <sstream>
 #include <string>
 
+#include "mongo/base/string_data.h"
 #include "mongo/bson/util/builder.h"
 
 namespace mongoutils {
@@ -64,6 +65,9 @@ public:
     operator std::string() const {
         return ss.str();
     }
+    operator mongo::StringData() const {
+        return ss.stringData();
+    }
 };
 
 inline bool startsWith(const char* str, const char* prefix) {